Documentation: fix netdev-FAQ.rst markup warning

[thirdparty/kernel/linux.git] / block / blk-flush.c

/*
 * Functions to sequence PREFLUSH and FUA writes.
 *
 * Copyright (C) 2011           Max Planck Institute for Gravitational Physics
 * Copyright (C) 2011           Tejun Heo <tj@kernel.org>
 *
 * This file is released under the GPLv2.
 *
 * REQ_{PREFLUSH|FUA} requests are decomposed to sequences consisted of three
 * optional steps - PREFLUSH, DATA and POSTFLUSH - according to the request
 * properties and hardware capability.
 *
 * If a request doesn't have data, only REQ_PREFLUSH makes sense, which
 * indicates a simple flush request.  If there is data, REQ_PREFLUSH indicates
 * that the device cache should be flushed before the data is executed, and
 * REQ_FUA means that the data must be on non-volatile media on request
 * completion.
 *
 * If the device doesn't have writeback cache, PREFLUSH and FUA don't make any
 * difference.  The requests are either completed immediately if there's no data
 * or executed as normal requests otherwise.
 *
 * If the device has writeback cache and supports FUA, REQ_PREFLUSH is
 * translated to PREFLUSH but REQ_FUA is passed down directly with DATA.
 *
 * If the device has writeback cache and doesn't support FUA, REQ_PREFLUSH
 * is translated to PREFLUSH and REQ_FUA to POSTFLUSH.
 *
 * The actual execution of flush is double buffered.  Whenever a request
 * needs to execute PRE or POSTFLUSH, it queues at
 * fq->flush_queue[fq->flush_pending_idx].  Once certain criteria are met, a
 * REQ_OP_FLUSH is issued and the pending_idx is toggled.  When the flush
 * completes, all the requests which were pending are proceeded to the next
 * step.  This allows arbitrary merging of different types of PREFLUSH/FUA
 * requests.
 *
 * Currently, the following conditions are used to determine when to issue
 * flush.
 *
 * C1. At any given time, only one flush shall be in progress.  This makes
 *     double buffering sufficient.
 *
 * C2. Flush is deferred if any request is executing DATA of its sequence.
 *     This avoids issuing separate POSTFLUSHes for requests which shared
 *     PREFLUSH.
 *
 * C3. The second condition is ignored if there is a request which has
 *     waited longer than FLUSH_PENDING_TIMEOUT.  This is to avoid
 *     starvation in the unlikely case where there are continuous stream of
 *     FUA (without PREFLUSH) requests.
 *
 * For devices which support FUA, it isn't clear whether C2 (and thus C3)
 * is beneficial.
 *
 * Note that a sequenced PREFLUSH/FUA request with DATA is completed twice.
 * Once while executing DATA and again after the whole sequence is
 * complete.  The first completion updates the contained bio but doesn't
 * finish it so that the bio submitter is notified only after the whole
 * sequence is complete.  This is implemented by testing RQF_FLUSH_SEQ in
 * req_bio_endio().
 *
 * The above peculiarity requires that each PREFLUSH/FUA request has only one
 * bio attached to it, which is guaranteed as they aren't allowed to be
 * merged in the usual way.
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/gfp.h>
#include <linux/blk-mq.h>

#include "blk.h"
#include "blk-mq.h"
#include "blk-mq-tag.h"
#include "blk-mq-sched.h"

/* PREFLUSH/FUA sequences */
enum {
        REQ_FSEQ_PREFLUSH       = (1 << 0), /* pre-flushing in progress */
        REQ_FSEQ_DATA           = (1 << 1), /* data write in progress */
        REQ_FSEQ_POSTFLUSH      = (1 << 2), /* post-flushing in progress */
        REQ_FSEQ_DONE           = (1 << 3),

        REQ_FSEQ_ACTIONS        = REQ_FSEQ_PREFLUSH | REQ_FSEQ_DATA |
                                  REQ_FSEQ_POSTFLUSH,

        /*
         * If flush has been pending longer than the following timeout,
         * it's issued even if flush_data requests are still in flight.
         */
        FLUSH_PENDING_TIMEOUT   = 5 * HZ,
};

static void blk_kick_flush(struct request_queue *q,
                           struct blk_flush_queue *fq, unsigned int flags);

static unsigned int blk_flush_policy(unsigned long fflags, struct request *rq)
{
        unsigned int policy = 0;

        if (blk_rq_sectors(rq))
                policy |= REQ_FSEQ_DATA;

