[thirdparty/linux.git] / block / blk-cgroup.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Common Block IO controller cgroup interface
 *
 * Based on ideas and code from CFQ, CFS and BFQ:
 * Copyright (C) 2003 Jens Axboe <axboe@kernel.dk>
 *
 * Copyright (C) 2008 Fabio Checconi <fabio@gandalf.sssup.it>
 *                    Paolo Valente <paolo.valente@unimore.it>
 *
 * Copyright (C) 2009 Vivek Goyal <vgoyal@redhat.com>
 *                    Nauman Rafique <nauman@google.com>
 *
 * For policy-specific per-blkcg data:
 * Copyright (C) 2015 Paolo Valente <paolo.valente@unimore.it>
 *                    Arianna Avanzini <avanzini.arianna@gmail.com>
 */
#include <linux/ioprio.h>
#include <linux/kdev_t.h>
#include <linux/module.h>
#include <linux/sched/signal.h>
#include <linux/err.h>
#include <linux/blkdev.h>
#include <linux/backing-dev.h>
#include <linux/slab.h>
#include <linux/delay.h>
#include <linux/atomic.h>
#include <linux/ctype.h>
#include <linux/resume_user_mode.h>
#include <linux/psi.h>
#include <linux/part_stat.h>
#include "blk.h"
#include "blk-cgroup.h"
#include "blk-ioprio.h"
#include "blk-throttle.h"

/*
 * blkcg_pol_mutex protects blkcg_policy[] and policy [de]activation.
 * blkcg_pol_register_mutex nests outside of it and synchronizes entire
 * policy [un]register operations including cgroup file additions /
 * removals.  Putting cgroup file registration outside blkcg_pol_mutex
 * allows grabbing it from cgroup callbacks.
 */
static DEFINE_MUTEX(blkcg_pol_register_mutex);
static DEFINE_MUTEX(blkcg_pol_mutex);

struct blkcg blkcg_root;
EXPORT_SYMBOL_GPL(blkcg_root);

struct cgroup_subsys_state * const blkcg_root_css = &blkcg_root.css;
EXPORT_SYMBOL_GPL(blkcg_root_css);

static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];

static LIST_HEAD(all_blkcgs);           /* protected by blkcg_pol_mutex */

bool blkcg_debug_stats = false;

#define BLKG_DESTROY_BATCH_SIZE  64

/*
 * Lockless lists for tracking IO stats update
 *
 * New IO stats are stored in the percpu iostat_cpu within blkcg_gq (blkg).
 * There are multiple blkg's (one for each block device) attached to each
 * blkcg. The rstat code keeps track of which cpu has IO stats updated,
 * but it doesn't know which blkg has the updated stats. If there are many
 * block devices in a system, the cost of iterating all the blkg's to flush
 * out the IO stats can be high. To reduce such overhead, a set of percpu
 * lockless lists (lhead) per blkcg are used to track the set of recently
 * updated iostat_cpu's since the last flush. An iostat_cpu will be put
 * onto the lockless list on the update side [blk_cgroup_bio_start()] if
 * not there yet and then removed when being flushed [blkcg_rstat_flush()].
 * References to blkg are gotten and then put back in the process to
 * protect against blkg removal.
 *
 * Return: 0 if successful or -ENOMEM if allocation fails.
 */
static int init_blkcg_llists(struct blkcg *blkcg)
{
        int cpu;

        blkcg->lhead = alloc_percpu_gfp(struct llist_head, GFP_KERNEL);
        if (!blkcg->lhead)
                return -ENOMEM;

        for_each_possible_cpu(cpu)
                init_llist_head(per_cpu_ptr(blkcg->lhead, cpu));
        return 0;
}

/**
 * blkcg_css - find the current css
 *
 * Find the css associated with either the kthread or the current task.
 * This may return a dying css, so it is up to the caller to use tryget logic
 * to confirm it is alive and well.
 */
static struct cgroup_subsys_state *blkcg_css(void)
{
        struct cgroup_subsys_state *css;

        css = kthread_blkcg();
        if (css)
                return css;
        return task_css(current, io_cgrp_id);
}

static bool blkcg_policy_enabled(struct request_queue *q,
                                 const struct blkcg_policy *pol)
{
        return pol && test_bit(pol->plid, q->blkcg_pols);
}

static void blkg_free_workfn(struct work_struct *work)
{
        struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
                                             free_work);
        struct request_queue *q = blkg->q;
        int i;

        /*
         * pd_free_fn() can also be called from blkcg_deactivate_policy(),
         * in order to make sure pd_free_fn() is called in order, the deletion
         * of the list blkg->q_node is delayed to here from blkg_destroy(), and
         * blkcg_mutex is used to synchronize blkg_free_workfn() and
         * blkcg_deactivate_policy().
         */
        mutex_lock(&q->blkcg_mutex);
        for (i = 0; i < BLKCG_MAX_POLS; i++)
                if (blkg->pd[i])
                        blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
        if (blkg->parent)
                blkg_put(blkg->parent);
        list_del_init(&blkg->q_node);
        mutex_unlock(&q->blkcg_mutex);

        blk_put_queue(q);
        free_percpu(blkg->iostat_cpu);
        percpu_ref_exit(&blkg->refcnt);
        kfree(blkg);
}

/**
 * blkg_free - free a blkg
 * @blkg: blkg to free
 *
 * Free @blkg which may be partially allocated.
 */
static void blkg_free(struct blkcg_gq *blkg)
{
        if (!blkg)
                return;

        /*
         * Both ->pd_free_fn() and request queue's release handler may
         * sleep, so free us by scheduling one work func
         */
        INIT_WORK(&blkg->free_work, blkg_free_workfn);
        schedule_work(&blkg->free_work);
}

static void __blkg_release(struct rcu_head *rcu)
{
        struct blkcg_gq *blkg = container_of(rcu, struct blkcg_gq, rcu_head);

#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
        WARN_ON(!bio_list_empty(&blkg->async_bios));
#endif

        /* release the blkcg and parent blkg refs this blkg has been holding */
        css_put(&blkg->blkcg->css);
        blkg_free(blkg);
}

/*
 * A group is RCU protected, but having an rcu lock does not mean that one
 * can access all the fields of blkg and assume these are valid.  For
 * example, don't try to follow throtl_data and request queue links.
 *
 * Having a reference to blkg under an rcu allows accesses to only values
 * local to groups like group stats and group rate limits.
 */
static void blkg_release(struct percpu_ref *ref)
{
        struct blkcg_gq *blkg = container_of(ref, struct blkcg_gq, refcnt);

        call_rcu(&blkg->rcu_head, __blkg_release);
}

#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
static struct workqueue_struct *blkcg_punt_bio_wq;

static void blkg_async_bio_workfn(struct work_struct *work)
{
        struct blkcg_gq *blkg = container_of(work, struct blkcg_gq,
                                             async_bio_work);
        struct bio_list bios = BIO_EMPTY_LIST;
        struct bio *bio;
        struct blk_plug plug;
        bool need_plug = false;

        /* as long as there are pending bios, @blkg can't go away */
        spin_lock(&blkg->async_bio_lock);
        bio_list_merge(&bios, &blkg->async_bios);
        bio_list_init(&blkg->async_bios);
        spin_unlock(&blkg->async_bio_lock);

        /* start plug only when bio_list contains at least 2 bios */
        if (bios.head && bios.head->bi_next) {
                need_plug = true;
                blk_start_plug(&plug);
        }
        while ((bio = bio_list_pop(&bios)))
                submit_bio(bio);
        if (need_plug)
                blk_finish_plug(&plug);
}

/*
 * When a shared kthread issues a bio for a cgroup, doing so synchronously can
 * lead to priority inversions as the kthread can be trapped waiting for that
 * cgroup.  Use this helper instead of submit_bio to punt the actual issuing to
 * a dedicated per-blkcg work item to avoid such priority inversions.
 */
void blkcg_punt_bio_submit(struct bio *bio)
{
        struct blkcg_gq *blkg = bio->bi_blkg;

        if (blkg->parent) {
                spin_lock(&blkg->async_bio_lock);
                bio_list_add(&blkg->async_bios, bio);
                spin_unlock(&blkg->async_bio_lock);
                queue_work(blkcg_punt_bio_wq, &blkg->async_bio_work);
        } else {
                /* never bounce for the root cgroup */
                submit_bio(bio);
        }
}
EXPORT_SYMBOL_GPL(blkcg_punt_bio_submit);

static int __init blkcg_punt_bio_init(void)
{
        blkcg_punt_bio_wq = alloc_workqueue("blkcg_punt_bio",
                                            WQ_MEM_RECLAIM | WQ_FREEZABLE |
                                            WQ_UNBOUND | WQ_SYSFS, 0);
        if (!blkcg_punt_bio_wq)
                return -ENOMEM;
        return 0;
}
subsys_initcall(blkcg_punt_bio_init);
#endif /* CONFIG_BLK_CGROUP_PUNT_BIO */

