Merge tag 'docs-5.0-fixes' of git://git.lwn.net/linux

[thirdparty/kernel/linux.git] / fs / btrfs / raid56.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2012 Fusion-io  All rights reserved.
 * Copyright (C) 2012 Intel Corp. All rights reserved.
 */

#include <linux/sched.h>
#include <linux/bio.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/raid/pq.h>
#include <linux/hash.h>
#include <linux/list_sort.h>
#include <linux/raid/xor.h>
#include <linux/mm.h>
#include "ctree.h"
#include "disk-io.h"
#include "volumes.h"
#include "raid56.h"
#include "async-thread.h"

/* set when additional merges to this rbio are not allowed */
#define RBIO_RMW_LOCKED_BIT     1

/*
 * set when this rbio is sitting in the hash, but it is just a cache
 * of past RMW
 */
#define RBIO_CACHE_BIT          2

/*
 * set when it is safe to trust the stripe_pages for caching
 */
#define RBIO_CACHE_READY_BIT    3

#define RBIO_CACHE_SIZE 1024

enum btrfs_rbio_ops {
        BTRFS_RBIO_WRITE,
        BTRFS_RBIO_READ_REBUILD,
        BTRFS_RBIO_PARITY_SCRUB,
        BTRFS_RBIO_REBUILD_MISSING,
};

struct btrfs_raid_bio {
        struct btrfs_fs_info *fs_info;
        struct btrfs_bio *bbio;

        /* while we're doing rmw on a stripe
         * we put it into a hash table so we can
         * lock the stripe and merge more rbios
         * into it.
         */
        struct list_head hash_list;

        /*
         * LRU list for the stripe cache
         */
        struct list_head stripe_cache;

        /*
         * for scheduling work in the helper threads
         */
        struct btrfs_work work;

        /*
         * bio list and bio_list_lock are used
         * to add more bios into the stripe
         * in hopes of avoiding the full rmw
         */
        struct bio_list bio_list;
        spinlock_t bio_list_lock;

        /* also protected by the bio_list_lock, the
         * plug list is used by the plugging code
         * to collect partial bios while plugged.  The
         * stripe locking code also uses it to hand off
         * the stripe lock to the next pending IO
         */
        struct list_head plug_list;

        /*
         * flags that tell us if it is safe to
         * merge with this bio
         */
        unsigned long flags;

        /* size of each individual stripe on disk */
        int stripe_len;

        /* number of data stripes (no p/q) */
        int nr_data;

        int real_stripes;

        int stripe_npages;
        /*
         * set if we're doing a parity rebuild
         * for a read from higher up, which is handled
         * differently from a parity rebuild as part of
         * rmw
         */
        enum btrfs_rbio_ops operation;

        /* first bad stripe */
        int faila;

        /* second bad stripe (for raid6 use) */
        int failb;

        int scrubp;
        /*
         * number of pages needed to represent the full
         * stripe
         */
        int nr_pages;

        /*
         * size of all the bios in the bio_list.  This
         * helps us decide if the rbio maps to a full
         * stripe or not
         */
        int bio_list_bytes;

        int generic_bio_cnt;

        refcount_t refs;

        atomic_t stripes_pending;

        atomic_t error;
        /*
         * these are two arrays of pointers.  We allocate the
         * rbio big enough to hold them both and setup their
         * locations when the rbio is allocated
         */

        /* pointers to pages that we allocated for
         * reading/writing stripes directly from the disk (including P/Q)
         */
        struct page **stripe_pages;

        /*
         * pointers to the pages in the bio_list.  Stored
         * here for faster lookup
         */
        struct page **bio_pages;

        /*
         * bitmap to record which horizontal stripe has data
         */
        unsigned long *dbitmap;

        /* allocated with real_stripes-many pointers for finish_*() calls */
        void **finish_pointers;

        /* allocated with stripe_npages-many bits for finish_*() calls */
        unsigned long *finish_pbitmap;
};

static int __raid56_parity_recover(struct btrfs_raid_bio *rbio);
static noinline void finish_rmw(struct btrfs_raid_bio *rbio);
static void rmw_work(struct btrfs_work *work);
static void read_rebuild_work(struct btrfs_work *work);
static int fail_bio_stripe(struct btrfs_raid_bio *rbio, struct bio *bio);
static int fail_rbio_index(struct btrfs_raid_bio *rbio, int failed);
static void __free_raid_bio(struct btrfs_raid_bio *rbio);
static void index_rbio_pages(struct btrfs_raid_bio *rbio);
static int alloc_rbio_pages(struct btrfs_raid_bio *rbio);

static noinline void finish_parity_scrub(struct btrfs_raid_bio *rbio,
                                         int need_check);
static void scrub_parity_work(struct btrfs_work *work);

static void start_async_work(struct btrfs_raid_bio *rbio, btrfs_func_t work_func)
{
        btrfs_init_work(&rbio->work, btrfs_rmw_helper, work_func, NULL, NULL);
        btrfs_queue_work(rbio->fs_info->rmw_workers, &rbio->work);
}

/*
 * the stripe hash table is used for locking, and to collect
 * bios in hopes of making a full stripe
 */
int btrfs_alloc_stripe_hash_table(struct btrfs_fs_info *info)
{
        struct btrfs_stripe_hash_table *table;
        struct btrfs_stripe_hash_table *x;
        struct btrfs_stripe_hash *cur;
        struct btrfs_stripe_hash *h;
        int num_entries = 1 << BTRFS_STRIPE_HASH_TABLE_BITS;
        int i;
        int table_size;

        if (info->stripe_hash_table)
                return 0;

        /*
         * The table is large, starting with order 4 and can go as high as
         * order 7 in case lock debugging is turned on.
         *
         * Try harder to allocate and fallback to vmalloc to lower the chance
         * of a failing mount.
         */
        table_size = sizeof(*table) + sizeof(*h) * num_entries;
        table = kvzalloc(table_size, GFP_KERNEL);
        if (!table)
                return -ENOMEM;

        spin_lock_init(&table->cache_lock);
        INIT_LIST_HEAD(&table->stripe_cache);

        h = table->table;

        for (i = 0; i < num_entries; i++) {
                cur = h + i;
                INIT_LIST_HEAD(&cur->hash_list);
                spin_lock_init(&cur->lock);
        }

        x = cmpxchg(&info->stripe_hash_table, NULL, table);
        if (x)
                kvfree(x);
        return 0;
}

/*
 * caching an rbio means to copy anything from the
 * bio_pages array into the stripe_pages array.  We
 * use the page uptodate bit in the stripe cache array
 * to indicate if it has valid data
 *
 * once the caching is done, we set the cache ready
 * bit.
 */
static void cache_rbio_pages(struct btrfs_raid_bio *rbio)
{
        int i;
        char *s;
        char *d;
        int ret;

        ret = alloc_rbio_pages(rbio);
        if (ret)
                return;

        for (i = 0; i < rbio->nr_pages; i++) {
                if (!rbio->bio_pages[i])
                        continue;

                s = kmap(rbio->bio_pages[i]);
                d = kmap(rbio->stripe_pages[i]);

                copy_page(d, s);

                kunmap(rbio->bio_pages[i]);
                kunmap(rbio->stripe_pages[i]);
                SetPageUptodate(rbio->stripe_pages[i]);
        }
        set_bit(RBIO_CACHE_READY_BIT, &rbio->flags);
}

/*
 * we hash on the first logical address of the stripe
 */
static int rbio_bucket(struct btrfs_raid_bio *rbio)
{
        u64 num = rbio->bbio->raid_map[0];

        /*
         * we shift down quite a bit.  We're using byte
         * addressing, and most of the lower bits are zeros.
         * This tends to upset hash_64, and it consistently
         * returns just one or two different values.
         *
         * shifting off the lower bits fixes things.
         */
        return hash_64(num >> 16, BTRFS_STRIPE_HASH_TABLE_BITS);
}

/*
 * stealing an rbio means taking all the uptodate pages from the stripe
 * array in the source rbio and putting them into the destination rbio
 */
static void steal_rbio(struct btrfs_raid_bio *src, struct btrfs_raid_bio *dest)
{
        int i;
        struct page *s;
        struct page *d;

        if (!test_bit(RBIO_CACHE_READY_BIT, &src->flags))
                return;

        for (i = 0; i < dest->nr_pages; i++) {
                s = src->stripe_pages[i];
                if (!s || !PageUptodate(s)) {
                        continue;
                }

                d = dest->stripe_pages[i];
                if (d)
                        __free_page(d);

                dest->stripe_pages[i] = s;
                src->stripe_pages[i] = NULL;
        }
}

/*
 * merging means we take the bio_list from the victim and
 * splice it into the destination.  The victim should
 * be discarded afterwards.
 *
 * must be called with dest->rbio_list_lock held
 */
static void merge_rbio(struct btrfs_raid_bio *dest,
                       struct btrfs_raid_bio *victim)
{
        bio_list_merge(&dest->bio_list, &victim->bio_list);
        dest->bio_list_bytes += victim->bio_list_bytes;
        dest->generic_bio_cnt += victim->generic_bio_cnt;
        bio_list_init(&victim->bio_list);
}

/*
 * used to prune items that are in the cache.  The caller
 * must hold the hash table lock.
 */
static void __remove_rbio_from_cache(struct btrfs_raid_bio *rbio)
{
        int bucket = rbio_bucket(rbio);
        struct btrfs_stripe_hash_table *table;
        struct btrfs_stripe_hash *h;
        int freeit = 0;

        /*
         * check the bit again under the hash table lock.
         */
        if (!test_bit(RBIO_CACHE_BIT, &rbio->flags))
                return;

        table = rbio->fs_info->stripe_hash_table;
        h = table->table + bucket;

        /* hold the lock for the bucket because we may be
         * removing it from the hash table
         */
        spin_lock(&h->lock);

        /*
         * hold the lock for the bio list because we need
         * to make sure the bio list is empty
         */
        spin_lock(&rbio->bio_list_lock);

        if (test_and_clear_bit(RBIO_CACHE_BIT, &rbio->flags)) {
                list_del_init(&rbio->stripe_cache);
                table->cache_size -= 1;
                freeit = 1;