        if (fflags & (1UL << QUEUE_FLAG_WC)) {
                if (rq->cmd_flags & REQ_PREFLUSH)
                        policy |= REQ_FSEQ_PREFLUSH;
                if (!(fflags & (1UL << QUEUE_FLAG_FUA)) &&
                    (rq->cmd_flags & REQ_FUA))
                        policy |= REQ_FSEQ_POSTFLUSH;
        }
        return policy;
}

static unsigned int blk_flush_cur_seq(struct request *rq)
{
        return 1 << ffz(rq->flush.seq);
}

static void blk_flush_restore_request(struct request *rq)
{
        /*
         * After flush data completion, @rq->bio is %NULL but we need to
         * complete the bio again.  @rq->biotail is guaranteed to equal the
         * original @rq->bio.  Restore it.
         */
        rq->bio = rq->biotail;

        /* make @rq a normal request */
        rq->rq_flags &= ~RQF_FLUSH_SEQ;
        rq->end_io = rq->flush.saved_end_io;
}

static void blk_flush_queue_rq(struct request *rq, bool add_front)
{
        blk_mq_add_to_requeue_list(rq, add_front, true);
}

/**
 * blk_flush_complete_seq - complete flush sequence
 * @rq: PREFLUSH/FUA request being sequenced
 * @fq: flush queue
 * @seq: sequences to complete (mask of %REQ_FSEQ_*, can be zero)
 * @error: whether an error occurred
 *
 * @rq just completed @seq part of its flush sequence, record the
 * completion and trigger the next step.
 *
 * CONTEXT:
 * spin_lock_irq(fq->mq_flush_lock)
 *
 * RETURNS:
 * %true if requests were added to the dispatch queue, %false otherwise.
 */
static void blk_flush_complete_seq(struct request *rq,
                                   struct blk_flush_queue *fq,
                                   unsigned int seq, blk_status_t error)
{
        struct request_queue *q = rq->q;
        struct list_head *pending = &fq->flush_queue[fq->flush_pending_idx];
        unsigned int cmd_flags;

        BUG_ON(rq->flush.seq & seq);
        rq->flush.seq |= seq;
        cmd_flags = rq->cmd_flags;

        if (likely(!error))
                seq = blk_flush_cur_seq(rq);
        else
                seq = REQ_FSEQ_DONE;

        switch (seq) {
        case REQ_FSEQ_PREFLUSH:
        case REQ_FSEQ_POSTFLUSH:
                /* queue for flush */
                if (list_empty(pending))
                        fq->flush_pending_since = jiffies;
                list_move_tail(&rq->flush.list, pending);
                break;

        case REQ_FSEQ_DATA:
                list_move_tail(&rq->flush.list, &fq->flush_data_in_flight);
                blk_flush_queue_rq(rq, true);
                break;

        case REQ_FSEQ_DONE:
                /*
                 * @rq was previously adjusted by blk_flush_issue() for
                 * flush sequencing and may already have gone through the
                 * flush data request completion path.  Restore @rq for
                 * normal completion and end it.
                 */
                BUG_ON(!list_empty(&rq->queuelist));
                list_del_init(&rq->flush.list);
                blk_flush_restore_request(rq);
                blk_mq_end_request(rq, error);
                break;

        default:
                BUG();
        }

        blk_kick_flush(q, fq, cmd_flags);
}

static void flush_end_io(struct request *flush_rq, blk_status_t error)
{
        struct request_queue *q = flush_rq->q;
        struct list_head *running;
        struct request *rq, *n;
        unsigned long flags = 0;
        struct blk_flush_queue *fq = blk_get_flush_queue(q, flush_rq->mq_ctx);
        struct blk_mq_hw_ctx *hctx;

        /* release the tag's ownership to the req cloned from */
        spin_lock_irqsave(&fq->mq_flush_lock, flags);
        hctx = flush_rq->mq_hctx;
        if (!q->elevator) {
                blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
                flush_rq->tag = -1;
        } else {
                blk_mq_put_driver_tag(flush_rq);
                flush_rq->internal_tag = -1;
        }

        running = &fq->flush_queue[fq->flush_running_idx];
        BUG_ON(fq->flush_pending_idx == fq->flush_running_idx);

        /* account completion of the flush request */
        fq->flush_running_idx ^= 1;

        /* and push the waiting requests to the next stage */
        list_for_each_entry_safe(rq, n, running, flush.list) {
                unsigned int seq = blk_flush_cur_seq(rq);

                BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
                blk_flush_complete_seq(rq, fq, seq, error);
        }

        fq->flush_queue_delayed = 0;
        spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
}

/**
 * blk_kick_flush - consider issuing flush request
 * @q: request_queue being kicked
 * @fq: flush queue
 * @flags: cmd_flags of the original request
 *
 * Flush related states of @q have changed, consider issuing flush request.
 * Please read the comment at the top of this file for more info.
 *
 * CONTEXT:
 * spin_lock_irq(fq->mq_flush_lock)
 *
 */
static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
                           unsigned int flags)
{
        struct list_head *pending = &fq->flush_queue[fq->flush_pending_idx];
        struct request *first_rq =
                list_first_entry(pending, struct request, flush.list);
        struct request *flush_rq = fq->flush_rq;

        /* C1 described at the top of this file */
        if (fq->flush_pending_idx != fq->flush_running_idx || list_empty(pending))
                return;

        /* C2 and C3
         *
         * For blk-mq + scheduling, we can risk having all driver tags
         * assigned to empty flushes, and we deadlock if we are expecting
         * other requests to make progress. Don't defer for that case.
         */
        if (!list_empty(&fq->flush_data_in_flight) && q->elevator &&
            time_before(jiffies,
                        fq->flush_pending_since + FLUSH_PENDING_TIMEOUT))
                return;

        /*
         * Issue flush and toggle pending_idx.  This makes pending_idx
         * different from running_idx, which means flush is in flight.
         */
        fq->flush_pending_idx ^= 1;

        blk_rq_init(q, flush_rq);

        /*
         * In case of none scheduler, borrow tag from the first request
         * since they can't be in flight at the same time. And acquire
         * the tag's ownership for flush req.
         *
         * In case of IO scheduler, flush rq need to borrow scheduler tag
         * just for cheating put/get driver tag.
         */
        flush_rq->mq_ctx = first_rq->mq_ctx;
        flush_rq->mq_hctx = first_rq->mq_hctx;

        if (!q->elevator) {
                fq->orig_rq = first_rq;
                flush_rq->tag = first_rq->tag;
                blk_mq_tag_set_rq(flush_rq->mq_hctx, first_rq->tag, flush_rq);
        } else {
                flush_rq->internal_tag = first_rq->internal_tag;
        }

        flush_rq->cmd_flags = REQ_OP_FLUSH | REQ_PREFLUSH;
        flush_rq->cmd_flags |= (flags & REQ_DRV) | (flags & REQ_FAILFAST_MASK);
        flush_rq->rq_flags |= RQF_FLUSH_SEQ;
        flush_rq->rq_disk = first_rq->rq_disk;
        flush_rq->end_io = flush_end_io;

        blk_flush_queue_rq(flush_rq, false);
}

static void mq_flush_data_end_io(struct request *rq, blk_status_t error)
{
        struct request_queue *q = rq->q;
        struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
        struct blk_mq_ctx *ctx = rq->mq_ctx;
        unsigned long flags;
        struct blk_flush_queue *fq = blk_get_flush_queue(q, ctx);

        if (q->elevator) {
                WARN_ON(rq->tag < 0);
                blk_mq_put_driver_tag(rq);
        }

        /*
         * After populating an empty queue, kick it to avoid stall.  Read
         * the comment in flush_end_io().
         */
        spin_lock_irqsave(&fq->mq_flush_lock, flags);
        blk_flush_complete_seq(rq, fq, REQ_FSEQ_DATA, error);
        spin_unlock_irqrestore(&fq->mq_flush_lock, flags);

        blk_mq_sched_restart(hctx);
}

/**
 * blk_insert_flush - insert a new PREFLUSH/FUA request
 * @rq: request to insert
 *
 * To be called from __elv_add_request() for %ELEVATOR_INSERT_FLUSH insertions.
 * or __blk_mq_run_hw_queue() to dispatch request.
 * @rq is being submitted.  Analyze what needs to be done and put it on the
 * right queue.
 */
void blk_insert_flush(struct request *rq)
{
        struct request_queue *q = rq->q;
        unsigned long fflags = q->queue_flags;  /* may change, cache */
        unsigned int policy = blk_flush_policy(fflags, rq);
        struct blk_flush_queue *fq = blk_get_flush_queue(q, rq->mq_ctx);