/**
 * bio_blkcg_css - return the blkcg CSS associated with a bio
 * @bio: target bio
 *
 * This returns the CSS for the blkcg associated with a bio, or %NULL if not
 * associated. Callers are expected to either handle %NULL or know association
 * has been done prior to calling this.
 */
struct cgroup_subsys_state *bio_blkcg_css(struct bio *bio)
{
        if (!bio || !bio->bi_blkg)
                return NULL;
        return &bio->bi_blkg->blkcg->css;
}
EXPORT_SYMBOL_GPL(bio_blkcg_css);

/**
 * blkcg_parent - get the parent of a blkcg
 * @blkcg: blkcg of interest
 *
 * Return the parent blkcg of @blkcg.  Can be called anytime.
 */
static inline struct blkcg *blkcg_parent(struct blkcg *blkcg)
{
        return css_to_blkcg(blkcg->css.parent);
}

/**
 * blkg_alloc - allocate a blkg
 * @blkcg: block cgroup the new blkg is associated with
 * @disk: gendisk the new blkg is associated with
 * @gfp_mask: allocation mask to use
 *
 * Allocate a new blkg assocating @blkcg and @q.
 */
static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct gendisk *disk,
                                   gfp_t gfp_mask)
{
        struct blkcg_gq *blkg;
        int i, cpu;

        /* alloc and init base part */
        blkg = kzalloc_node(sizeof(*blkg), gfp_mask, disk->queue->node);
        if (!blkg)
                return NULL;
        if (percpu_ref_init(&blkg->refcnt, blkg_release, 0, gfp_mask))
                goto out_free_blkg;
        blkg->iostat_cpu = alloc_percpu_gfp(struct blkg_iostat_set, gfp_mask);
        if (!blkg->iostat_cpu)
                goto out_exit_refcnt;
        if (!blk_get_queue(disk->queue))
                goto out_free_iostat;

        blkg->q = disk->queue;
        INIT_LIST_HEAD(&blkg->q_node);
        blkg->blkcg = blkcg;
#ifdef CONFIG_BLK_CGROUP_PUNT_BIO
        spin_lock_init(&blkg->async_bio_lock);
        bio_list_init(&blkg->async_bios);
        INIT_WORK(&blkg->async_bio_work, blkg_async_bio_workfn);
#endif

        u64_stats_init(&blkg->iostat.sync);
        for_each_possible_cpu(cpu) {
                u64_stats_init(&per_cpu_ptr(blkg->iostat_cpu, cpu)->sync);
                per_cpu_ptr(blkg->iostat_cpu, cpu)->blkg = blkg;
        }

        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];
                struct blkg_policy_data *pd;

                if (!blkcg_policy_enabled(disk->queue, pol))
                        continue;

                /* alloc per-policy data and attach it to blkg */
                pd = pol->pd_alloc_fn(disk, blkcg, gfp_mask);
                if (!pd)
                        goto out_free_pds;
                blkg->pd[i] = pd;
                pd->blkg = blkg;
                pd->plid = i;
                pd->online = false;
        }

        return blkg;

out_free_pds:
        while (--i >= 0)
                if (blkg->pd[i])
                        blkcg_policy[i]->pd_free_fn(blkg->pd[i]);
        blk_put_queue(disk->queue);
out_free_iostat:
        free_percpu(blkg->iostat_cpu);
out_exit_refcnt:
        percpu_ref_exit(&blkg->refcnt);
out_free_blkg:
        kfree(blkg);
        return NULL;
}

/*
 * If @new_blkg is %NULL, this function tries to allocate a new one as
 * necessary using %GFP_NOWAIT.  @new_blkg is always consumed on return.
 */
static struct blkcg_gq *blkg_create(struct blkcg *blkcg, struct gendisk *disk,
                                    struct blkcg_gq *new_blkg)
{
        struct blkcg_gq *blkg;
        int i, ret;

        lockdep_assert_held(&disk->queue->queue_lock);

        /* request_queue is dying, do not create/recreate a blkg */
        if (blk_queue_dying(disk->queue)) {
                ret = -ENODEV;
                goto err_free_blkg;
        }

        /* blkg holds a reference to blkcg */
        if (!css_tryget_online(&blkcg->css)) {
                ret = -ENODEV;
                goto err_free_blkg;
        }

        /* allocate */
        if (!new_blkg) {
                new_blkg = blkg_alloc(blkcg, disk, GFP_NOWAIT | __GFP_NOWARN);
                if (unlikely(!new_blkg)) {
                        ret = -ENOMEM;
                        goto err_put_css;
                }
        }
        blkg = new_blkg;

        /* link parent */
        if (blkcg_parent(blkcg)) {
                blkg->parent = blkg_lookup(blkcg_parent(blkcg), disk->queue);
                if (WARN_ON_ONCE(!blkg->parent)) {
                        ret = -ENODEV;
                        goto err_put_css;
                }
                blkg_get(blkg->parent);
        }

        /* invoke per-policy init */
        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];

                if (blkg->pd[i] && pol->pd_init_fn)
                        pol->pd_init_fn(blkg->pd[i]);
        }

        /* insert */
        spin_lock(&blkcg->lock);
        ret = radix_tree_insert(&blkcg->blkg_tree, disk->queue->id, blkg);
        if (likely(!ret)) {
                hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
                list_add(&blkg->q_node, &disk->queue->blkg_list);

                for (i = 0; i < BLKCG_MAX_POLS; i++) {
                        struct blkcg_policy *pol = blkcg_policy[i];

                        if (blkg->pd[i]) {
                                if (pol->pd_online_fn)
                                        pol->pd_online_fn(blkg->pd[i]);
                                blkg->pd[i]->online = true;
                        }
                }
        }
        blkg->online = true;
        spin_unlock(&blkcg->lock);

        if (!ret)
                return blkg;

        /* @blkg failed fully initialized, use the usual release path */
        blkg_put(blkg);
        return ERR_PTR(ret);

err_put_css:
        css_put(&blkcg->css);
err_free_blkg:
        if (new_blkg)
                blkg_free(new_blkg);
        return ERR_PTR(ret);
}

/**
 * blkg_lookup_create - lookup blkg, try to create one if not there
 * @blkcg: blkcg of interest
 * @disk: gendisk of interest
 *
 * Lookup blkg for the @blkcg - @disk pair.  If it doesn't exist, try to
 * create one.  blkg creation is performed recursively from blkcg_root such
 * that all non-root blkg's have access to the parent blkg.  This function
 * should be called under RCU read lock and takes @disk->queue->queue_lock.
 *
 * Returns the blkg or the closest blkg if blkg_create() fails as it walks
 * down from root.
 */
static struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
                struct gendisk *disk)
{
        struct request_queue *q = disk->queue;
        struct blkcg_gq *blkg;
        unsigned long flags;

        WARN_ON_ONCE(!rcu_read_lock_held());

        blkg = blkg_lookup(blkcg, q);
        if (blkg)
                return blkg;

        spin_lock_irqsave(&q->queue_lock, flags);
        blkg = blkg_lookup(blkcg, q);
        if (blkg) {
                if (blkcg != &blkcg_root &&
                    blkg != rcu_dereference(blkcg->blkg_hint))
                        rcu_assign_pointer(blkcg->blkg_hint, blkg);
                goto found;
        }

        /*
         * Create blkgs walking down from blkcg_root to @blkcg, so that all
         * non-root blkgs have access to their parents.  Returns the closest
         * blkg to the intended blkg should blkg_create() fail.
         */
        while (true) {
                struct blkcg *pos = blkcg;
                struct blkcg *parent = blkcg_parent(blkcg);
                struct blkcg_gq *ret_blkg = q->root_blkg;

                while (parent) {
                        blkg = blkg_lookup(parent, q);
                        if (blkg) {
                                /* remember closest blkg */
                                ret_blkg = blkg;
                                break;
                        }
                        pos = parent;
                        parent = blkcg_parent(parent);
                }

                blkg = blkg_create(pos, disk, NULL);
                if (IS_ERR(blkg)) {
                        blkg = ret_blkg;
                        break;
                }
                if (pos == blkcg)
                        break;
        }

found:
        spin_unlock_irqrestore(&q->queue_lock, flags);
        return blkg;
}

static void blkg_destroy(struct blkcg_gq *blkg)
{
        struct blkcg *blkcg = blkg->blkcg;
        int i;

        lockdep_assert_held(&blkg->q->queue_lock);
        lockdep_assert_held(&blkcg->lock);

        /*
         * blkg stays on the queue list until blkg_free_workfn(), see details in
         * blkg_free_workfn(), hence this function can be called from
         * blkcg_destroy_blkgs() first and again from blkg_destroy_all() before
         * blkg_free_workfn().
         */
        if (hlist_unhashed(&blkg->blkcg_node))
                return;