                /* if the bio list isn't empty, this rbio is
                 * still involved in an IO.  We take it out
                 * of the cache list, and drop the ref that
                 * was held for the list.
                 *
                 * If the bio_list was empty, we also remove
                 * the rbio from the hash_table, and drop
                 * the corresponding ref
                 */
                if (bio_list_empty(&rbio->bio_list)) {
                        if (!list_empty(&rbio->hash_list)) {
                                list_del_init(&rbio->hash_list);
                                refcount_dec(&rbio->refs);
                                BUG_ON(!list_empty(&rbio->plug_list));
                        }
                }
        }

        spin_unlock(&rbio->bio_list_lock);
        spin_unlock(&h->lock);

        if (freeit)
                __free_raid_bio(rbio);
}

/*
 * prune a given rbio from the cache
 */
static void remove_rbio_from_cache(struct btrfs_raid_bio *rbio)
{
        struct btrfs_stripe_hash_table *table;
        unsigned long flags;

        if (!test_bit(RBIO_CACHE_BIT, &rbio->flags))
                return;

        table = rbio->fs_info->stripe_hash_table;

        spin_lock_irqsave(&table->cache_lock, flags);
        __remove_rbio_from_cache(rbio);
        spin_unlock_irqrestore(&table->cache_lock, flags);
}

/*
 * remove everything in the cache
 */
static void btrfs_clear_rbio_cache(struct btrfs_fs_info *info)
{
        struct btrfs_stripe_hash_table *table;
        unsigned long flags;
        struct btrfs_raid_bio *rbio;

        table = info->stripe_hash_table;

        spin_lock_irqsave(&table->cache_lock, flags);
        while (!list_empty(&table->stripe_cache)) {
                rbio = list_entry(table->stripe_cache.next,
                                  struct btrfs_raid_bio,
                                  stripe_cache);
                __remove_rbio_from_cache(rbio);
        }
        spin_unlock_irqrestore(&table->cache_lock, flags);
}

/*
 * remove all cached entries and free the hash table
 * used by unmount
 */
void btrfs_free_stripe_hash_table(struct btrfs_fs_info *info)
{
        if (!info->stripe_hash_table)
                return;
        btrfs_clear_rbio_cache(info);
        kvfree(info->stripe_hash_table);
        info->stripe_hash_table = NULL;
}

/*
 * insert an rbio into the stripe cache.  It
 * must have already been prepared by calling
 * cache_rbio_pages
 *
 * If this rbio was already cached, it gets
 * moved to the front of the lru.
 *
 * If the size of the rbio cache is too big, we
 * prune an item.
 */
static void cache_rbio(struct btrfs_raid_bio *rbio)
{
        struct btrfs_stripe_hash_table *table;
        unsigned long flags;

        if (!test_bit(RBIO_CACHE_READY_BIT, &rbio->flags))
                return;

        table = rbio->fs_info->stripe_hash_table;

        spin_lock_irqsave(&table->cache_lock, flags);
        spin_lock(&rbio->bio_list_lock);

        /* bump our ref if we were not in the list before */
        if (!test_and_set_bit(RBIO_CACHE_BIT, &rbio->flags))
                refcount_inc(&rbio->refs);

        if (!list_empty(&rbio->stripe_cache)){
                list_move(&rbio->stripe_cache, &table->stripe_cache);
        } else {
                list_add(&rbio->stripe_cache, &table->stripe_cache);
                table->cache_size += 1;
        }

        spin_unlock(&rbio->bio_list_lock);

        if (table->cache_size > RBIO_CACHE_SIZE) {
                struct btrfs_raid_bio *found;

                found = list_entry(table->stripe_cache.prev,
                                  struct btrfs_raid_bio,
                                  stripe_cache);

                if (found != rbio)
                        __remove_rbio_from_cache(found);
        }

        spin_unlock_irqrestore(&table->cache_lock, flags);
}

/*
 * helper function to run the xor_blocks api.  It is only
 * able to do MAX_XOR_BLOCKS at a time, so we need to
 * loop through.
 */
static void run_xor(void **pages, int src_cnt, ssize_t len)
{
        int src_off = 0;
        int xor_src_cnt = 0;
        void *dest = pages[src_cnt];

        while(src_cnt > 0) {
                xor_src_cnt = min(src_cnt, MAX_XOR_BLOCKS);
                xor_blocks(xor_src_cnt, len, dest, pages + src_off);

                src_cnt -= xor_src_cnt;
                src_off += xor_src_cnt;
        }
}

/*
 * Returns true if the bio list inside this rbio covers an entire stripe (no
 * rmw required).
 */
static int rbio_is_full(struct btrfs_raid_bio *rbio)
{
        unsigned long flags;
        unsigned long size = rbio->bio_list_bytes;
        int ret = 1;

        spin_lock_irqsave(&rbio->bio_list_lock, flags);
        if (size != rbio->nr_data * rbio->stripe_len)
                ret = 0;
        BUG_ON(size > rbio->nr_data * rbio->stripe_len);
        spin_unlock_irqrestore(&rbio->bio_list_lock, flags);

        return ret;
}

/*
 * returns 1 if it is safe to merge two rbios together.
 * The merging is safe if the two rbios correspond to
 * the same stripe and if they are both going in the same
 * direction (read vs write), and if neither one is
 * locked for final IO
 *
 * The caller is responsible for locking such that
 * rmw_locked is safe to test
 */
static int rbio_can_merge(struct btrfs_raid_bio *last,
                          struct btrfs_raid_bio *cur)
{
        if (test_bit(RBIO_RMW_LOCKED_BIT, &last->flags) ||
            test_bit(RBIO_RMW_LOCKED_BIT, &cur->flags))
                return 0;

        /*
         * we can't merge with cached rbios, since the
         * idea is that when we merge the destination
         * rbio is going to run our IO for us.  We can
         * steal from cached rbios though, other functions
         * handle that.
         */
        if (test_bit(RBIO_CACHE_BIT, &last->flags) ||
            test_bit(RBIO_CACHE_BIT, &cur->flags))
                return 0;

        if (last->bbio->raid_map[0] !=
            cur->bbio->raid_map[0])
                return 0;

        /* we can't merge with different operations */
        if (last->operation != cur->operation)
                return 0;
        /*
         * We've need read the full stripe from the drive.
         * check and repair the parity and write the new results.
         *
         * We're not allowed to add any new bios to the
         * bio list here, anyone else that wants to
         * change this stripe needs to do their own rmw.
         */
        if (last->operation == BTRFS_RBIO_PARITY_SCRUB)
                return 0;

        if (last->operation == BTRFS_RBIO_REBUILD_MISSING)
                return 0;

        if (last->operation == BTRFS_RBIO_READ_REBUILD) {
                int fa = last->faila;
                int fb = last->failb;
                int cur_fa = cur->faila;
                int cur_fb = cur->failb;

                if (last->faila >= last->failb) {
                        fa = last->failb;
                        fb = last->faila;
                }

                if (cur->faila >= cur->failb) {
                        cur_fa = cur->failb;
                        cur_fb = cur->faila;
                }

                if (fa != cur_fa || fb != cur_fb)
                        return 0;
        }
        return 1;
}

static int rbio_stripe_page_index(struct btrfs_raid_bio *rbio, int stripe,
                                  int index)
{
        return stripe * rbio->stripe_npages + index;
}

/*
 * these are just the pages from the rbio array, not from anything
 * the FS sent down to us
 */
static struct page *rbio_stripe_page(struct btrfs_raid_bio *rbio, int stripe,
                                     int index)
{
        return rbio->stripe_pages[rbio_stripe_page_index(rbio, stripe, index)];
}

/*
 * helper to index into the pstripe
 */
static struct page *rbio_pstripe_page(struct btrfs_raid_bio *rbio, int index)
{
        return rbio_stripe_page(rbio, rbio->nr_data, index);
}

/*
 * helper to index into the qstripe, returns null
 * if there is no qstripe
 */
static struct page *rbio_qstripe_page(struct btrfs_raid_bio *rbio, int index)
{
        if (rbio->nr_data + 1 == rbio->real_stripes)
                return NULL;
        return rbio_stripe_page(rbio, rbio->nr_data + 1, index);
}

/*
 * The first stripe in the table for a logical address
 * has the lock.  rbios are added in one of three ways:
 *
 * 1) Nobody has the stripe locked yet.  The rbio is given
 * the lock and 0 is returned.  The caller must start the IO
 * themselves.
 *
 * 2) Someone has the stripe locked, but we're able to merge
 * with the lock owner.  The rbio is freed and the IO will
 * start automatically along with the existing rbio.  1 is returned.
 *
 * 3) Someone has the stripe locked, but we're not able to merge.
 * The rbio is added to the lock owner's plug list, or merged into
 * an rbio already on the plug list.  When the lock owner unlocks,
 * the next rbio on the list is run and the IO is started automatically.
 * 1 is returned
 *
 * If we return 0, the caller still owns the rbio and must continue with
 * IO submission.  If we return 1, the caller must assume the rbio has
 * already been freed.
 */
static noinline int lock_stripe_add(struct btrfs_raid_bio *rbio)
{
        int bucket = rbio_bucket(rbio);
        struct btrfs_stripe_hash *h = rbio->fs_info->stripe_hash_table->table + bucket;
        struct btrfs_raid_bio *cur;
        struct btrfs_raid_bio *pending;
        unsigned long flags;
        struct btrfs_raid_bio *freeit = NULL;
        struct btrfs_raid_bio *cache_drop = NULL;
        int ret = 0;

        spin_lock_irqsave(&h->lock, flags);
        list_for_each_entry(cur, &h->hash_list, hash_list) {
                if (cur->bbio->raid_map[0] == rbio->bbio->raid_map[0]) {
                        spin_lock(&cur->bio_list_lock);

                        /* can we steal this cached rbio's pages? */
                        if (bio_list_empty(&cur->bio_list) &&
                            list_empty(&cur->plug_list) &&
                            test_bit(RBIO_CACHE_BIT, &cur->flags) &&
                            !test_bit(RBIO_RMW_LOCKED_BIT, &cur->flags)) {
                                list_del_init(&cur->hash_list);
                                refcount_dec(&cur->refs);

                                steal_rbio(cur, rbio);
                                cache_drop = cur;
                                spin_unlock(&cur->bio_list_lock);

                                goto lockit;
                        }

                        /* can we merge into the lock owner? */
                        if (rbio_can_merge(cur, rbio)) {
                                merge_rbio(cur, rbio);
                                spin_unlock(&cur->bio_list_lock);
                                freeit = rbio;
                                ret = 1;
                                goto out;
                        }


                        /*
                         * we couldn't merge with the running
                         * rbio, see if we can merge with the
                         * pending ones.  We don't have to
                         * check for rmw_locked because there
                         * is no way they are inside finish_rmw
                         * right now
                         */
                        list_for_each_entry(pending, &cur->plug_list,
                                            plug_list) {
                                if (rbio_can_merge(pending, rbio)) {
                                        merge_rbio(pending, rbio);
                                        spin_unlock(&cur->bio_list_lock);
                                        freeit = rbio;
                                        ret = 1;
                                        goto out;
                                }
                        }