        /*
         * @policy now records what operations need to be done.  Adjust
         * REQ_PREFLUSH and FUA for the driver.
         */
        rq->cmd_flags &= ~REQ_PREFLUSH;
        if (!(fflags & (1UL << QUEUE_FLAG_FUA)))
                rq->cmd_flags &= ~REQ_FUA;

        /*
         * REQ_PREFLUSH|REQ_FUA implies REQ_SYNC, so if we clear any
         * of those flags, we have to set REQ_SYNC to avoid skewing
         * the request accounting.
         */
        rq->cmd_flags |= REQ_SYNC;

        /*
         * An empty flush handed down from a stacking driver may
         * translate into nothing if the underlying device does not
         * advertise a write-back cache.  In this case, simply
         * complete the request.
         */
        if (!policy) {
                blk_mq_end_request(rq, 0);
                return;
        }

        BUG_ON(rq->bio != rq->biotail); /*assumes zero or single bio rq */

        /*
         * If there's data but flush is not necessary, the request can be
         * processed directly without going through flush machinery.  Queue
         * for normal execution.
         */
        if ((policy & REQ_FSEQ_DATA) &&
            !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
                blk_mq_request_bypass_insert(rq, false);
                return;
        }

        /*
         * @rq should go through flush machinery.  Mark it part of flush
         * sequence and submit for further processing.
         */
        memset(&rq->flush, 0, sizeof(rq->flush));
        INIT_LIST_HEAD(&rq->flush.list);
        rq->rq_flags |= RQF_FLUSH_SEQ;
        rq->flush.saved_end_io = rq->end_io; /* Usually NULL */

        rq->end_io = mq_flush_data_end_io;

        spin_lock_irq(&fq->mq_flush_lock);
        blk_flush_complete_seq(rq, fq, REQ_FSEQ_ACTIONS & ~policy, 0);
        spin_unlock_irq(&fq->mq_flush_lock);
}

/**
 * blkdev_issue_flush - queue a flush
 * @bdev:       blockdev to issue flush for
 * @gfp_mask:   memory allocation flags (for bio_alloc)
 * @error_sector:       error sector
 *
 * Description:
 *    Issue a flush for the block device in question. Caller can supply
 *    room for storing the error offset in case of a flush error, if they
 *    wish to.
 */
int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
                sector_t *error_sector)
{
        struct request_queue *q;
        struct bio *bio;
        int ret = 0;

        if (bdev->bd_disk == NULL)
                return -ENXIO;

        q = bdev_get_queue(bdev);
        if (!q)
                return -ENXIO;

        /*
         * some block devices may not have their queue correctly set up here
         * (e.g. loop device without a backing file) and so issuing a flush
         * here will panic. Ensure there is a request function before issuing
         * the flush.
         */
        if (!q->make_request_fn)
                return -ENXIO;

        bio = bio_alloc(gfp_mask, 0);
        bio_set_dev(bio, bdev);
        bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH;

        ret = submit_bio_wait(bio);

        /*
         * The driver must store the error location in ->bi_sector, if
         * it supports it. For non-stacked drivers, this should be
         * copied from blk_rq_pos(rq).
         */
        if (error_sector)
                *error_sector = bio->bi_iter.bi_sector;

        bio_put(bio);
        return ret;
}
EXPORT_SYMBOL(blkdev_issue_flush);

struct blk_flush_queue *blk_alloc_flush_queue(struct request_queue *q,
                int node, int cmd_size, gfp_t flags)
{
        struct blk_flush_queue *fq;
        int rq_sz = sizeof(struct request);

        fq = kzalloc_node(sizeof(*fq), flags, node);
        if (!fq)
                goto fail;

        spin_lock_init(&fq->mq_flush_lock);

        rq_sz = round_up(rq_sz + cmd_size, cache_line_size());
        fq->flush_rq = kzalloc_node(rq_sz, flags, node);
        if (!fq->flush_rq)
                goto fail_rq;

        INIT_LIST_HEAD(&fq->flush_queue[0]);
        INIT_LIST_HEAD(&fq->flush_queue[1]);
        INIT_LIST_HEAD(&fq->flush_data_in_flight);

        return fq;

 fail_rq:
        kfree(fq);
 fail:
        return NULL;
}

void blk_free_flush_queue(struct blk_flush_queue *fq)
{
        /* bio based request queue hasn't flush queue */
        if (!fq)
                return;

        kfree(fq->flush_rq);
        kfree(fq);
}