        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];

                if (blkg->pd[i] && blkg->pd[i]->online) {
                        blkg->pd[i]->online = false;
                        if (pol->pd_offline_fn)
                                pol->pd_offline_fn(blkg->pd[i]);
                }
        }

        blkg->online = false;

        radix_tree_delete(&blkcg->blkg_tree, blkg->q->id);
        hlist_del_init_rcu(&blkg->blkcg_node);

        /*
         * Both setting lookup hint to and clearing it from @blkg are done
         * under queue_lock.  If it's not pointing to @blkg now, it never
         * will.  Hint assignment itself can race safely.
         */
        if (rcu_access_pointer(blkcg->blkg_hint) == blkg)
                rcu_assign_pointer(blkcg->blkg_hint, NULL);

        /*
         * Put the reference taken at the time of creation so that when all
         * queues are gone, group can be destroyed.
         */
        percpu_ref_kill(&blkg->refcnt);
}

static void blkg_destroy_all(struct gendisk *disk)
{
        struct request_queue *q = disk->queue;
        struct blkcg_gq *blkg, *n;
        int count = BLKG_DESTROY_BATCH_SIZE;

restart:
        spin_lock_irq(&q->queue_lock);
        list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
                struct blkcg *blkcg = blkg->blkcg;

                if (hlist_unhashed(&blkg->blkcg_node))
                        continue;

                spin_lock(&blkcg->lock);
                blkg_destroy(blkg);
                spin_unlock(&blkcg->lock);

                /*
                 * in order to avoid holding the spin lock for too long, release
                 * it when a batch of blkgs are destroyed.
                 */
                if (!(--count)) {
                        count = BLKG_DESTROY_BATCH_SIZE;
                        spin_unlock_irq(&q->queue_lock);
                        cond_resched();
                        goto restart;
                }
        }

        q->root_blkg = NULL;
        spin_unlock_irq(&q->queue_lock);
}

static int blkcg_reset_stats(struct cgroup_subsys_state *css,
                             struct cftype *cftype, u64 val)
{
        struct blkcg *blkcg = css_to_blkcg(css);
        struct blkcg_gq *blkg;
        int i, cpu;

        mutex_lock(&blkcg_pol_mutex);
        spin_lock_irq(&blkcg->lock);

        /*
         * Note that stat reset is racy - it doesn't synchronize against
         * stat updates.  This is a debug feature which shouldn't exist
         * anyway.  If you get hit by a race, retry.
         */
        hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
                for_each_possible_cpu(cpu) {
                        struct blkg_iostat_set *bis =
                                per_cpu_ptr(blkg->iostat_cpu, cpu);
                        memset(bis, 0, sizeof(*bis));
                }
                memset(&blkg->iostat, 0, sizeof(blkg->iostat));

                for (i = 0; i < BLKCG_MAX_POLS; i++) {
                        struct blkcg_policy *pol = blkcg_policy[i];

                        if (blkg->pd[i] && pol->pd_reset_stats_fn)
                                pol->pd_reset_stats_fn(blkg->pd[i]);
                }
        }

        spin_unlock_irq(&blkcg->lock);
        mutex_unlock(&blkcg_pol_mutex);
        return 0;
}

const char *blkg_dev_name(struct blkcg_gq *blkg)
{
        if (!blkg->q->disk)
                return NULL;
        return bdi_dev_name(blkg->q->disk->bdi);
}

/**
 * blkcg_print_blkgs - helper for printing per-blkg data
 * @sf: seq_file to print to
 * @blkcg: blkcg of interest
 * @prfill: fill function to print out a blkg
 * @pol: policy in question
 * @data: data to be passed to @prfill
 * @show_total: to print out sum of prfill return values or not
 *
 * This function invokes @prfill on each blkg of @blkcg if pd for the
 * policy specified by @pol exists.  @prfill is invoked with @sf, the
 * policy data and @data and the matching queue lock held.  If @show_total
 * is %true, the sum of the return values from @prfill is printed with
 * "Total" label at the end.
 *
 * This is to be used to construct print functions for
 * cftype->read_seq_string method.
 */
void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
                       u64 (*prfill)(struct seq_file *,
                                     struct blkg_policy_data *, int),
                       const struct blkcg_policy *pol, int data,
                       bool show_total)
{
        struct blkcg_gq *blkg;
        u64 total = 0;

        rcu_read_lock();
        hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
                spin_lock_irq(&blkg->q->queue_lock);
                if (blkcg_policy_enabled(blkg->q, pol))
                        total += prfill(sf, blkg->pd[pol->plid], data);
                spin_unlock_irq(&blkg->q->queue_lock);
        }
        rcu_read_unlock();

        if (show_total)
                seq_printf(sf, "Total %llu\n", (unsigned long long)total);
}
EXPORT_SYMBOL_GPL(blkcg_print_blkgs);

/**
 * __blkg_prfill_u64 - prfill helper for a single u64 value
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @v: value to print
 *
 * Print @v to @sf for the device associated with @pd.
 */
u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v)
{
        const char *dname = blkg_dev_name(pd->blkg);

        if (!dname)
                return 0;

        seq_printf(sf, "%s %llu\n", dname, (unsigned long long)v);
        return v;
}
EXPORT_SYMBOL_GPL(__blkg_prfill_u64);

/**
 * blkg_conf_init - initialize a blkg_conf_ctx
 * @ctx: blkg_conf_ctx to initialize
 * @input: input string
 *
 * Initialize @ctx which can be used to parse blkg config input string @input.
 * Once initialized, @ctx can be used with blkg_conf_open_bdev() and
 * blkg_conf_prep(), and must be cleaned up with blkg_conf_exit().
 */
void blkg_conf_init(struct blkg_conf_ctx *ctx, char *input)
{
        *ctx = (struct blkg_conf_ctx){ .input = input };
}
EXPORT_SYMBOL_GPL(blkg_conf_init);

/**
 * blkg_conf_open_bdev - parse and open bdev for per-blkg config update
 * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
 *
 * Parse the device node prefix part, MAJ:MIN, of per-blkg config update from
 * @ctx->input and get and store the matching bdev in @ctx->bdev. @ctx->body is
 * set to point past the device node prefix.
 *
 * This function may be called multiple times on @ctx and the extra calls become
 * NOOPs. blkg_conf_prep() implicitly calls this function. Use this function
 * explicitly if bdev access is needed without resolving the blkcg / policy part
 * of @ctx->input. Returns -errno on error.
 */
int blkg_conf_open_bdev(struct blkg_conf_ctx *ctx)
{
        char *input = ctx->input;
        unsigned int major, minor;
        struct block_device *bdev;
        int key_len;

        if (ctx->bdev)
                return 0;

        if (sscanf(input, "%u:%u%n", &major, &minor, &key_len) != 2)
                return -EINVAL;

        input += key_len;
        if (!isspace(*input))
                return -EINVAL;
        input = skip_spaces(input);

        bdev = blkdev_get_no_open(MKDEV(major, minor));
        if (!bdev)
                return -ENODEV;
        if (bdev_is_partition(bdev)) {
                blkdev_put_no_open(bdev);
                return -ENODEV;
        }

        ctx->body = input;
        ctx->bdev = bdev;
        return 0;
}

/**
 * blkg_conf_prep - parse and prepare for per-blkg config update
 * @blkcg: target block cgroup
 * @pol: target policy
 * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
 *
 * Parse per-blkg config update from @ctx->input and initialize @ctx
 * accordingly. On success, @ctx->body points to the part of @ctx->input
 * following MAJ:MIN, @ctx->bdev points to the target block device and
 * @ctx->blkg to the blkg being configured.
 *
 * blkg_conf_open_bdev() may be called on @ctx beforehand. On success, this
 * function returns with queue lock held and must be followed by
 * blkg_conf_exit().
 */
int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
                   struct blkg_conf_ctx *ctx)
        __acquires(&bdev->bd_queue->queue_lock)
{
        struct gendisk *disk;
        struct request_queue *q;
        struct blkcg_gq *blkg;
        int ret;

        ret = blkg_conf_open_bdev(ctx);
        if (ret)
                return ret;

        disk = ctx->bdev->bd_disk;
        q = disk->queue;

        /*
         * blkcg_deactivate_policy() requires queue to be frozen, we can grab
         * q_usage_counter to prevent concurrent with blkcg_deactivate_policy().
         */
        ret = blk_queue_enter(q, 0);
        if (ret)
                goto fail;

        spin_lock_irq(&q->queue_lock);

        if (!blkcg_policy_enabled(q, pol)) {
                ret = -EOPNOTSUPP;
                goto fail_unlock;
        }

        blkg = blkg_lookup(blkcg, q);
        if (blkg)
                goto success;