                        /* no merging, put us on the tail of the plug list,
                         * our rbio will be started with the currently
                         * running rbio unlocks
                         */
                        list_add_tail(&rbio->plug_list, &cur->plug_list);
                        spin_unlock(&cur->bio_list_lock);
                        ret = 1;
                        goto out;
                }
        }
lockit:
        refcount_inc(&rbio->refs);
        list_add(&rbio->hash_list, &h->hash_list);
out:
        spin_unlock_irqrestore(&h->lock, flags);
        if (cache_drop)
                remove_rbio_from_cache(cache_drop);
        if (freeit)
                __free_raid_bio(freeit);
        return ret;
}

/*
 * called as rmw or parity rebuild is completed.  If the plug list has more
 * rbios waiting for this stripe, the next one on the list will be started
 */
static noinline void unlock_stripe(struct btrfs_raid_bio *rbio)
{
        int bucket;
        struct btrfs_stripe_hash *h;
        unsigned long flags;
        int keep_cache = 0;

        bucket = rbio_bucket(rbio);
        h = rbio->fs_info->stripe_hash_table->table + bucket;

        if (list_empty(&rbio->plug_list))
                cache_rbio(rbio);

        spin_lock_irqsave(&h->lock, flags);
        spin_lock(&rbio->bio_list_lock);

        if (!list_empty(&rbio->hash_list)) {
                /*
                 * if we're still cached and there is no other IO
                 * to perform, just leave this rbio here for others
                 * to steal from later
                 */
                if (list_empty(&rbio->plug_list) &&
                    test_bit(RBIO_CACHE_BIT, &rbio->flags)) {
                        keep_cache = 1;
                        clear_bit(RBIO_RMW_LOCKED_BIT, &rbio->flags);
                        BUG_ON(!bio_list_empty(&rbio->bio_list));
                        goto done;
                }

                list_del_init(&rbio->hash_list);
                refcount_dec(&rbio->refs);

                /*
                 * we use the plug list to hold all the rbios
                 * waiting for the chance to lock this stripe.
                 * hand the lock over to one of them.
                 */
                if (!list_empty(&rbio->plug_list)) {
                        struct btrfs_raid_bio *next;
                        struct list_head *head = rbio->plug_list.next;

                        next = list_entry(head, struct btrfs_raid_bio,
                                          plug_list);

                        list_del_init(&rbio->plug_list);

                        list_add(&next->hash_list, &h->hash_list);
                        refcount_inc(&next->refs);
                        spin_unlock(&rbio->bio_list_lock);
                        spin_unlock_irqrestore(&h->lock, flags);

                        if (next->operation == BTRFS_RBIO_READ_REBUILD)
                                start_async_work(next, read_rebuild_work);
                        else if (next->operation == BTRFS_RBIO_REBUILD_MISSING) {
                                steal_rbio(rbio, next);
                                start_async_work(next, read_rebuild_work);
                        } else if (next->operation == BTRFS_RBIO_WRITE) {
                                steal_rbio(rbio, next);
                                start_async_work(next, rmw_work);
                        } else if (next->operation == BTRFS_RBIO_PARITY_SCRUB) {
                                steal_rbio(rbio, next);
                                start_async_work(next, scrub_parity_work);
                        }

                        goto done_nolock;
                }
        }
done:
        spin_unlock(&rbio->bio_list_lock);
        spin_unlock_irqrestore(&h->lock, flags);

done_nolock:
        if (!keep_cache)
                remove_rbio_from_cache(rbio);
}

static void __free_raid_bio(struct btrfs_raid_bio *rbio)
{
        int i;

        if (!refcount_dec_and_test(&rbio->refs))
                return;

        WARN_ON(!list_empty(&rbio->stripe_cache));
        WARN_ON(!list_empty(&rbio->hash_list));
        WARN_ON(!bio_list_empty(&rbio->bio_list));

        for (i = 0; i < rbio->nr_pages; i++) {
                if (rbio->stripe_pages[i]) {
                        __free_page(rbio->stripe_pages[i]);
                        rbio->stripe_pages[i] = NULL;
                }
        }

        btrfs_put_bbio(rbio->bbio);
        kfree(rbio);
}

static void rbio_endio_bio_list(struct bio *cur, blk_status_t err)
{
        struct bio *next;

        while (cur) {
                next = cur->bi_next;
                cur->bi_next = NULL;
                cur->bi_status = err;
                bio_endio(cur);
                cur = next;
        }
}

/*
 * this frees the rbio and runs through all the bios in the
 * bio_list and calls end_io on them
 */
static void rbio_orig_end_io(struct btrfs_raid_bio *rbio, blk_status_t err)
{
        struct bio *cur = bio_list_get(&rbio->bio_list);
        struct bio *extra;

        if (rbio->generic_bio_cnt)
                btrfs_bio_counter_sub(rbio->fs_info, rbio->generic_bio_cnt);

        /*
         * At this moment, rbio->bio_list is empty, however since rbio does not
         * always have RBIO_RMW_LOCKED_BIT set and rbio is still linked on the
         * hash list, rbio may be merged with others so that rbio->bio_list
         * becomes non-empty.
         * Once unlock_stripe() is done, rbio->bio_list will not be updated any
         * more and we can call bio_endio() on all queued bios.
         */
        unlock_stripe(rbio);
        extra = bio_list_get(&rbio->bio_list);
        __free_raid_bio(rbio);

        rbio_endio_bio_list(cur, err);
        if (extra)
                rbio_endio_bio_list(extra, err);
}

/*
 * end io function used by finish_rmw.  When we finally
 * get here, we've written a full stripe
 */
static void raid_write_end_io(struct bio *bio)
{
        struct btrfs_raid_bio *rbio = bio->bi_private;
        blk_status_t err = bio->bi_status;
        int max_errors;

        if (err)
                fail_bio_stripe(rbio, bio);

        bio_put(bio);

        if (!atomic_dec_and_test(&rbio->stripes_pending))
                return;

        err = BLK_STS_OK;

        /* OK, we have read all the stripes we need to. */
        max_errors = (rbio->operation == BTRFS_RBIO_PARITY_SCRUB) ?
                     0 : rbio->bbio->max_errors;
        if (atomic_read(&rbio->error) > max_errors)
                err = BLK_STS_IOERR;

        rbio_orig_end_io(rbio, err);
}

/*
 * the read/modify/write code wants to use the original bio for
 * any pages it included, and then use the rbio for everything
 * else.  This function decides if a given index (stripe number)
 * and page number in that stripe fall inside the original bio
 * or the rbio.
 *
 * if you set bio_list_only, you'll get a NULL back for any ranges
 * that are outside the bio_list
 *
 * This doesn't take any refs on anything, you get a bare page pointer
 * and the caller must bump refs as required.
 *
 * You must call index_rbio_pages once before you can trust
 * the answers from this function.
 */
static struct page *page_in_rbio(struct btrfs_raid_bio *rbio,
                                 int index, int pagenr, int bio_list_only)
{
        int chunk_page;
        struct page *p = NULL;

        chunk_page = index * (rbio->stripe_len >> PAGE_SHIFT) + pagenr;

        spin_lock_irq(&rbio->bio_list_lock);
        p = rbio->bio_pages[chunk_page];
        spin_unlock_irq(&rbio->bio_list_lock);

        if (p || bio_list_only)
                return p;

        return rbio->stripe_pages[chunk_page];
}

/*
 * number of pages we need for the entire stripe across all the
 * drives
 */
static unsigned long rbio_nr_pages(unsigned long stripe_len, int nr_stripes)
{
        return DIV_ROUND_UP(stripe_len, PAGE_SIZE) * nr_stripes;
}

/*
 * allocation and initial setup for the btrfs_raid_bio.  Not
 * this does not allocate any pages for rbio->pages.
 */
static struct btrfs_raid_bio *alloc_rbio(struct btrfs_fs_info *fs_info,
                                         struct btrfs_bio *bbio,
                                         u64 stripe_len)
{
        struct btrfs_raid_bio *rbio;
        int nr_data = 0;
        int real_stripes = bbio->num_stripes - bbio->num_tgtdevs;
        int num_pages = rbio_nr_pages(stripe_len, real_stripes);
        int stripe_npages = DIV_ROUND_UP(stripe_len, PAGE_SIZE);
        void *p;

        rbio = kzalloc(sizeof(*rbio) +
                       sizeof(*rbio->stripe_pages) * num_pages +
                       sizeof(*rbio->bio_pages) * num_pages +
                       sizeof(*rbio->finish_pointers) * real_stripes +
                       sizeof(*rbio->dbitmap) * BITS_TO_LONGS(stripe_npages) +
                       sizeof(*rbio->finish_pbitmap) *
                                BITS_TO_LONGS(stripe_npages),
                       GFP_NOFS);
        if (!rbio)
                return ERR_PTR(-ENOMEM);

        bio_list_init(&rbio->bio_list);
        INIT_LIST_HEAD(&rbio->plug_list);
        spin_lock_init(&rbio->bio_list_lock);
        INIT_LIST_HEAD(&rbio->stripe_cache);
        INIT_LIST_HEAD(&rbio->hash_list);
        rbio->bbio = bbio;
        rbio->fs_info = fs_info;
        rbio->stripe_len = stripe_len;
        rbio->nr_pages = num_pages;
        rbio->real_stripes = real_stripes;
        rbio->stripe_npages = stripe_npages;
        rbio->faila = -1;
        rbio->failb = -1;
        refcount_set(&rbio->refs, 1);
        atomic_set(&rbio->error, 0);
        atomic_set(&rbio->stripes_pending, 0);

        /*
         * the stripe_pages, bio_pages, etc arrays point to the extra
         * memory we allocated past the end of the rbio
         */
        p = rbio + 1;
#define CONSUME_ALLOC(ptr, count)       do {                            \
                ptr = p;                                                \
                p = (unsigned char *)p + sizeof(*(ptr)) * (count);      \
        } while (0)
        CONSUME_ALLOC(rbio->stripe_pages, num_pages);
        CONSUME_ALLOC(rbio->bio_pages, num_pages);
        CONSUME_ALLOC(rbio->finish_pointers, real_stripes);
        CONSUME_ALLOC(rbio->dbitmap, BITS_TO_LONGS(stripe_npages));
        CONSUME_ALLOC(rbio->finish_pbitmap, BITS_TO_LONGS(stripe_npages));
#undef  CONSUME_ALLOC

        if (bbio->map_type & BTRFS_BLOCK_GROUP_RAID5)
                nr_data = real_stripes - 1;
        else if (bbio->map_type & BTRFS_BLOCK_GROUP_RAID6)
                nr_data = real_stripes - 2;
        else
                BUG();

        rbio->nr_data = nr_data;
        return rbio;
}

/* allocate pages for all the stripes in the bio, including parity */
static int alloc_rbio_pages(struct btrfs_raid_bio *rbio)
{
        int i;
        struct page *page;

        for (i = 0; i < rbio->nr_pages; i++) {
                if (rbio->stripe_pages[i])
                        continue;
                page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
                if (!page)
                        return -ENOMEM;
                rbio->stripe_pages[i] = page;
        }
        return 0;
}

/* only allocate pages for p/q stripes */
static int alloc_rbio_parity_pages(struct btrfs_raid_bio *rbio)
{
        int i;
        struct page *page;

        i = rbio_stripe_page_index(rbio, rbio->nr_data, 0);

        for (; i < rbio->nr_pages; i++) {
                if (rbio->stripe_pages[i])
                        continue;
                page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
                if (!page)
                        return -ENOMEM;
                rbio->stripe_pages[i] = page;
        }
        return 0;
}

/*
 * add a single page from a specific stripe into our list of bios for IO
 * this will try to merge into existing bios if possible, and returns
 * zero if all went well.
 */
static int rbio_add_io_page(struct btrfs_raid_bio *rbio,
                            struct bio_list *bio_list,
                            struct page *page,
                            int stripe_nr,
                            unsigned long page_index,
                            unsigned long bio_max_len)
{
        struct bio *last = bio_list->tail;
        u64 last_end = 0;
        int ret;
        struct bio *bio;
        struct btrfs_bio_stripe *stripe;
        u64 disk_start;

        stripe = &rbio->bbio->stripes[stripe_nr];
        disk_start = stripe->physical + (page_index << PAGE_SHIFT);

        /* if the device is missing, just fail this stripe */
        if (!stripe->dev->bdev)
                return fail_rbio_index(rbio, stripe_nr);

        /* see if we can add this page onto our existing bio */
        if (last) {
                last_end = (u64)last->bi_iter.bi_sector << 9;
                last_end += last->bi_iter.bi_size;

                /*
                 * we can't merge these if they are from different
                 * devices or if they are not contiguous
                 */
                if (last_end == disk_start && stripe->dev->bdev &&
                    !last->bi_status &&
                    last->bi_disk == stripe->dev->bdev->bd_disk &&
                    last->bi_partno == stripe->dev->bdev->bd_partno) {
                        ret = bio_add_page(last, page, PAGE_SIZE, 0);
                        if (ret == PAGE_SIZE)
                                return 0;
                }
        }

        /* put a new bio on the list */
        bio = btrfs_io_bio_alloc(bio_max_len >> PAGE_SHIFT ?: 1);
        bio->bi_iter.bi_size = 0;
        bio_set_dev(bio, stripe->dev->bdev);
        bio->bi_iter.bi_sector = disk_start >> 9;

        bio_add_page(bio, page, PAGE_SIZE, 0);
        bio_list_add(bio_list, bio);
        return 0;
}

/*
 * while we're doing the read/modify/write cycle, we could
 * have errors in reading pages off the disk.  This checks
 * for errors and if we're not able to read the page it'll
 * trigger parity reconstruction.  The rmw will be finished
 * after we've reconstructed the failed stripes
 */
static void validate_rbio_for_rmw(struct btrfs_raid_bio *rbio)
{
        if (rbio->faila >= 0 || rbio->failb >= 0) {
                BUG_ON(rbio->faila == rbio->real_stripes - 1);
                __raid56_parity_recover(rbio);
        } else {
                finish_rmw(rbio);
        }
}

/*
 * helper function to walk our bio list and populate the bio_pages array with
 * the result.  This seems expensive, but it is faster than constantly
 * searching through the bio list as we setup the IO in finish_rmw or stripe
 * reconstruction.
 *
 * This must be called before you trust the answers from page_in_rbio
 */
static void index_rbio_pages(struct btrfs_raid_bio *rbio)
{
        struct bio *bio;
        u64 start;
        unsigned long stripe_offset;
        unsigned long page_index;

        spin_lock_irq(&rbio->bio_list_lock);
        bio_list_for_each(bio, &rbio->bio_list) {
                struct bio_vec bvec;
                struct bvec_iter iter;
                int i = 0;

                start = (u64)bio->bi_iter.bi_sector << 9;
                stripe_offset = start - rbio->bbio->raid_map[0];
                page_index = stripe_offset >> PAGE_SHIFT;

                if (bio_flagged(bio, BIO_CLONED))
                        bio->bi_iter = btrfs_io_bio(bio)->iter;

                bio_for_each_segment(bvec, bio, iter) {
                        rbio->bio_pages[page_index + i] = bvec.bv_page;
                        i++;
                }
        }
        spin_unlock_irq(&rbio->bio_list_lock);
}

/*
 * this is called from one of two situations.  We either
 * have a full stripe from the higher layers, or we've read all
 * the missing bits off disk.
 *
 * This will calculate the parity and then send down any
 * changed blocks.
 */
static noinline void finish_rmw(struct btrfs_raid_bio *rbio)
{
        struct btrfs_bio *bbio = rbio->bbio;
        void **pointers = rbio->finish_pointers;
        int nr_data = rbio->nr_data;
        int stripe;
        int pagenr;
        int p_stripe = -1;
        int q_stripe = -1;
        struct bio_list bio_list;
        struct bio *bio;
        int ret;

        bio_list_init(&bio_list);

        if (rbio->real_stripes - rbio->nr_data == 1) {
                p_stripe = rbio->real_stripes - 1;
        } else if (rbio->real_stripes - rbio->nr_data == 2) {
                p_stripe = rbio->real_stripes - 2;
                q_stripe = rbio->real_stripes - 1;
        } else {
                BUG();
        }

        /* at this point we either have a full stripe,
         * or we've read the full stripe from the drive.
         * recalculate the parity and write the new results.
         *
         * We're not allowed to add any new bios to the
         * bio list here, anyone else that wants to
         * change this stripe needs to do their own rmw.
         */
        spin_lock_irq(&rbio->bio_list_lock);
        set_bit(RBIO_RMW_LOCKED_BIT, &rbio->flags);
        spin_unlock_irq(&rbio->bio_list_lock);

        atomic_set(&rbio->error, 0);

        /*
         * now that we've set rmw_locked, run through the
         * bio list one last time and map the page pointers
         *
         * We don't cache full rbios because we're assuming
         * the higher layers are unlikely to use this area of
         * the disk again soon.  If they do use it again,
         * hopefully they will send another full bio.
         */
        index_rbio_pages(rbio);
        if (!rbio_is_full(rbio))
                cache_rbio_pages(rbio);
        else
                clear_bit(RBIO_CACHE_READY_BIT, &rbio->flags);

        for (pagenr = 0; pagenr < rbio->stripe_npages; pagenr++) {
                struct page *p;
                /* first collect one page from each data stripe */
                for (stripe = 0; stripe < nr_data; stripe++) {
                        p = page_in_rbio(rbio, stripe, pagenr, 0);
                        pointers[stripe] = kmap(p);
                }

                /* then add the parity stripe */
                p = rbio_pstripe_page(rbio, pagenr);
                SetPageUptodate(p);
                pointers[stripe++] = kmap(p);

                if (q_stripe != -1) {

                        /*
                         * raid6, add the qstripe and call the
                         * library function to fill in our p/q
                         */
                        p = rbio_qstripe_page(rbio, pagenr);
                        SetPageUptodate(p);
                        pointers[stripe++] = kmap(p);

                        raid6_call.gen_syndrome(rbio->real_stripes, PAGE_SIZE,
                                                pointers);
                } else {
                        /* raid5 */
                        copy_page(pointers[nr_data], pointers[0]);
                        run_xor(pointers + 1, nr_data - 1, PAGE_SIZE);
                }


                for (stripe = 0; stripe < rbio->real_stripes; stripe++)
                        kunmap(page_in_rbio(rbio, stripe, pagenr, 0));
        }

        /*
         * time to start writing.  Make bios for everything from the
         * higher layers (the bio_list in our rbio) and our p/q.  Ignore
         * everything else.
         */
        for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
                for (pagenr = 0; pagenr < rbio->stripe_npages; pagenr++) {
                        struct page *page;
                        if (stripe < rbio->nr_data) {
                                page = page_in_rbio(rbio, stripe, pagenr, 1);
                                if (!page)
                                        continue;
                        } else {
                               page = rbio_stripe_page(rbio, stripe, pagenr);
                        }

                        ret = rbio_add_io_page(rbio, &bio_list,
                                       page, stripe, pagenr, rbio->stripe_len);
                        if (ret)
                                goto cleanup;
                }
        }

        if (likely(!bbio->num_tgtdevs))
                goto write_data;

        for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
                if (!bbio->tgtdev_map[stripe])
                        continue;

                for (pagenr = 0; pagenr < rbio->stripe_npages; pagenr++) {
                        struct page *page;
                        if (stripe < rbio->nr_data) {
                                page = page_in_rbio(rbio, stripe, pagenr, 1);
                                if (!page)
                                        continue;
                        } else {
                               page = rbio_stripe_page(rbio, stripe, pagenr);
                        }

                        ret = rbio_add_io_page(rbio, &bio_list, page,
                                               rbio->bbio->tgtdev_map[stripe],
                                               pagenr, rbio->stripe_len);
                        if (ret)
                                goto cleanup;
                }
        }

write_data:
        atomic_set(&rbio->stripes_pending, bio_list_size(&bio_list));
        BUG_ON(atomic_read(&rbio->stripes_pending) == 0);

        while (1) {
                bio = bio_list_pop(&bio_list);
                if (!bio)
                        break;

                bio->bi_private = rbio;
                bio->bi_end_io = raid_write_end_io;
                bio->bi_opf = REQ_OP_WRITE;

                submit_bio(bio);
        }
        return;

cleanup:
        rbio_orig_end_io(rbio, BLK_STS_IOERR);

        while ((bio = bio_list_pop(&bio_list)))
                bio_put(bio);
}

/*
 * helper to find the stripe number for a given bio.  Used to figure out which
 * stripe has failed.  This expects the bio to correspond to a physical disk,
 * so it looks up based on physical sector numbers.
 */
static int find_bio_stripe(struct btrfs_raid_bio *rbio,
                           struct bio *bio)
{
        u64 physical = bio->bi_iter.bi_sector;
        u64 stripe_start;
        int i;
        struct btrfs_bio_stripe *stripe;

        physical <<= 9;

        for (i = 0; i < rbio->bbio->num_stripes; i++) {
                stripe = &rbio->bbio->stripes[i];
                stripe_start = stripe->physical;
                if (physical >= stripe_start &&
                    physical < stripe_start + rbio->stripe_len &&
                    stripe->dev->bdev &&
                    bio->bi_disk == stripe->dev->bdev->bd_disk &&
                    bio->bi_partno == stripe->dev->bdev->bd_partno) {
                        return i;
                }
        }
        return -1;
}