        /*
         * Create blkgs walking down from blkcg_root to @blkcg, so that all
         * non-root blkgs have access to their parents.
         */
        while (true) {
                struct blkcg *pos = blkcg;
                struct blkcg *parent;
                struct blkcg_gq *new_blkg;

                parent = blkcg_parent(blkcg);
                while (parent && !blkg_lookup(parent, q)) {
                        pos = parent;
                        parent = blkcg_parent(parent);
                }

                /* Drop locks to do new blkg allocation with GFP_KERNEL. */
                spin_unlock_irq(&q->queue_lock);

                new_blkg = blkg_alloc(pos, disk, GFP_KERNEL);
                if (unlikely(!new_blkg)) {
                        ret = -ENOMEM;
                        goto fail_exit_queue;
                }

                if (radix_tree_preload(GFP_KERNEL)) {
                        blkg_free(new_blkg);
                        ret = -ENOMEM;
                        goto fail_exit_queue;
                }

                spin_lock_irq(&q->queue_lock);

                if (!blkcg_policy_enabled(q, pol)) {
                        blkg_free(new_blkg);
                        ret = -EOPNOTSUPP;
                        goto fail_preloaded;
                }

                blkg = blkg_lookup(pos, q);
                if (blkg) {
                        blkg_free(new_blkg);
                } else {
                        blkg = blkg_create(pos, disk, new_blkg);
                        if (IS_ERR(blkg)) {
                                ret = PTR_ERR(blkg);
                                goto fail_preloaded;
                        }
                }

                radix_tree_preload_end();

                if (pos == blkcg)
                        goto success;
        }
success:
        blk_queue_exit(q);
        ctx->blkg = blkg;
        return 0;

fail_preloaded:
        radix_tree_preload_end();
fail_unlock:
        spin_unlock_irq(&q->queue_lock);
fail_exit_queue:
        blk_queue_exit(q);
fail:
        /*
         * If queue was bypassing, we should retry.  Do so after a
         * short msleep().  It isn't strictly necessary but queue
         * can be bypassing for some time and it's always nice to
         * avoid busy looping.
         */
        if (ret == -EBUSY) {
                msleep(10);
                ret = restart_syscall();
        }
        return ret;
}
EXPORT_SYMBOL_GPL(blkg_conf_prep);

/**
 * blkg_conf_exit - clean up per-blkg config update
 * @ctx: blkg_conf_ctx initialized with blkg_conf_init()
 *
 * Clean up after per-blkg config update. This function must be called on all
 * blkg_conf_ctx's initialized with blkg_conf_init().
 */
void blkg_conf_exit(struct blkg_conf_ctx *ctx)
        __releases(&ctx->bdev->bd_queue->queue_lock)
{
        if (ctx->blkg) {
                spin_unlock_irq(&bdev_get_queue(ctx->bdev)->queue_lock);
                ctx->blkg = NULL;
        }

        if (ctx->bdev) {
                blkdev_put_no_open(ctx->bdev);
                ctx->body = NULL;
                ctx->bdev = NULL;
        }
}
EXPORT_SYMBOL_GPL(blkg_conf_exit);

static void blkg_iostat_set(struct blkg_iostat *dst, struct blkg_iostat *src)
{
        int i;

        for (i = 0; i < BLKG_IOSTAT_NR; i++) {
                dst->bytes[i] = src->bytes[i];
                dst->ios[i] = src->ios[i];
        }
}

static void blkg_iostat_add(struct blkg_iostat *dst, struct blkg_iostat *src)
{
        int i;

        for (i = 0; i < BLKG_IOSTAT_NR; i++) {
                dst->bytes[i] += src->bytes[i];
                dst->ios[i] += src->ios[i];
        }
}

static void blkg_iostat_sub(struct blkg_iostat *dst, struct blkg_iostat *src)
{
        int i;

        for (i = 0; i < BLKG_IOSTAT_NR; i++) {
                dst->bytes[i] -= src->bytes[i];
                dst->ios[i] -= src->ios[i];
        }
}

static void blkcg_iostat_update(struct blkcg_gq *blkg, struct blkg_iostat *cur,
                                struct blkg_iostat *last)
{
        struct blkg_iostat delta;
        unsigned long flags;

        /* propagate percpu delta to global */
        flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
        blkg_iostat_set(&delta, cur);
        blkg_iostat_sub(&delta, last);
        blkg_iostat_add(&blkg->iostat.cur, &delta);
        blkg_iostat_add(last, &delta);
        u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
}

static void blkcg_rstat_flush(struct cgroup_subsys_state *css, int cpu)
{
        struct blkcg *blkcg = css_to_blkcg(css);
        struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
        struct llist_node *lnode;
        struct blkg_iostat_set *bisc, *next_bisc;

        /* Root-level stats are sourced from system-wide IO stats */
        if (!cgroup_parent(css->cgroup))
                return;

        rcu_read_lock();

        lnode = llist_del_all(lhead);
        if (!lnode)
                goto out;

        /*
         * Iterate only the iostat_cpu's queued in the lockless list.
         */
        llist_for_each_entry_safe(bisc, next_bisc, lnode, lnode) {
                struct blkcg_gq *blkg = bisc->blkg;
                struct blkcg_gq *parent = blkg->parent;
                struct blkg_iostat cur;
                unsigned int seq;

                WRITE_ONCE(bisc->lqueued, false);

                /* fetch the current per-cpu values */
                do {
                        seq = u64_stats_fetch_begin(&bisc->sync);
                        blkg_iostat_set(&cur, &bisc->cur);
                } while (u64_stats_fetch_retry(&bisc->sync, seq));

                blkcg_iostat_update(blkg, &cur, &bisc->last);

                /* propagate global delta to parent (unless that's root) */
                if (parent && parent->parent)
                        blkcg_iostat_update(parent, &blkg->iostat.cur,
                                            &blkg->iostat.last);
                percpu_ref_put(&blkg->refcnt);
        }

out:
        rcu_read_unlock();
}

/*
 * We source root cgroup stats from the system-wide stats to avoid
 * tracking the same information twice and incurring overhead when no
 * cgroups are defined. For that reason, cgroup_rstat_flush in
 * blkcg_print_stat does not actually fill out the iostat in the root
 * cgroup's blkcg_gq.
 *
 * However, we would like to re-use the printing code between the root and
 * non-root cgroups to the extent possible. For that reason, we simulate
 * flushing the root cgroup's stats by explicitly filling in the iostat
 * with disk level statistics.
 */
static void blkcg_fill_root_iostats(void)
{
        struct class_dev_iter iter;
        struct device *dev;

        class_dev_iter_init(&iter, &block_class, NULL, &disk_type);
        while ((dev = class_dev_iter_next(&iter))) {
                struct block_device *bdev = dev_to_bdev(dev);
                struct blkcg_gq *blkg = bdev->bd_disk->queue->root_blkg;
                struct blkg_iostat tmp;
                int cpu;
                unsigned long flags;

                memset(&tmp, 0, sizeof(tmp));
                for_each_possible_cpu(cpu) {
                        struct disk_stats *cpu_dkstats;

                        cpu_dkstats = per_cpu_ptr(bdev->bd_stats, cpu);
                        tmp.ios[BLKG_IOSTAT_READ] +=
                                cpu_dkstats->ios[STAT_READ];
                        tmp.ios[BLKG_IOSTAT_WRITE] +=
                                cpu_dkstats->ios[STAT_WRITE];
                        tmp.ios[BLKG_IOSTAT_DISCARD] +=
                                cpu_dkstats->ios[STAT_DISCARD];
                        // convert sectors to bytes
                        tmp.bytes[BLKG_IOSTAT_READ] +=
                                cpu_dkstats->sectors[STAT_READ] << 9;
                        tmp.bytes[BLKG_IOSTAT_WRITE] +=
                                cpu_dkstats->sectors[STAT_WRITE] << 9;
                        tmp.bytes[BLKG_IOSTAT_DISCARD] +=
                                cpu_dkstats->sectors[STAT_DISCARD] << 9;
                }

                flags = u64_stats_update_begin_irqsave(&blkg->iostat.sync);
                blkg_iostat_set(&blkg->iostat.cur, &tmp);
                u64_stats_update_end_irqrestore(&blkg->iostat.sync, flags);
        }
}

static void blkcg_print_one_stat(struct blkcg_gq *blkg, struct seq_file *s)
{
        struct blkg_iostat_set *bis = &blkg->iostat;
        u64 rbytes, wbytes, rios, wios, dbytes, dios;
        const char *dname;
        unsigned seq;
        int i;

        if (!blkg->online)
                return;

        dname = blkg_dev_name(blkg);
        if (!dname)
                return;

        seq_printf(s, "%s ", dname);

        do {
                seq = u64_stats_fetch_begin(&bis->sync);