/*
 * helper to find the stripe number for a given
 * bio (before mapping).  Used to figure out which stripe has
 * failed.  This looks up based on logical block numbers.
 */
static int find_logical_bio_stripe(struct btrfs_raid_bio *rbio,
                                   struct bio *bio)
{
        u64 logical = bio->bi_iter.bi_sector;
        u64 stripe_start;
        int i;

        logical <<= 9;

        for (i = 0; i < rbio->nr_data; i++) {
                stripe_start = rbio->bbio->raid_map[i];
                if (logical >= stripe_start &&
                    logical < stripe_start + rbio->stripe_len) {
                        return i;
                }
        }
        return -1;
}

/*
 * returns -EIO if we had too many failures
 */
static int fail_rbio_index(struct btrfs_raid_bio *rbio, int failed)
{
        unsigned long flags;
        int ret = 0;

        spin_lock_irqsave(&rbio->bio_list_lock, flags);

        /* we already know this stripe is bad, move on */
        if (rbio->faila == failed || rbio->failb == failed)
                goto out;

        if (rbio->faila == -1) {
                /* first failure on this rbio */
                rbio->faila = failed;
                atomic_inc(&rbio->error);
        } else if (rbio->failb == -1) {
                /* second failure on this rbio */
                rbio->failb = failed;
                atomic_inc(&rbio->error);
        } else {
                ret = -EIO;
        }
out:
        spin_unlock_irqrestore(&rbio->bio_list_lock, flags);

        return ret;
}

/*
 * helper to fail a stripe based on a physical disk
 * bio.
 */
static int fail_bio_stripe(struct btrfs_raid_bio *rbio,
                           struct bio *bio)
{
        int failed = find_bio_stripe(rbio, bio);

        if (failed < 0)
                return -EIO;

        return fail_rbio_index(rbio, failed);
}

/*
 * this sets each page in the bio uptodate.  It should only be used on private
 * rbio pages, nothing that comes in from the higher layers
 */
static void set_bio_pages_uptodate(struct bio *bio)
{
        struct bio_vec *bvec;
        int i;

        ASSERT(!bio_flagged(bio, BIO_CLONED));

        bio_for_each_segment_all(bvec, bio, i)
                SetPageUptodate(bvec->bv_page);
}

/*
 * end io for the read phase of the rmw cycle.  All the bios here are physical
 * stripe bios we've read from the disk so we can recalculate the parity of the
 * stripe.
 *
 * This will usually kick off finish_rmw once all the bios are read in, but it
 * may trigger parity reconstruction if we had any errors along the way
 */
static void raid_rmw_end_io(struct bio *bio)
{
        struct btrfs_raid_bio *rbio = bio->bi_private;

        if (bio->bi_status)
                fail_bio_stripe(rbio, bio);
        else
                set_bio_pages_uptodate(bio);

        bio_put(bio);

        if (!atomic_dec_and_test(&rbio->stripes_pending))
                return;

        if (atomic_read(&rbio->error) > rbio->bbio->max_errors)
                goto cleanup;

        /*
         * this will normally call finish_rmw to start our write
         * but if there are any failed stripes we'll reconstruct
         * from parity first
         */
        validate_rbio_for_rmw(rbio);
        return;

cleanup:

        rbio_orig_end_io(rbio, BLK_STS_IOERR);
}

/*
 * the stripe must be locked by the caller.  It will
 * unlock after all the writes are done
 */
static int raid56_rmw_stripe(struct btrfs_raid_bio *rbio)
{
        int bios_to_read = 0;
        struct bio_list bio_list;
        int ret;
        int pagenr;
        int stripe;
        struct bio *bio;

        bio_list_init(&bio_list);

        ret = alloc_rbio_pages(rbio);
        if (ret)
                goto cleanup;

        index_rbio_pages(rbio);

        atomic_set(&rbio->error, 0);
        /*
         * build a list of bios to read all the missing parts of this
         * stripe
         */
        for (stripe = 0; stripe < rbio->nr_data; stripe++) {
                for (pagenr = 0; pagenr < rbio->stripe_npages; pagenr++) {
                        struct page *page;
                        /*
                         * we want to find all the pages missing from
                         * the rbio and read them from the disk.  If
                         * page_in_rbio finds a page in the bio list
                         * we don't need to read it off the stripe.
                         */
                        page = page_in_rbio(rbio, stripe, pagenr, 1);
                        if (page)
                                continue;

                        page = rbio_stripe_page(rbio, stripe, pagenr);
                        /*
                         * the bio cache may have handed us an uptodate
                         * page.  If so, be happy and use it
                         */
                        if (PageUptodate(page))
                                continue;

                        ret = rbio_add_io_page(rbio, &bio_list, page,
                                       stripe, pagenr, rbio->stripe_len);
                        if (ret)
                                goto cleanup;
                }
        }

        bios_to_read = bio_list_size(&bio_list);
        if (!bios_to_read) {
                /*
                 * this can happen if others have merged with
                 * us, it means there is nothing left to read.
                 * But if there are missing devices it may not be
                 * safe to do the full stripe write yet.
                 */
                goto finish;
        }

        /*
         * the bbio may be freed once we submit the last bio.  Make sure
         * not to touch it after that
         */
        atomic_set(&rbio->stripes_pending, bios_to_read);
        while (1) {
                bio = bio_list_pop(&bio_list);
                if (!bio)
                        break;

                bio->bi_private = rbio;
                bio->bi_end_io = raid_rmw_end_io;
                bio->bi_opf = REQ_OP_READ;

                btrfs_bio_wq_end_io(rbio->fs_info, bio, BTRFS_WQ_ENDIO_RAID56);

                submit_bio(bio);
        }
        /* the actual write will happen once the reads are done */
        return 0;

cleanup:
        rbio_orig_end_io(rbio, BLK_STS_IOERR);

        while ((bio = bio_list_pop(&bio_list)))
                bio_put(bio);

        return -EIO;

finish:
        validate_rbio_for_rmw(rbio);
        return 0;
}

/*
 * if the upper layers pass in a full stripe, we thank them by only allocating
 * enough pages to hold the parity, and sending it all down quickly.
 */
static int full_stripe_write(struct btrfs_raid_bio *rbio)
{
        int ret;

        ret = alloc_rbio_parity_pages(rbio);
        if (ret) {
                __free_raid_bio(rbio);
                return ret;
        }

        ret = lock_stripe_add(rbio);
        if (ret == 0)
                finish_rmw(rbio);
        return 0;
}

/*
 * partial stripe writes get handed over to async helpers.
 * We're really hoping to merge a few more writes into this
 * rbio before calculating new parity
 */
static int partial_stripe_write(struct btrfs_raid_bio *rbio)
{
        int ret;

        ret = lock_stripe_add(rbio);
        if (ret == 0)
                start_async_work(rbio, rmw_work);
        return 0;
}

/*
 * sometimes while we were reading from the drive to
 * recalculate parity, enough new bios come into create
 * a full stripe.  So we do a check here to see if we can
 * go directly to finish_rmw
 */
static int __raid56_parity_write(struct btrfs_raid_bio *rbio)
{
        /* head off into rmw land if we don't have a full stripe */
        if (!rbio_is_full(rbio))
                return partial_stripe_write(rbio);
        return full_stripe_write(rbio);
}

/*
 * We use plugging call backs to collect full stripes.
 * Any time we get a partial stripe write while plugged
 * we collect it into a list.  When the unplug comes down,
 * we sort the list by logical block number and merge
 * everything we can into the same rbios
 */
struct btrfs_plug_cb {
        struct blk_plug_cb cb;
        struct btrfs_fs_info *info;
        struct list_head rbio_list;
        struct btrfs_work work;
};

/*
 * rbios on the plug list are sorted for easier merging.
 */
static int plug_cmp(void *priv, struct list_head *a, struct list_head *b)
{
        struct btrfs_raid_bio *ra = container_of(a, struct btrfs_raid_bio,
                                                 plug_list);
        struct btrfs_raid_bio *rb = container_of(b, struct btrfs_raid_bio,
                                                 plug_list);
        u64 a_sector = ra->bio_list.head->bi_iter.bi_sector;
        u64 b_sector = rb->bio_list.head->bi_iter.bi_sector;

        if (a_sector < b_sector)
                return -1;
        if (a_sector > b_sector)
                return 1;
        return 0;
}

static void run_plug(struct btrfs_plug_cb *plug)
{
        struct btrfs_raid_bio *cur;
        struct btrfs_raid_bio *last = NULL;

        /*
         * sort our plug list then try to merge
         * everything we can in hopes of creating full
         * stripes.
         */
        list_sort(NULL, &plug->rbio_list, plug_cmp);
        while (!list_empty(&plug->rbio_list)) {
                cur = list_entry(plug->rbio_list.next,
                                 struct btrfs_raid_bio, plug_list);
                list_del_init(&cur->plug_list);

                if (rbio_is_full(cur)) {
                        int ret;

                        /* we have a full stripe, send it down */
                        ret = full_stripe_write(cur);
                        BUG_ON(ret);
                        continue;
                }
                if (last) {
                        if (rbio_can_merge(last, cur)) {
                                merge_rbio(last, cur);
                                __free_raid_bio(cur);
                                continue;

                        }
                        __raid56_parity_write(last);
                }
                last = cur;
        }
        if (last) {
                __raid56_parity_write(last);
        }
        kfree(plug);
}

/*
 * if the unplug comes from schedule, we have to push the
 * work off to a helper thread
 */
static void unplug_work(struct btrfs_work *work)
{
        struct btrfs_plug_cb *plug;
        plug = container_of(work, struct btrfs_plug_cb, work);
        run_plug(plug);
}

static void btrfs_raid_unplug(struct blk_plug_cb *cb, bool from_schedule)
{
        struct btrfs_plug_cb *plug;
        plug = container_of(cb, struct btrfs_plug_cb, cb);

        if (from_schedule) {
                btrfs_init_work(&plug->work, btrfs_rmw_helper,
                                unplug_work, NULL, NULL);
                btrfs_queue_work(plug->info->rmw_workers,
                                 &plug->work);
                return;
        }
        run_plug(plug);
}