                rbytes = bis->cur.bytes[BLKG_IOSTAT_READ];
                wbytes = bis->cur.bytes[BLKG_IOSTAT_WRITE];
                dbytes = bis->cur.bytes[BLKG_IOSTAT_DISCARD];
                rios = bis->cur.ios[BLKG_IOSTAT_READ];
                wios = bis->cur.ios[BLKG_IOSTAT_WRITE];
                dios = bis->cur.ios[BLKG_IOSTAT_DISCARD];
        } while (u64_stats_fetch_retry(&bis->sync, seq));

        if (rbytes || wbytes || rios || wios) {
                seq_printf(s, "rbytes=%llu wbytes=%llu rios=%llu wios=%llu dbytes=%llu dios=%llu",
                        rbytes, wbytes, rios, wios,
                        dbytes, dios);
        }

        if (blkcg_debug_stats && atomic_read(&blkg->use_delay)) {
                seq_printf(s, " use_delay=%d delay_nsec=%llu",
                        atomic_read(&blkg->use_delay),
                        atomic64_read(&blkg->delay_nsec));
        }

        for (i = 0; i < BLKCG_MAX_POLS; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];

                if (!blkg->pd[i] || !pol->pd_stat_fn)
                        continue;

                pol->pd_stat_fn(blkg->pd[i], s);
        }

        seq_puts(s, "\n");
}

static int blkcg_print_stat(struct seq_file *sf, void *v)
{
        struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
        struct blkcg_gq *blkg;

        if (!seq_css(sf)->parent)
                blkcg_fill_root_iostats();
        else
                cgroup_rstat_flush(blkcg->css.cgroup);

        rcu_read_lock();
        hlist_for_each_entry_rcu(blkg, &blkcg->blkg_list, blkcg_node) {
                spin_lock_irq(&blkg->q->queue_lock);
                blkcg_print_one_stat(blkg, sf);
                spin_unlock_irq(&blkg->q->queue_lock);
        }
        rcu_read_unlock();
        return 0;
}

static struct cftype blkcg_files[] = {
        {
                .name = "stat",
                .seq_show = blkcg_print_stat,
        },
        { }     /* terminate */
};

static struct cftype blkcg_legacy_files[] = {
        {
                .name = "reset_stats",
                .write_u64 = blkcg_reset_stats,
        },
        { }     /* terminate */
};

#ifdef CONFIG_CGROUP_WRITEBACK
struct list_head *blkcg_get_cgwb_list(struct cgroup_subsys_state *css)
{
        return &css_to_blkcg(css)->cgwb_list;
}
#endif

/*
 * blkcg destruction is a three-stage process.
 *
 * 1. Destruction starts.  The blkcg_css_offline() callback is invoked
 *    which offlines writeback.  Here we tie the next stage of blkg destruction
 *    to the completion of writeback associated with the blkcg.  This lets us
 *    avoid punting potentially large amounts of outstanding writeback to root
 *    while maintaining any ongoing policies.  The next stage is triggered when
 *    the nr_cgwbs count goes to zero.
 *
 * 2. When the nr_cgwbs count goes to zero, blkcg_destroy_blkgs() is called
 *    and handles the destruction of blkgs.  Here the css reference held by
 *    the blkg is put back eventually allowing blkcg_css_free() to be called.
 *    This work may occur in cgwb_release_workfn() on the cgwb_release
 *    workqueue.  Any submitted ios that fail to get the blkg ref will be
 *    punted to the root_blkg.
 *
 * 3. Once the blkcg ref count goes to zero, blkcg_css_free() is called.
 *    This finally frees the blkcg.
 */

/**
 * blkcg_destroy_blkgs - responsible for shooting down blkgs
 * @blkcg: blkcg of interest
 *
 * blkgs should be removed while holding both q and blkcg locks.  As blkcg lock
 * is nested inside q lock, this function performs reverse double lock dancing.
 * Destroying the blkgs releases the reference held on the blkcg's css allowing
 * blkcg_css_free to eventually be called.
 *
 * This is the blkcg counterpart of ioc_release_fn().
 */
static void blkcg_destroy_blkgs(struct blkcg *blkcg)
{
        might_sleep();

        spin_lock_irq(&blkcg->lock);

        while (!hlist_empty(&blkcg->blkg_list)) {
                struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
                                                struct blkcg_gq, blkcg_node);
                struct request_queue *q = blkg->q;

                if (need_resched() || !spin_trylock(&q->queue_lock)) {
                        /*
                         * Given that the system can accumulate a huge number
                         * of blkgs in pathological cases, check to see if we
                         * need to rescheduling to avoid softlockup.
                         */
                        spin_unlock_irq(&blkcg->lock);
                        cond_resched();
                        spin_lock_irq(&blkcg->lock);
                        continue;
                }

                blkg_destroy(blkg);
                spin_unlock(&q->queue_lock);
        }

        spin_unlock_irq(&blkcg->lock);
}

/**
 * blkcg_pin_online - pin online state
 * @blkcg_css: blkcg of interest
 *
 * While pinned, a blkcg is kept online.  This is primarily used to
 * impedance-match blkg and cgwb lifetimes so that blkg doesn't go offline
 * while an associated cgwb is still active.
 */
void blkcg_pin_online(struct cgroup_subsys_state *blkcg_css)
{
        refcount_inc(&css_to_blkcg(blkcg_css)->online_pin);
}

/**
 * blkcg_unpin_online - unpin online state
 * @blkcg_css: blkcg of interest
 *
 * This is primarily used to impedance-match blkg and cgwb lifetimes so
 * that blkg doesn't go offline while an associated cgwb is still active.
 * When this count goes to zero, all active cgwbs have finished so the
 * blkcg can continue destruction by calling blkcg_destroy_blkgs().
 */
void blkcg_unpin_online(struct cgroup_subsys_state *blkcg_css)
{
        struct blkcg *blkcg = css_to_blkcg(blkcg_css);

        do {
                if (!refcount_dec_and_test(&blkcg->online_pin))
                        break;
                blkcg_destroy_blkgs(blkcg);
                blkcg = blkcg_parent(blkcg);
        } while (blkcg);
}

/**
 * blkcg_css_offline - cgroup css_offline callback
 * @css: css of interest
 *
 * This function is called when @css is about to go away.  Here the cgwbs are
 * offlined first and only once writeback associated with the blkcg has
 * finished do we start step 2 (see above).
 */
static void blkcg_css_offline(struct cgroup_subsys_state *css)
{
        /* this prevents anyone from attaching or migrating to this blkcg */
        wb_blkcg_offline(css);

        /* put the base online pin allowing step 2 to be triggered */
        blkcg_unpin_online(css);
}

static void blkcg_css_free(struct cgroup_subsys_state *css)
{
        struct blkcg *blkcg = css_to_blkcg(css);
        int i;

        mutex_lock(&blkcg_pol_mutex);

        list_del(&blkcg->all_blkcgs_node);

        for (i = 0; i < BLKCG_MAX_POLS; i++)
                if (blkcg->cpd[i])
                        blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);

        mutex_unlock(&blkcg_pol_mutex);

        free_percpu(blkcg->lhead);
        kfree(blkcg);
}

static struct cgroup_subsys_state *
blkcg_css_alloc(struct cgroup_subsys_state *parent_css)
{
        struct blkcg *blkcg;
        int i;

        mutex_lock(&blkcg_pol_mutex);

        if (!parent_css) {
                blkcg = &blkcg_root;
        } else {
                blkcg = kzalloc(sizeof(*blkcg), GFP_KERNEL);
                if (!blkcg)
                        goto unlock;
        }

        if (init_blkcg_llists(blkcg))
                goto free_blkcg;

        for (i = 0; i < BLKCG_MAX_POLS ; i++) {
                struct blkcg_policy *pol = blkcg_policy[i];
                struct blkcg_policy_data *cpd;

                /*
                 * If the policy hasn't been attached yet, wait for it
                 * to be attached before doing anything else. Otherwise,
                 * check if the policy requires any specific per-cgroup
                 * data: if it does, allocate and initialize it.
                 */
                if (!pol || !pol->cpd_alloc_fn)
                        continue;

                cpd = pol->cpd_alloc_fn(GFP_KERNEL);
                if (!cpd)
                        goto free_pd_blkcg;

                blkcg->cpd[i] = cpd;
                cpd->blkcg = blkcg;
                cpd->plid = i;
        }

        spin_lock_init(&blkcg->lock);
        refcount_set(&blkcg->online_pin, 1);
        INIT_RADIX_TREE(&blkcg->blkg_tree, GFP_NOWAIT | __GFP_NOWARN);
        INIT_HLIST_HEAD(&blkcg->blkg_list);
#ifdef CONFIG_CGROUP_WRITEBACK
        INIT_LIST_HEAD(&blkcg->cgwb_list);
#endif
        list_add_tail(&blkcg->all_blkcgs_node, &all_blkcgs);

        mutex_unlock(&blkcg_pol_mutex);
        return &blkcg->css;

free_pd_blkcg:
        for (i--; i >= 0; i--)
                if (blkcg->cpd[i])
                        blkcg_policy[i]->cpd_free_fn(blkcg->cpd[i]);
        free_percpu(blkcg->lhead);
free_blkcg:
        if (blkcg != &blkcg_root)
                kfree(blkcg);
unlock:
        mutex_unlock(&blkcg_pol_mutex);
        return ERR_PTR(-ENOMEM);
}

static int blkcg_css_online(struct cgroup_subsys_state *css)
{
        struct blkcg *parent = blkcg_parent(css_to_blkcg(css));