/*
 * our main entry point for writes from the rest of the FS.
 */
int raid56_parity_write(struct btrfs_fs_info *fs_info, struct bio *bio,
                        struct btrfs_bio *bbio, u64 stripe_len)
{
        struct btrfs_raid_bio *rbio;
        struct btrfs_plug_cb *plug = NULL;
        struct blk_plug_cb *cb;
        int ret;

        rbio = alloc_rbio(fs_info, bbio, stripe_len);
        if (IS_ERR(rbio)) {
                btrfs_put_bbio(bbio);
                return PTR_ERR(rbio);
        }
        bio_list_add(&rbio->bio_list, bio);
        rbio->bio_list_bytes = bio->bi_iter.bi_size;
        rbio->operation = BTRFS_RBIO_WRITE;

        btrfs_bio_counter_inc_noblocked(fs_info);
        rbio->generic_bio_cnt = 1;

        /*
         * don't plug on full rbios, just get them out the door
         * as quickly as we can
         */
        if (rbio_is_full(rbio)) {
                ret = full_stripe_write(rbio);
                if (ret)
                        btrfs_bio_counter_dec(fs_info);
                return ret;
        }

        cb = blk_check_plugged(btrfs_raid_unplug, fs_info, sizeof(*plug));
        if (cb) {
                plug = container_of(cb, struct btrfs_plug_cb, cb);
                if (!plug->info) {
                        plug->info = fs_info;
                        INIT_LIST_HEAD(&plug->rbio_list);
                }
                list_add_tail(&rbio->plug_list, &plug->rbio_list);
                ret = 0;
        } else {
                ret = __raid56_parity_write(rbio);
                if (ret)
                        btrfs_bio_counter_dec(fs_info);
        }
        return ret;
}

/*
 * all parity reconstruction happens here.  We've read in everything
 * we can find from the drives and this does the heavy lifting of
 * sorting the good from the bad.
 */
static void __raid_recover_end_io(struct btrfs_raid_bio *rbio)
{
        int pagenr, stripe;
        void **pointers;
        int faila = -1, failb = -1;
        struct page *page;
        blk_status_t err;
        int i;

        pointers = kcalloc(rbio->real_stripes, sizeof(void *), GFP_NOFS);
        if (!pointers) {
                err = BLK_STS_RESOURCE;
                goto cleanup_io;
        }

        faila = rbio->faila;
        failb = rbio->failb;

        if (rbio->operation == BTRFS_RBIO_READ_REBUILD ||
            rbio->operation == BTRFS_RBIO_REBUILD_MISSING) {
                spin_lock_irq(&rbio->bio_list_lock);
                set_bit(RBIO_RMW_LOCKED_BIT, &rbio->flags);
                spin_unlock_irq(&rbio->bio_list_lock);
        }

        index_rbio_pages(rbio);

        for (pagenr = 0; pagenr < rbio->stripe_npages; pagenr++) {
                /*
                 * Now we just use bitmap to mark the horizontal stripes in
                 * which we have data when doing parity scrub.
                 */
                if (rbio->operation == BTRFS_RBIO_PARITY_SCRUB &&
                    !test_bit(pagenr, rbio->dbitmap))
                        continue;

                /* setup our array of pointers with pages
                 * from each stripe
                 */
                for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
                        /*
                         * if we're rebuilding a read, we have to use
                         * pages from the bio list
                         */
                        if ((rbio->operation == BTRFS_RBIO_READ_REBUILD ||
                             rbio->operation == BTRFS_RBIO_REBUILD_MISSING) &&
                            (stripe == faila || stripe == failb)) {
                                page = page_in_rbio(rbio, stripe, pagenr, 0);
                        } else {
                                page = rbio_stripe_page(rbio, stripe, pagenr);
                        }
                        pointers[stripe] = kmap(page);
                }

                /* all raid6 handling here */
                if (rbio->bbio->map_type & BTRFS_BLOCK_GROUP_RAID6) {
                        /*
                         * single failure, rebuild from parity raid5
                         * style
                         */
                        if (failb < 0) {
                                if (faila == rbio->nr_data) {
                                        /*
                                         * Just the P stripe has failed, without
                                         * a bad data or Q stripe.
                                         * TODO, we should redo the xor here.
                                         */
                                        err = BLK_STS_IOERR;
                                        goto cleanup;
                                }
                                /*
                                 * a single failure in raid6 is rebuilt
                                 * in the pstripe code below
                                 */
                                goto pstripe;
                        }

                        /* make sure our ps and qs are in order */
                        if (faila > failb) {
                                int tmp = failb;
                                failb = faila;
                                faila = tmp;
                        }

                        /* if the q stripe is failed, do a pstripe reconstruction
                         * from the xors.
                         * If both the q stripe and the P stripe are failed, we're
                         * here due to a crc mismatch and we can't give them the
                         * data they want
                         */
                        if (rbio->bbio->raid_map[failb] == RAID6_Q_STRIPE) {
                                if (rbio->bbio->raid_map[faila] ==
                                    RAID5_P_STRIPE) {
                                        err = BLK_STS_IOERR;
                                        goto cleanup;
                                }
                                /*
                                 * otherwise we have one bad data stripe and
                                 * a good P stripe.  raid5!
                                 */
                                goto pstripe;
                        }

                        if (rbio->bbio->raid_map[failb] == RAID5_P_STRIPE) {
                                raid6_datap_recov(rbio->real_stripes,
                                                  PAGE_SIZE, faila, pointers);
                        } else {
                                raid6_2data_recov(rbio->real_stripes,
                                                  PAGE_SIZE, faila, failb,
                                                  pointers);
                        }
                } else {
                        void *p;

                        /* rebuild from P stripe here (raid5 or raid6) */
                        BUG_ON(failb != -1);
pstripe:
                        /* Copy parity block into failed block to start with */
                        copy_page(pointers[faila], pointers[rbio->nr_data]);

                        /* rearrange the pointer array */
                        p = pointers[faila];
                        for (stripe = faila; stripe < rbio->nr_data - 1; stripe++)
                                pointers[stripe] = pointers[stripe + 1];
                        pointers[rbio->nr_data - 1] = p;

                        /* xor in the rest */
                        run_xor(pointers, rbio->nr_data - 1, PAGE_SIZE);
                }
                /* if we're doing this rebuild as part of an rmw, go through
                 * and set all of our private rbio pages in the
                 * failed stripes as uptodate.  This way finish_rmw will
                 * know they can be trusted.  If this was a read reconstruction,
                 * other endio functions will fiddle the uptodate bits
                 */
                if (rbio->operation == BTRFS_RBIO_WRITE) {
                        for (i = 0;  i < rbio->stripe_npages; i++) {
                                if (faila != -1) {
                                        page = rbio_stripe_page(rbio, faila, i);
                                        SetPageUptodate(page);
                                }
                                if (failb != -1) {
                                        page = rbio_stripe_page(rbio, failb, i);
                                        SetPageUptodate(page);
                                }
                        }
                }
                for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
                        /*
                         * if we're rebuilding a read, we have to use
                         * pages from the bio list
                         */
                        if ((rbio->operation == BTRFS_RBIO_READ_REBUILD ||
                             rbio->operation == BTRFS_RBIO_REBUILD_MISSING) &&
                            (stripe == faila || stripe == failb)) {
                                page = page_in_rbio(rbio, stripe, pagenr, 0);
                        } else {
                                page = rbio_stripe_page(rbio, stripe, pagenr);
                        }
                        kunmap(page);
                }
        }

        err = BLK_STS_OK;
cleanup:
        kfree(pointers);

cleanup_io:
        /*
         * Similar to READ_REBUILD, REBUILD_MISSING at this point also has a
         * valid rbio which is consistent with ondisk content, thus such a
         * valid rbio can be cached to avoid further disk reads.
         */
        if (rbio->operation == BTRFS_RBIO_READ_REBUILD ||
            rbio->operation == BTRFS_RBIO_REBUILD_MISSING) {
                /*
                 * - In case of two failures, where rbio->failb != -1:
                 *
                 *   Do not cache this rbio since the above read reconstruction
                 *   (raid6_datap_recov() or raid6_2data_recov()) may have
                 *   changed some content of stripes which are not identical to
                 *   on-disk content any more, otherwise, a later write/recover
                 *   may steal stripe_pages from this rbio and end up with
                 *   corruptions or rebuild failures.
                 *
                 * - In case of single failure, where rbio->failb == -1:
                 *
                 *   Cache this rbio iff the above read reconstruction is
                 *   executed without problems.
                 */
                if (err == BLK_STS_OK && rbio->failb < 0)
                        cache_rbio_pages(rbio);
                else
                        clear_bit(RBIO_CACHE_READY_BIT, &rbio->flags);

                rbio_orig_end_io(rbio, err);
        } else if (err == BLK_STS_OK) {
                rbio->faila = -1;
                rbio->failb = -1;

                if (rbio->operation == BTRFS_RBIO_WRITE)
                        finish_rmw(rbio);
                else if (rbio->operation == BTRFS_RBIO_PARITY_SCRUB)
                        finish_parity_scrub(rbio, 0);
                else
                        BUG();
        } else {
                rbio_orig_end_io(rbio, err);
        }
}

/*
 * This is called only for stripes we've read from disk to
 * reconstruct the parity.
 */
static void raid_recover_end_io(struct bio *bio)
{
        struct btrfs_raid_bio *rbio = bio->bi_private;

        /*
         * we only read stripe pages off the disk, set them
         * up to date if there were no errors
         */
        if (bio->bi_status)
                fail_bio_stripe(rbio, bio);
        else
                set_bio_pages_uptodate(bio);
        bio_put(bio);

        if (!atomic_dec_and_test(&rbio->stripes_pending))
                return;

        if (atomic_read(&rbio->error) > rbio->bbio->max_errors)
                rbio_orig_end_io(rbio, BLK_STS_IOERR);
        else
                __raid_recover_end_io(rbio);
}

/*
 * reads everything we need off the disk to reconstruct
 * the parity. endio handlers trigger final reconstruction
 * when the IO is done.
 *
 * This is used both for reads from the higher layers and for
 * parity construction required to finish a rmw cycle.
 */
static int __raid56_parity_recover(struct btrfs_raid_bio *rbio)
{
        int bios_to_read = 0;
        struct bio_list bio_list;
        int ret;
        int pagenr;
        int stripe;
        struct bio *bio;

        bio_list_init(&bio_list);

        ret = alloc_rbio_pages(rbio);
        if (ret)
                goto cleanup;

        atomic_set(&rbio->error, 0);

        /*
         * read everything that hasn't failed.  Thanks to the
         * stripe cache, it is possible that some or all of these
         * pages are going to be uptodate.
         */
        for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
                if (rbio->faila == stripe || rbio->failb == stripe) {
                        atomic_inc(&rbio->error);
                        continue;
                }

                for (pagenr = 0; pagenr < rbio->stripe_npages; pagenr++) {
                        struct page *p;

                        /*
                         * the rmw code may have already read this
                         * page in
                         */
                        p = rbio_stripe_page(rbio, stripe, pagenr);
                        if (PageUptodate(p))
                                continue;

                        ret = rbio_add_io_page(rbio, &bio_list,
                                       rbio_stripe_page(rbio, stripe, pagenr),
                                       stripe, pagenr, rbio->stripe_len);
                        if (ret < 0)
                                goto cleanup;
                }
        }

        bios_to_read = bio_list_size(&bio_list);
        if (!bios_to_read) {
                /*
                 * we might have no bios to read just because the pages
                 * were up to date, or we might have no bios to read because
                 * the devices were gone.
                 */
                if (atomic_read(&rbio->error) <= rbio->bbio->max_errors) {
                        __raid_recover_end_io(rbio);
                        goto out;
                } else {
                        goto cleanup;
                }
        }

        /*
         * the bbio may be freed once we submit the last bio.  Make sure
         * not to touch it after that
         */
        atomic_set(&rbio->stripes_pending, bios_to_read);
        while (1) {
                bio = bio_list_pop(&bio_list);
                if (!bio)
                        break;

                bio->bi_private = rbio;
                bio->bi_end_io = raid_recover_end_io;
                bio->bi_opf = REQ_OP_READ;

                btrfs_bio_wq_end_io(rbio->fs_info, bio, BTRFS_WQ_ENDIO_RAID56);

                submit_bio(bio);
        }
out:
        return 0;

cleanup:
        if (rbio->operation == BTRFS_RBIO_READ_REBUILD ||
            rbio->operation == BTRFS_RBIO_REBUILD_MISSING)
                rbio_orig_end_io(rbio, BLK_STS_IOERR);

        while ((bio = bio_list_pop(&bio_list)))
                bio_put(bio);

        return -EIO;
}

/*
 * the main entry point for reads from the higher layers.  This
 * is really only called when the normal read path had a failure,
 * so we assume the bio they send down corresponds to a failed part
 * of the drive.
 */
int raid56_parity_recover(struct btrfs_fs_info *fs_info, struct bio *bio,
                          struct btrfs_bio *bbio, u64 stripe_len,
                          int mirror_num, int generic_io)
{
        struct btrfs_raid_bio *rbio;
        int ret;

        if (generic_io) {
                ASSERT(bbio->mirror_num == mirror_num);
                btrfs_io_bio(bio)->mirror_num = mirror_num;
        }

        rbio = alloc_rbio(fs_info, bbio, stripe_len);
        if (IS_ERR(rbio)) {
                if (generic_io)
                        btrfs_put_bbio(bbio);
                return PTR_ERR(rbio);
        }

        rbio->operation = BTRFS_RBIO_READ_REBUILD;
        bio_list_add(&rbio->bio_list, bio);
        rbio->bio_list_bytes = bio->bi_iter.bi_size;

        rbio->faila = find_logical_bio_stripe(rbio, bio);
        if (rbio->faila == -1) {
                btrfs_warn(fs_info,
        "%s could not find the bad stripe in raid56 so that we cannot recover any more (bio has logical %llu len %llu, bbio has map_type %llu)",
                           __func__, (u64)bio->bi_iter.bi_sector << 9,
                           (u64)bio->bi_iter.bi_size, bbio->map_type);
                if (generic_io)
                        btrfs_put_bbio(bbio);
                kfree(rbio);
                return -EIO;
        }

        if (generic_io) {
                btrfs_bio_counter_inc_noblocked(fs_info);
                rbio->generic_bio_cnt = 1;
        } else {
                btrfs_get_bbio(bbio);
        }

        /*
         * Loop retry:
         * for 'mirror == 2', reconstruct from all other stripes.
         * for 'mirror_num > 2', select a stripe to fail on every retry.
         */
        if (mirror_num > 2) {
                /*
                 * 'mirror == 3' is to fail the p stripe and
                 * reconstruct from the q stripe.  'mirror > 3' is to
                 * fail a data stripe and reconstruct from p+q stripe.
                 */
                rbio->failb = rbio->real_stripes - (mirror_num - 1);
                ASSERT(rbio->failb > 0);
                if (rbio->failb <= rbio->faila)
                        rbio->failb--;
        }

        ret = lock_stripe_add(rbio);

        /*
         * __raid56_parity_recover will end the bio with
         * any errors it hits.  We don't want to return
         * its error value up the stack because our caller
         * will end up calling bio_endio with any nonzero
         * return
         */
        if (ret == 0)
                __raid56_parity_recover(rbio);
        /*
         * our rbio has been added to the list of
         * rbios that will be handled after the
         * currently lock owner is done
         */
        return 0;

}

static void rmw_work(struct btrfs_work *work)
{
        struct btrfs_raid_bio *rbio;

        rbio = container_of(work, struct btrfs_raid_bio, work);
        raid56_rmw_stripe(rbio);
}

static void read_rebuild_work(struct btrfs_work *work)
{
        struct btrfs_raid_bio *rbio;

        rbio = container_of(work, struct btrfs_raid_bio, work);
        __raid56_parity_recover(rbio);
}

/*
 * The following code is used to scrub/replace the parity stripe
 *
 * Caller must have already increased bio_counter for getting @bbio.
 *
 * Note: We need make sure all the pages that add into the scrub/replace
 * raid bio are correct and not be changed during the scrub/replace. That
 * is those pages just hold metadata or file data with checksum.
 */

struct btrfs_raid_bio *
raid56_parity_alloc_scrub_rbio(struct btrfs_fs_info *fs_info, struct bio *bio,
                               struct btrfs_bio *bbio, u64 stripe_len,
                               struct btrfs_device *scrub_dev,
                               unsigned long *dbitmap, int stripe_nsectors)
{
        struct btrfs_raid_bio *rbio;
        int i;

        rbio = alloc_rbio(fs_info, bbio, stripe_len);
        if (IS_ERR(rbio))
                return NULL;
        bio_list_add(&rbio->bio_list, bio);
        /*
         * This is a special bio which is used to hold the completion handler
         * and make the scrub rbio is similar to the other types
         */
        ASSERT(!bio->bi_iter.bi_size);
        rbio->operation = BTRFS_RBIO_PARITY_SCRUB;

        /*
         * After mapping bbio with BTRFS_MAP_WRITE, parities have been sorted
         * to the end position, so this search can start from the first parity
         * stripe.
         */
        for (i = rbio->nr_data; i < rbio->real_stripes; i++) {
                if (bbio->stripes[i].dev == scrub_dev) {
                        rbio->scrubp = i;
                        break;
                }
        }
        ASSERT(i < rbio->real_stripes);

        /* Now we just support the sectorsize equals to page size */
        ASSERT(fs_info->sectorsize == PAGE_SIZE);
        ASSERT(rbio->stripe_npages == stripe_nsectors);
        bitmap_copy(rbio->dbitmap, dbitmap, stripe_nsectors);

        /*
         * We have already increased bio_counter when getting bbio, record it
         * so we can free it at rbio_orig_end_io().
         */
        rbio->generic_bio_cnt = 1;

        return rbio;
}

/* Used for both parity scrub and missing. */
void raid56_add_scrub_pages(struct btrfs_raid_bio *rbio, struct page *page,
                            u64 logical)
{
        int stripe_offset;
        int index;

        ASSERT(logical >= rbio->bbio->raid_map[0]);
        ASSERT(logical + PAGE_SIZE <= rbio->bbio->raid_map[0] +
                                rbio->stripe_len * rbio->nr_data);
        stripe_offset = (int)(logical - rbio->bbio->raid_map[0]);
        index = stripe_offset >> PAGE_SHIFT;
        rbio->bio_pages[index] = page;
}

/*
 * We just scrub the parity that we have correct data on the same horizontal,
 * so we needn't allocate all pages for all the stripes.
 */
static int alloc_rbio_essential_pages(struct btrfs_raid_bio *rbio)
{
        int i;
        int bit;
        int index;
        struct page *page;

        for_each_set_bit(bit, rbio->dbitmap, rbio->stripe_npages) {
                for (i = 0; i < rbio->real_stripes; i++) {
                        index = i * rbio->stripe_npages + bit;
                        if (rbio->stripe_pages[index])
                                continue;

                        page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
                        if (!page)
                                return -ENOMEM;
                        rbio->stripe_pages[index] = page;
                }
        }
        return 0;
}

static noinline void finish_parity_scrub(struct btrfs_raid_bio *rbio,
                                         int need_check)
{
        struct btrfs_bio *bbio = rbio->bbio;
        void **pointers = rbio->finish_pointers;
        unsigned long *pbitmap = rbio->finish_pbitmap;
        int nr_data = rbio->nr_data;
        int stripe;
        int pagenr;
        int p_stripe = -1;
        int q_stripe = -1;
        struct page *p_page = NULL;
        struct page *q_page = NULL;
        struct bio_list bio_list;
        struct bio *bio;
        int is_replace = 0;
        int ret;

        bio_list_init(&bio_list);

        if (rbio->real_stripes - rbio->nr_data == 1) {
                p_stripe = rbio->real_stripes - 1;
        } else if (rbio->real_stripes - rbio->nr_data == 2) {
                p_stripe = rbio->real_stripes - 2;
                q_stripe = rbio->real_stripes - 1;
        } else {
                BUG();
        }

        if (bbio->num_tgtdevs && bbio->tgtdev_map[rbio->scrubp]) {
                is_replace = 1;
                bitmap_copy(pbitmap, rbio->dbitmap, rbio->stripe_npages);
        }