        /*
         * blkcg_pin_online() is used to delay blkcg offline so that blkgs
         * don't go offline while cgwbs are still active on them.  Pin the
         * parent so that offline always happens towards the root.
         */
        if (parent)
                blkcg_pin_online(&parent->css);
        return 0;
}

int blkcg_init_disk(struct gendisk *disk)
{
        struct request_queue *q = disk->queue;
        struct blkcg_gq *new_blkg, *blkg;
        bool preloaded;
        int ret;

        INIT_LIST_HEAD(&q->blkg_list);
        mutex_init(&q->blkcg_mutex);

        new_blkg = blkg_alloc(&blkcg_root, disk, GFP_KERNEL);
        if (!new_blkg)
                return -ENOMEM;

        preloaded = !radix_tree_preload(GFP_KERNEL);

        /* Make sure the root blkg exists. */
        /* spin_lock_irq can serve as RCU read-side critical section. */
        spin_lock_irq(&q->queue_lock);
        blkg = blkg_create(&blkcg_root, disk, new_blkg);
        if (IS_ERR(blkg))
                goto err_unlock;
        q->root_blkg = blkg;
        spin_unlock_irq(&q->queue_lock);

        if (preloaded)
                radix_tree_preload_end();

        ret = blk_ioprio_init(disk);
        if (ret)
                goto err_destroy_all;

        ret = blk_throtl_init(disk);
        if (ret)
                goto err_ioprio_exit;

        return 0;

err_ioprio_exit:
        blk_ioprio_exit(disk);
err_destroy_all:
        blkg_destroy_all(disk);
        return ret;
err_unlock:
        spin_unlock_irq(&q->queue_lock);
        if (preloaded)
                radix_tree_preload_end();
        return PTR_ERR(blkg);
}

void blkcg_exit_disk(struct gendisk *disk)
{
        blkg_destroy_all(disk);
        blk_throtl_exit(disk);
}

static void blkcg_exit(struct task_struct *tsk)
{
        if (tsk->throttle_disk)
                put_disk(tsk->throttle_disk);
        tsk->throttle_disk = NULL;
}

struct cgroup_subsys io_cgrp_subsys = {
        .css_alloc = blkcg_css_alloc,
        .css_online = blkcg_css_online,
        .css_offline = blkcg_css_offline,
        .css_free = blkcg_css_free,
        .css_rstat_flush = blkcg_rstat_flush,
        .dfl_cftypes = blkcg_files,
        .legacy_cftypes = blkcg_legacy_files,
        .legacy_name = "blkio",
        .exit = blkcg_exit,
#ifdef CONFIG_MEMCG
        /*
         * This ensures that, if available, memcg is automatically enabled
         * together on the default hierarchy so that the owner cgroup can
         * be retrieved from writeback pages.
         */
        .depends_on = 1 << memory_cgrp_id,
#endif
};
EXPORT_SYMBOL_GPL(io_cgrp_subsys);

/**
 * blkcg_activate_policy - activate a blkcg policy on a gendisk
 * @disk: gendisk of interest
 * @pol: blkcg policy to activate
 *
 * Activate @pol on @disk.  Requires %GFP_KERNEL context.  @disk goes through
 * bypass mode to populate its blkgs with policy_data for @pol.
 *
 * Activation happens with @disk bypassed, so nobody would be accessing blkgs
 * from IO path.  Update of each blkg is protected by both queue and blkcg
 * locks so that holding either lock and testing blkcg_policy_enabled() is
 * always enough for dereferencing policy data.
 *
 * The caller is responsible for synchronizing [de]activations and policy
 * [un]registerations.  Returns 0 on success, -errno on failure.
 */
int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
{
        struct request_queue *q = disk->queue;
        struct blkg_policy_data *pd_prealloc = NULL;
        struct blkcg_gq *blkg, *pinned_blkg = NULL;
        int ret;

        if (blkcg_policy_enabled(q, pol))
                return 0;

        if (queue_is_mq(q))
                blk_mq_freeze_queue(q);
retry:
        spin_lock_irq(&q->queue_lock);

        /* blkg_list is pushed at the head, reverse walk to allocate parents first */
        list_for_each_entry_reverse(blkg, &q->blkg_list, q_node) {
                struct blkg_policy_data *pd;

                if (blkg->pd[pol->plid])
                        continue;

                /* If prealloc matches, use it; otherwise try GFP_NOWAIT */
                if (blkg == pinned_blkg) {
                        pd = pd_prealloc;
                        pd_prealloc = NULL;
                } else {
                        pd = pol->pd_alloc_fn(disk, blkg->blkcg,
                                              GFP_NOWAIT | __GFP_NOWARN);
                }

                if (!pd) {
                        /*
                         * GFP_NOWAIT failed.  Free the existing one and
                         * prealloc for @blkg w/ GFP_KERNEL.
                         */
                        if (pinned_blkg)
                                blkg_put(pinned_blkg);
                        blkg_get(blkg);
                        pinned_blkg = blkg;

                        spin_unlock_irq(&q->queue_lock);

                        if (pd_prealloc)
                                pol->pd_free_fn(pd_prealloc);
                        pd_prealloc = pol->pd_alloc_fn(disk, blkg->blkcg,
                                                       GFP_KERNEL);
                        if (pd_prealloc)
                                goto retry;
                        else
                                goto enomem;
                }

                blkg->pd[pol->plid] = pd;
                pd->blkg = blkg;
                pd->plid = pol->plid;
                pd->online = false;
        }

        /* all allocated, init in the same order */
        if (pol->pd_init_fn)
                list_for_each_entry_reverse(blkg, &q->blkg_list, q_node)
                        pol->pd_init_fn(blkg->pd[pol->plid]);

        list_for_each_entry_reverse(blkg, &q->blkg_list, q_node) {
                if (pol->pd_online_fn)
                        pol->pd_online_fn(blkg->pd[pol->plid]);
                blkg->pd[pol->plid]->online = true;
        }

        __set_bit(pol->plid, q->blkcg_pols);
        ret = 0;

        spin_unlock_irq(&q->queue_lock);
out:
        if (queue_is_mq(q))
                blk_mq_unfreeze_queue(q);
        if (pinned_blkg)
                blkg_put(pinned_blkg);
        if (pd_prealloc)
                pol->pd_free_fn(pd_prealloc);
        return ret;

enomem:
        /* alloc failed, nothing's initialized yet, free everything */
        spin_lock_irq(&q->queue_lock);
        list_for_each_entry(blkg, &q->blkg_list, q_node) {
                struct blkcg *blkcg = blkg->blkcg;

                spin_lock(&blkcg->lock);
                if (blkg->pd[pol->plid]) {
                        pol->pd_free_fn(blkg->pd[pol->plid]);
                        blkg->pd[pol->plid] = NULL;
                }
                spin_unlock(&blkcg->lock);
        }
        spin_unlock_irq(&q->queue_lock);
        ret = -ENOMEM;
        goto out;
}
EXPORT_SYMBOL_GPL(blkcg_activate_policy);

/**
 * blkcg_deactivate_policy - deactivate a blkcg policy on a gendisk
 * @disk: gendisk of interest
 * @pol: blkcg policy to deactivate
 *
 * Deactivate @pol on @disk.  Follows the same synchronization rules as
 * blkcg_activate_policy().
 */
void blkcg_deactivate_policy(struct gendisk *disk,
                             const struct blkcg_policy *pol)
{
        struct request_queue *q = disk->queue;
        struct blkcg_gq *blkg;

        if (!blkcg_policy_enabled(q, pol))
                return;

        if (queue_is_mq(q))
                blk_mq_freeze_queue(q);

        mutex_lock(&q->blkcg_mutex);
        spin_lock_irq(&q->queue_lock);