        /*
         * Because the higher layers(scrubber) are unlikely to
         * use this area of the disk again soon, so don't cache
         * it.
         */
        clear_bit(RBIO_CACHE_READY_BIT, &rbio->flags);

        if (!need_check)
                goto writeback;

        p_page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
        if (!p_page)
                goto cleanup;
        SetPageUptodate(p_page);

        if (q_stripe != -1) {
                q_page = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
                if (!q_page) {
                        __free_page(p_page);
                        goto cleanup;
                }
                SetPageUptodate(q_page);
        }

        atomic_set(&rbio->error, 0);

        for_each_set_bit(pagenr, rbio->dbitmap, rbio->stripe_npages) {
                struct page *p;
                void *parity;
                /* first collect one page from each data stripe */
                for (stripe = 0; stripe < nr_data; stripe++) {
                        p = page_in_rbio(rbio, stripe, pagenr, 0);
                        pointers[stripe] = kmap(p);
                }

                /* then add the parity stripe */
                pointers[stripe++] = kmap(p_page);

                if (q_stripe != -1) {

                        /*
                         * raid6, add the qstripe and call the
                         * library function to fill in our p/q
                         */
                        pointers[stripe++] = kmap(q_page);

                        raid6_call.gen_syndrome(rbio->real_stripes, PAGE_SIZE,
                                                pointers);
                } else {
                        /* raid5 */
                        copy_page(pointers[nr_data], pointers[0]);
                        run_xor(pointers + 1, nr_data - 1, PAGE_SIZE);
                }

                /* Check scrubbing parity and repair it */
                p = rbio_stripe_page(rbio, rbio->scrubp, pagenr);
                parity = kmap(p);
                if (memcmp(parity, pointers[rbio->scrubp], PAGE_SIZE))
                        copy_page(parity, pointers[rbio->scrubp]);
                else
                        /* Parity is right, needn't writeback */
                        bitmap_clear(rbio->dbitmap, pagenr, 1);
                kunmap(p);

                for (stripe = 0; stripe < rbio->real_stripes; stripe++)
                        kunmap(page_in_rbio(rbio, stripe, pagenr, 0));
        }

        __free_page(p_page);
        if (q_page)
                __free_page(q_page);

writeback:
        /*
         * time to start writing.  Make bios for everything from the
         * higher layers (the bio_list in our rbio) and our p/q.  Ignore
         * everything else.
         */
        for_each_set_bit(pagenr, rbio->dbitmap, rbio->stripe_npages) {
                struct page *page;

                page = rbio_stripe_page(rbio, rbio->scrubp, pagenr);
                ret = rbio_add_io_page(rbio, &bio_list,
                               page, rbio->scrubp, pagenr, rbio->stripe_len);
                if (ret)
                        goto cleanup;
        }

        if (!is_replace)
                goto submit_write;

        for_each_set_bit(pagenr, pbitmap, rbio->stripe_npages) {
                struct page *page;

                page = rbio_stripe_page(rbio, rbio->scrubp, pagenr);
                ret = rbio_add_io_page(rbio, &bio_list, page,
                                       bbio->tgtdev_map[rbio->scrubp],
                                       pagenr, rbio->stripe_len);
                if (ret)
                        goto cleanup;
        }

submit_write:
        nr_data = bio_list_size(&bio_list);
        if (!nr_data) {
                /* Every parity is right */
                rbio_orig_end_io(rbio, BLK_STS_OK);
                return;
        }

        atomic_set(&rbio->stripes_pending, nr_data);

        while (1) {
                bio = bio_list_pop(&bio_list);
                if (!bio)
                        break;

                bio->bi_private = rbio;
                bio->bi_end_io = raid_write_end_io;
                bio->bi_opf = REQ_OP_WRITE;

                submit_bio(bio);
        }
        return;

cleanup:
        rbio_orig_end_io(rbio, BLK_STS_IOERR);

        while ((bio = bio_list_pop(&bio_list)))
                bio_put(bio);
}

static inline int is_data_stripe(struct btrfs_raid_bio *rbio, int stripe)
{
        if (stripe >= 0 && stripe < rbio->nr_data)
                return 1;
        return 0;
}

/*
 * While we're doing the parity check and repair, we could have errors
 * in reading pages off the disk.  This checks for errors and if we're
 * not able to read the page it'll trigger parity reconstruction.  The
 * parity scrub will be finished after we've reconstructed the failed
 * stripes
 */
static void validate_rbio_for_parity_scrub(struct btrfs_raid_bio *rbio)
{
        if (atomic_read(&rbio->error) > rbio->bbio->max_errors)
                goto cleanup;

        if (rbio->faila >= 0 || rbio->failb >= 0) {
                int dfail = 0, failp = -1;

                if (is_data_stripe(rbio, rbio->faila))
                        dfail++;
                else if (is_parity_stripe(rbio->faila))
                        failp = rbio->faila;

                if (is_data_stripe(rbio, rbio->failb))
                        dfail++;
                else if (is_parity_stripe(rbio->failb))
                        failp = rbio->failb;

                /*
                 * Because we can not use a scrubbing parity to repair
                 * the data, so the capability of the repair is declined.
                 * (In the case of RAID5, we can not repair anything)
                 */
                if (dfail > rbio->bbio->max_errors - 1)
                        goto cleanup;

                /*
                 * If all data is good, only parity is correctly, just
                 * repair the parity.
                 */
                if (dfail == 0) {
                        finish_parity_scrub(rbio, 0);
                        return;
                }

                /*
                 * Here means we got one corrupted data stripe and one
                 * corrupted parity on RAID6, if the corrupted parity
                 * is scrubbing parity, luckily, use the other one to repair
                 * the data, or we can not repair the data stripe.
                 */
                if (failp != rbio->scrubp)
                        goto cleanup;

                __raid_recover_end_io(rbio);
        } else {
                finish_parity_scrub(rbio, 1);
        }
        return;

cleanup:
        rbio_orig_end_io(rbio, BLK_STS_IOERR);
}

/*
 * end io for the read phase of the rmw cycle.  All the bios here are physical
 * stripe bios we've read from the disk so we can recalculate the parity of the
 * stripe.
 *
 * This will usually kick off finish_rmw once all the bios are read in, but it
 * may trigger parity reconstruction if we had any errors along the way
 */
static void raid56_parity_scrub_end_io(struct bio *bio)
{
        struct btrfs_raid_bio *rbio = bio->bi_private;

        if (bio->bi_status)
                fail_bio_stripe(rbio, bio);
        else
                set_bio_pages_uptodate(bio);

        bio_put(bio);

        if (!atomic_dec_and_test(&rbio->stripes_pending))
                return;

        /*
         * this will normally call finish_rmw to start our write
         * but if there are any failed stripes we'll reconstruct
         * from parity first
         */
        validate_rbio_for_parity_scrub(rbio);
}

static void raid56_parity_scrub_stripe(struct btrfs_raid_bio *rbio)
{
        int bios_to_read = 0;
        struct bio_list bio_list;
        int ret;
        int pagenr;
        int stripe;
        struct bio *bio;

        bio_list_init(&bio_list);

        ret = alloc_rbio_essential_pages(rbio);
        if (ret)
                goto cleanup;

        atomic_set(&rbio->error, 0);
        /*
         * build a list of bios to read all the missing parts of this
         * stripe
         */
        for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
                for_each_set_bit(pagenr, rbio->dbitmap, rbio->stripe_npages) {
                        struct page *page;
                        /*
                         * we want to find all the pages missing from
                         * the rbio and read them from the disk.  If
                         * page_in_rbio finds a page in the bio list
                         * we don't need to read it off the stripe.
                         */
                        page = page_in_rbio(rbio, stripe, pagenr, 1);
                        if (page)
                                continue;

                        page = rbio_stripe_page(rbio, stripe, pagenr);
                        /*
                         * the bio cache may have handed us an uptodate
                         * page.  If so, be happy and use it
                         */
                        if (PageUptodate(page))
                                continue;

                        ret = rbio_add_io_page(rbio, &bio_list, page,
                                       stripe, pagenr, rbio->stripe_len);
                        if (ret)
                                goto cleanup;
                }
        }

        bios_to_read = bio_list_size(&bio_list);
        if (!bios_to_read) {
                /*
                 * this can happen if others have merged with
                 * us, it means there is nothing left to read.
                 * But if there are missing devices it may not be
                 * safe to do the full stripe write yet.
                 */
                goto finish;
        }

        /*
         * the bbio may be freed once we submit the last bio.  Make sure
         * not to touch it after that
         */
        atomic_set(&rbio->stripes_pending, bios_to_read);
        while (1) {
                bio = bio_list_pop(&bio_list);
                if (!bio)
                        break;

                bio->bi_private = rbio;
                bio->bi_end_io = raid56_parity_scrub_end_io;
                bio->bi_opf = REQ_OP_READ;

                btrfs_bio_wq_end_io(rbio->fs_info, bio, BTRFS_WQ_ENDIO_RAID56);

                submit_bio(bio);
        }
        /* the actual write will happen once the reads are done */
        return;

cleanup:
        rbio_orig_end_io(rbio, BLK_STS_IOERR);

        while ((bio = bio_list_pop(&bio_list)))
                bio_put(bio);

        return;

finish:
        validate_rbio_for_parity_scrub(rbio);
}

static void scrub_parity_work(struct btrfs_work *work)
{
        struct btrfs_raid_bio *rbio;

        rbio = container_of(work, struct btrfs_raid_bio, work);
        raid56_parity_scrub_stripe(rbio);
}

void raid56_parity_submit_scrub_rbio(struct btrfs_raid_bio *rbio)
{
        if (!lock_stripe_add(rbio))
                start_async_work(rbio, scrub_parity_work);
}

/* The following code is used for dev replace of a missing RAID 5/6 device. */

struct btrfs_raid_bio *
raid56_alloc_missing_rbio(struct btrfs_fs_info *fs_info, struct bio *bio,
                          struct btrfs_bio *bbio, u64 length)
{
        struct btrfs_raid_bio *rbio;

        rbio = alloc_rbio(fs_info, bbio, length);
        if (IS_ERR(rbio))
                return NULL;

        rbio->operation = BTRFS_RBIO_REBUILD_MISSING;
        bio_list_add(&rbio->bio_list, bio);
        /*
         * This is a special bio which is used to hold the completion handler
         * and make the scrub rbio is similar to the other types
         */
        ASSERT(!bio->bi_iter.bi_size);

        rbio->faila = find_logical_bio_stripe(rbio, bio);
        if (rbio->faila == -1) {
                BUG();
                kfree(rbio);
                return NULL;
        }

        /*
         * When we get bbio, we have already increased bio_counter, record it
         * so we can free it at rbio_orig_end_io()
         */
        rbio->generic_bio_cnt = 1;

        return rbio;
}

void raid56_submit_missing_rbio(struct btrfs_raid_bio *rbio)
{
        if (!lock_stripe_add(rbio))
                start_async_work(rbio, read_rebuild_work);
}