        __clear_bit(pol->plid, q->blkcg_pols);

        list_for_each_entry(blkg, &q->blkg_list, q_node) {
                struct blkcg *blkcg = blkg->blkcg;

                spin_lock(&blkcg->lock);
                if (blkg->pd[pol->plid]) {
                        if (blkg->pd[pol->plid]->online && pol->pd_offline_fn)
                                pol->pd_offline_fn(blkg->pd[pol->plid]);
                        pol->pd_free_fn(blkg->pd[pol->plid]);
                        blkg->pd[pol->plid] = NULL;
                }
                spin_unlock(&blkcg->lock);
        }

        spin_unlock_irq(&q->queue_lock);
        mutex_unlock(&q->blkcg_mutex);

        if (queue_is_mq(q))
                blk_mq_unfreeze_queue(q);
}
EXPORT_SYMBOL_GPL(blkcg_deactivate_policy);

static void blkcg_free_all_cpd(struct blkcg_policy *pol)
{
        struct blkcg *blkcg;

        list_for_each_entry(blkcg, &all_blkcgs, all_blkcgs_node) {
                if (blkcg->cpd[pol->plid]) {
                        pol->cpd_free_fn(blkcg->cpd[pol->plid]);
                        blkcg->cpd[pol->plid] = NULL;
                }
        }
}

/**
 * blkcg_policy_register - register a blkcg policy
 * @pol: blkcg policy to register
 *
 * Register @pol with blkcg core.  Might sleep and @pol may be modified on
 * successful registration.  Returns 0 on success and -errno on failure.
 */
int blkcg_policy_register(struct blkcg_policy *pol)
{
        struct blkcg *blkcg;
        int i, ret;

        mutex_lock(&blkcg_pol_register_mutex);
        mutex_lock(&blkcg_pol_mutex);

        /* find an empty slot */
        ret = -ENOSPC;
        for (i = 0; i < BLKCG_MAX_POLS; i++)
                if (!blkcg_policy[i])
                        break;
        if (i >= BLKCG_MAX_POLS) {
                pr_warn("blkcg_policy_register: BLKCG_MAX_POLS too small\n");
                goto err_unlock;
        }

        /* Make sure cpd/pd_alloc_fn and cpd/pd_free_fn in pairs */
        if ((!pol->cpd_alloc_fn ^ !pol->cpd_free_fn) ||
                (!pol->pd_alloc_fn ^ !pol->pd_free_fn))
                goto err_unlock;

        /* register @pol */
        pol->plid = i;
        blkcg_policy[pol->plid] = pol;

        /* allocate and install cpd's */
        if (pol->cpd_alloc_fn) {
                list_for_each_entry(blkcg, &all_blkcgs, all_blkcgs_node) {
                        struct blkcg_policy_data *cpd;

                        cpd = pol->cpd_alloc_fn(GFP_KERNEL);
                        if (!cpd)
                                goto err_free_cpds;

                        blkcg->cpd[pol->plid] = cpd;
                        cpd->blkcg = blkcg;
                        cpd->plid = pol->plid;
                }
        }

        mutex_unlock(&blkcg_pol_mutex);

        /* everything is in place, add intf files for the new policy */
        if (pol->dfl_cftypes)
                WARN_ON(cgroup_add_dfl_cftypes(&io_cgrp_subsys,
                                               pol->dfl_cftypes));
        if (pol->legacy_cftypes)
                WARN_ON(cgroup_add_legacy_cftypes(&io_cgrp_subsys,
                                                  pol->legacy_cftypes));
        mutex_unlock(&blkcg_pol_register_mutex);
        return 0;

err_free_cpds:
        if (pol->cpd_free_fn)
                blkcg_free_all_cpd(pol);

        blkcg_policy[pol->plid] = NULL;
err_unlock:
        mutex_unlock(&blkcg_pol_mutex);
        mutex_unlock(&blkcg_pol_register_mutex);
        return ret;
}
EXPORT_SYMBOL_GPL(blkcg_policy_register);

/**
 * blkcg_policy_unregister - unregister a blkcg policy
 * @pol: blkcg policy to unregister
 *
 * Undo blkcg_policy_register(@pol).  Might sleep.
 */
void blkcg_policy_unregister(struct blkcg_policy *pol)
{
        mutex_lock(&blkcg_pol_register_mutex);

        if (WARN_ON(blkcg_policy[pol->plid] != pol))
                goto out_unlock;

        /* kill the intf files first */
        if (pol->dfl_cftypes)
                cgroup_rm_cftypes(pol->dfl_cftypes);
        if (pol->legacy_cftypes)
                cgroup_rm_cftypes(pol->legacy_cftypes);

        /* remove cpds and unregister */
        mutex_lock(&blkcg_pol_mutex);

        if (pol->cpd_free_fn)
                blkcg_free_all_cpd(pol);

        blkcg_policy[pol->plid] = NULL;

        mutex_unlock(&blkcg_pol_mutex);
out_unlock:
        mutex_unlock(&blkcg_pol_register_mutex);
}
EXPORT_SYMBOL_GPL(blkcg_policy_unregister);

/*
 * Scale the accumulated delay based on how long it has been since we updated
 * the delay.  We only call this when we are adding delay, in case it's been a
 * while since we added delay, and when we are checking to see if we need to
 * delay a task, to account for any delays that may have occurred.
 */
static void blkcg_scale_delay(struct blkcg_gq *blkg, u64 now)
{
        u64 old = atomic64_read(&blkg->delay_start);

        /* negative use_delay means no scaling, see blkcg_set_delay() */
        if (atomic_read(&blkg->use_delay) < 0)
                return;

        /*
         * We only want to scale down every second.  The idea here is that we
         * want to delay people for min(delay_nsec, NSEC_PER_SEC) in a certain
         * time window.  We only want to throttle tasks for recent delay that
         * has occurred, in 1 second time windows since that's the maximum
         * things can be throttled.  We save the current delay window in
         * blkg->last_delay so we know what amount is still left to be charged
         * to the blkg from this point onward.  blkg->last_use keeps track of
         * the use_delay counter.  The idea is if we're unthrottling the blkg we
         * are ok with whatever is happening now, and we can take away more of
         * the accumulated delay as we've already throttled enough that
         * everybody is happy with their IO latencies.
         */
        if (time_before64(old + NSEC_PER_SEC, now) &&
            atomic64_try_cmpxchg(&blkg->delay_start, &old, now)) {
                u64 cur = atomic64_read(&blkg->delay_nsec);
                u64 sub = min_t(u64, blkg->last_delay, now - old);
                int cur_use = atomic_read(&blkg->use_delay);

                /*
                 * We've been unthrottled, subtract a larger chunk of our
                 * accumulated delay.
                 */
                if (cur_use < blkg->last_use)
                        sub = max_t(u64, sub, blkg->last_delay >> 1);

                /*
                 * This shouldn't happen, but handle it anyway.  Our delay_nsec
                 * should only ever be growing except here where we subtract out
                 * min(last_delay, 1 second), but lord knows bugs happen and I'd
                 * rather not end up with negative numbers.
                 */
                if (unlikely(cur < sub)) {
                        atomic64_set(&blkg->delay_nsec, 0);
                        blkg->last_delay = 0;
                } else {
                        atomic64_sub(sub, &blkg->delay_nsec);
                        blkg->last_delay = cur - sub;
                }
                blkg->last_use = cur_use;
        }
}

/*
 * This is called when we want to actually walk up the hierarchy and check to
 * see if we need to throttle, and then actually throttle if there is some
 * accumulated delay.  This should only be called upon return to user space so
 * we're not holding some lock that would induce a priority inversion.
 */
static void blkcg_maybe_throttle_blkg(struct blkcg_gq *blkg, bool use_memdelay)
{
        unsigned long pflags;
        bool clamp;
        u64 now = ktime_to_ns(ktime_get());
        u64 exp;
        u64 delay_nsec = 0;
        int tok;

        while (blkg->parent) {
                int use_delay = atomic_read(&blkg->use_delay);

                if (use_delay) {
                        u64 this_delay;

                        blkcg_scale_delay(blkg, now);
                        this_delay = atomic64_read(&blkg->delay_nsec);
                        if (this_delay > delay_nsec) {
                                delay_nsec = this_delay;
                                clamp = use_delay > 0;
                        }
                }
                blkg = blkg->parent;
        }

        if (!delay_nsec)
                return;

        /*
         * Let's not sleep for all eternity if we've amassed a huge delay.
         * Swapping or metadata IO can accumulate 10's of seconds worth of
         * delay, and we want userspace to be able to do _something_ so cap the
         * delays at 0.25s. If there's 10's of seconds worth of delay then the
         * tasks will be delayed for 0.25 second for every syscall. If
         * blkcg_set_delay() was used as indicated by negative use_delay, the
         * caller is responsible for regulating the range.
         */
        if (clamp)
                delay_nsec = min_t(u64, delay_nsec, 250 * NSEC_PER_MSEC);

        if (use_memdelay)
                psi_memstall_enter(&pflags);

        exp = ktime_add_ns(now, delay_nsec);
        tok = io_schedule_prepare();
        do {
                __set_current_state(TASK_KILLABLE);
                if (!schedule_hrtimeout(&exp, HRTIMER_MODE_ABS))
                        break;
        } while (!fatal_signal_pending(current));
        io_schedule_finish(tok);

        if (use_memdelay)
                psi_memstall_leave(&pflags);
}

/**
 * blkcg_maybe_throttle_current - throttle the current task if it has been marked
 *
 * This is only called if we've been marked with set_notify_resume().  Obviously
 * we can be set_notify_resume() for reasons other than blkcg throttling, so we
 * check to see if current->throttle_disk is set and if not this doesn't do
 * anything.  This should only ever be called by the resume code, it's not meant
 * to be called by people willy-nilly as it will actually do the work to
 * throttle the task if it is setup for throttling.
 */
void blkcg_maybe_throttle_current(void)
{
        struct gendisk *disk = current->throttle_disk;
        struct blkcg *blkcg;
        struct blkcg_gq *blkg;
        bool use_memdelay = current->use_memdelay;

        if (!disk)
                return;

        current->throttle_disk = NULL;
        current->use_memdelay = false;

        rcu_read_lock();
        blkcg = css_to_blkcg(blkcg_css());
        if (!blkcg)
                goto out;
        blkg = blkg_lookup(blkcg, disk->queue);
        if (!blkg)
                goto out;
        if (!blkg_tryget(blkg))
                goto out;
        rcu_read_unlock();

        blkcg_maybe_throttle_blkg(blkg, use_memdelay);
        blkg_put(blkg);
        put_disk(disk);
        return;
out:
        rcu_read_unlock();
}

/**
 * blkcg_schedule_throttle - this task needs to check for throttling
 * @disk: disk to throttle
 * @use_memdelay: do we charge this to memory delay for PSI
 *
 * This is called by the IO controller when we know there's delay accumulated
 * for the blkg for this task.  We do not pass the blkg because there are places
 * we call this that may not have that information, the swapping code for
 * instance will only have a block_device at that point.  This set's the
 * notify_resume for the task to check and see if it requires throttling before
 * returning to user space.
 *
 * We will only schedule once per syscall.  You can call this over and over
 * again and it will only do the check once upon return to user space, and only
 * throttle once.  If the task needs to be throttled again it'll need to be
 * re-set at the next time we see the task.
 */
void blkcg_schedule_throttle(struct gendisk *disk, bool use_memdelay)
{
        if (unlikely(current->flags & PF_KTHREAD))
                return;

        if (current->throttle_disk != disk) {
                if (test_bit(GD_DEAD, &disk->state))
                        return;
                get_device(disk_to_dev(disk));

                if (current->throttle_disk)
                        put_disk(current->throttle_disk);
                current->throttle_disk = disk;
        }

        if (use_memdelay)
                current->use_memdelay = use_memdelay;
        set_notify_resume(current);
}

/**
 * blkcg_add_delay - add delay to this blkg
 * @blkg: blkg of interest
 * @now: the current time in nanoseconds
 * @delta: how many nanoseconds of delay to add
 *
 * Charge @delta to the blkg's current delay accumulation.  This is used to
 * throttle tasks if an IO controller thinks we need more throttling.
 */
void blkcg_add_delay(struct blkcg_gq *blkg, u64 now, u64 delta)
{
        if (WARN_ON_ONCE(atomic_read(&blkg->use_delay) < 0))
                return;
        blkcg_scale_delay(blkg, now);
        atomic64_add(delta, &blkg->delay_nsec);
}

/**
 * blkg_tryget_closest - try and get a blkg ref on the closet blkg
 * @bio: target bio
 * @css: target css
 *
 * As the failure mode here is to walk up the blkg tree, this ensure that the
 * blkg->parent pointers are always valid.  This returns the blkg that it ended
 * up taking a reference on or %NULL if no reference was taken.
 */
static inline struct blkcg_gq *blkg_tryget_closest(struct bio *bio,
                struct cgroup_subsys_state *css)
{
        struct blkcg_gq *blkg, *ret_blkg = NULL;

        rcu_read_lock();
        blkg = blkg_lookup_create(css_to_blkcg(css), bio->bi_bdev->bd_disk);
        while (blkg) {
                if (blkg_tryget(blkg)) {
                        ret_blkg = blkg;
                        break;
                }
                blkg = blkg->parent;
        }
        rcu_read_unlock();

        return ret_blkg;
}

/**
 * bio_associate_blkg_from_css - associate a bio with a specified css
 * @bio: target bio
 * @css: target css
 *
 * Associate @bio with the blkg found by combining the css's blkg and the
 * request_queue of the @bio.  An association failure is handled by walking up
 * the blkg tree.  Therefore, the blkg associated can be anything between @blkg
 * and q->root_blkg.  This situation only happens when a cgroup is dying and
 * then the remaining bios will spill to the closest alive blkg.
 *
 * A reference will be taken on the blkg and will be released when @bio is
 * freed.
 */
void bio_associate_blkg_from_css(struct bio *bio,
                                 struct cgroup_subsys_state *css)
{
        if (bio->bi_blkg)
                blkg_put(bio->bi_blkg);

        if (css && css->parent) {
                bio->bi_blkg = blkg_tryget_closest(bio, css);
        } else {
                blkg_get(bdev_get_queue(bio->bi_bdev)->root_blkg);
                bio->bi_blkg = bdev_get_queue(bio->bi_bdev)->root_blkg;
        }
}
EXPORT_SYMBOL_GPL(bio_associate_blkg_from_css);

/**
 * bio_associate_blkg - associate a bio with a blkg
 * @bio: target bio
 *
 * Associate @bio with the blkg found from the bio's css and request_queue.
 * If one is not found, bio_lookup_blkg() creates the blkg.  If a blkg is
 * already associated, the css is reused and association redone as the
 * request_queue may have changed.
 */
void bio_associate_blkg(struct bio *bio)
{
        struct cgroup_subsys_state *css;

        rcu_read_lock();

        if (bio->bi_blkg)
                css = bio_blkcg_css(bio);
        else
                css = blkcg_css();

        bio_associate_blkg_from_css(bio, css);

        rcu_read_unlock();
}
EXPORT_SYMBOL_GPL(bio_associate_blkg);

/**
 * bio_clone_blkg_association - clone blkg association from src to dst bio
 * @dst: destination bio
 * @src: source bio
 */
void bio_clone_blkg_association(struct bio *dst, struct bio *src)
{
        if (src->bi_blkg)
                bio_associate_blkg_from_css(dst, bio_blkcg_css(src));
}
EXPORT_SYMBOL_GPL(bio_clone_blkg_association);

static int blk_cgroup_io_type(struct bio *bio)
{
        if (op_is_discard(bio->bi_opf))
                return BLKG_IOSTAT_DISCARD;
        if (op_is_write(bio->bi_opf))
                return BLKG_IOSTAT_WRITE;
        return BLKG_IOSTAT_READ;
}

void blk_cgroup_bio_start(struct bio *bio)
{
        struct blkcg *blkcg = bio->bi_blkg->blkcg;
        int rwd = blk_cgroup_io_type(bio), cpu;
        struct blkg_iostat_set *bis;
        unsigned long flags;

        /* Root-level stats are sourced from system-wide IO stats */
        if (!cgroup_parent(blkcg->css.cgroup))
                return;

        cpu = get_cpu();
        bis = per_cpu_ptr(bio->bi_blkg->iostat_cpu, cpu);
        flags = u64_stats_update_begin_irqsave(&bis->sync);

        /*
         * If the bio is flagged with BIO_CGROUP_ACCT it means this is a split
         * bio and we would have already accounted for the size of the bio.
         */
        if (!bio_flagged(bio, BIO_CGROUP_ACCT)) {
                bio_set_flag(bio, BIO_CGROUP_ACCT);
                bis->cur.bytes[rwd] += bio->bi_iter.bi_size;
        }
        bis->cur.ios[rwd]++;

        /*
         * If the iostat_cpu isn't in a lockless list, put it into the
         * list to indicate that a stat update is pending.
         */
        if (!READ_ONCE(bis->lqueued)) {
                struct llist_head *lhead = this_cpu_ptr(blkcg->lhead);

                llist_add(&bis->lnode, lhead);
                WRITE_ONCE(bis->lqueued, true);
                percpu_ref_get(&bis->blkg->refcnt);
        }

        u64_stats_update_end_irqrestore(&bis->sync, flags);
        if (cgroup_subsys_on_dfl(io_cgrp_subsys))
                cgroup_rstat_updated(blkcg->css.cgroup, cpu);
        put_cpu();
}

bool blk_cgroup_congested(void)
{
        struct cgroup_subsys_state *css;
        bool ret = false;

        rcu_read_lock();
        for (css = blkcg_css(); css; css = css->parent) {
                if (atomic_read(&css->cgroup->congestion_count)) {
                        ret = true;
                        break;
                }
        }
        rcu_read_unlock();
        return ret;
}

module_param(blkcg_debug_stats, bool, 0644);
MODULE_PARM_DESC(blkcg_debug_stats, "True if you want debug stats, false if not");