blk-flush.c

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/*
 * Functions to sequence FLUSH and FUA writes.
 *
 * Copyright (C) 2011       Max Planck Institute for Gravitational Physics
 * Copyright (C) 2011       Tejun Heo <[email protected]>
 *
 * This file is released under the GPLv2.
 *
 * REQ_{FLUSH|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_FLUSH makes sense, which
 * indicates a simple flush request.  If there is data, REQ_FLUSH 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, FLUSH 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_FLUSH 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_FLUSH 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
 * q->flush_queue[q->flush_pending_idx].  Once certain criteria are met, a
 * 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 FLUSH/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 FLUSH) requests.
 *
 * For devices which support FUA, it isn't clear whether C2 (and thus C3)
 * is beneficial.
 *
 * Note that a sequenced FLUSH/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 REQ_FLUSH_SEQ in
 * req_bio_endio().
 *
 * The above peculiarity requires that each FLUSH/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 "blk.h"

/* FLUSH/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 bool blk_kick_flush(struct request_queue *q);

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

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

    if (fflags & REQ_FLUSH) {
        if (rq->cmd_flags & REQ_FLUSH)
            policy |= REQ_FSEQ_PREFLUSH;
        if (!(fflags & REQ_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->cmd_flags &= ~REQ_FLUSH_SEQ;
    rq->end_io = rq->flush.saved_end_io;
}

static bool blk_flush_complete_seq(struct request *rq, unsigned int seq,
                   int error)
{
    struct request_queue *q = rq->q;
    struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
    bool queued = false;

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

    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))
            q->flush_pending_since = jiffies;
        list_move_tail(&rq->flush.list, pending);
        break;

    case REQ_FSEQ_DATA:
        list_move_tail(&rq->flush.list, &q->flush_data_in_flight);
        list_add(&rq->queuelist, &q->queue_head);
        queued = 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_end_request_all(rq, error);
        break;

    default:
        BUG();
    }

    return blk_kick_flush(q) | queued;
}

static void flush_end_io(struct request *flush_rq, int error)
{
    struct request_queue *q = flush_rq->q;
    struct list_head *running = &q->flush_queue[q->flush_running_idx];
    bool queued = false;
    struct request *rq, *n;

    BUG_ON(q->flush_pending_idx == q->flush_running_idx);

    /* account completion of the flush request */
    q->flush_running_idx ^= 1;
    elv_completed_request(q, flush_rq);

    /* 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);
        queued |= blk_flush_complete_seq(rq, seq, error);
    }

    /*
     * Kick the queue to avoid stall for two cases:
     * 1. Moving a request silently to empty queue_head may stall the
     * queue.
     * 2. When flush request is running in non-queueable queue, the
     * queue is hold. Restart the queue after flush request is finished
     * to avoid stall.
     * This function is called from request completion path and calling
     * directly into request_fn may confuse the driver.  Always use
     * kblockd.
     */
    if (queued || q->flush_queue_delayed)
        blk_run_queue_async(q);
    q->flush_queue_delayed = 0;
}

static bool blk_kick_flush(struct request_queue *q)
{
    struct list_head *pending = &q->flush_queue[q->flush_pending_idx];
    struct request *first_rq =
        list_first_entry(pending, struct request, flush.list);

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

    /* C2 and C3 */
    if (!list_empty(&q->flush_data_in_flight) &&
        time_before(jiffies,
            q->flush_pending_since + FLUSH_PENDING_TIMEOUT))
        return false;

    /*
     * Issue flush and toggle pending_idx.  This makes pending_idx
     * different from running_idx, which means flush is in flight.
     */
    blk_rq_init(q, &q->flush_rq);
    q->flush_rq.cmd_type = REQ_TYPE_FS;
    q->flush_rq.cmd_flags = WRITE_FLUSH | REQ_FLUSH_SEQ;
    q->flush_rq.rq_disk = first_rq->rq_disk;
    q->flush_rq.end_io = flush_end_io;

    q->flush_pending_idx ^= 1;
    list_add_tail(&q->flush_rq.queuelist, &q->queue_head);
    return true;
}

static void flush_data_end_io(struct request *rq, int error)
{
    struct request_queue *q = rq->q;

    /*
     * After populating an empty queue, kick it to avoid stall.  Read
     * the comment in flush_end_io().
     */
    if (blk_flush_complete_seq(rq, REQ_FSEQ_DATA, error))
        blk_run_queue_async(q);
}

void blk_insert_flush(struct request *rq)
{
    struct request_queue *q = rq->q;
    unsigned int fflags = q->flush_flags;   /* may change, cache */
    unsigned int policy = blk_flush_policy(fflags, rq);

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

    /*
     * 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_end_bidi_request(rq, 0, 0, 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))) {
        list_add_tail(&rq->queuelist, &q->queue_head);
        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->cmd_flags |= REQ_FLUSH_SEQ;
    rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
    rq->end_io = flush_data_end_io;

    blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);
}

void blk_abort_flushes(struct request_queue *q)
{
    struct request *rq, *n;
    int i;

    /*
     * Requests in flight for data are already owned by the dispatch
     * queue or the device driver.  Just restore for normal completion.
     */
    list_for_each_entry_safe(rq, n, &q->flush_data_in_flight, flush.list) {
        list_del_init(&rq->flush.list);
        blk_flush_restore_request(rq);
    }

    /*
     * We need to give away requests on flush queues.  Restore for
     * normal completion and put them on the dispatch queue.
     */
    for (i = 0; i < ARRAY_SIZE(q->flush_queue); i++) {
        list_for_each_entry_safe(rq, n, &q->flush_queue[i],
                     flush.list) {
            list_del_init(&rq->flush.list);
            blk_flush_restore_request(rq);
            list_add_tail(&rq->queuelist, &q->queue_head);
        }
    }
}

static void bio_end_flush(struct bio *bio, int err)
{
    if (err)
        clear_bit(BIO_UPTODATE, &bio->bi_flags);
    if (bio->bi_private)
        complete(bio->bi_private);
    bio_put(bio);
}

int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
        sector_t *error_sector)
{
    DECLARE_COMPLETION_ONSTACK(wait);
    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->bi_end_io = bio_end_flush;
    bio->bi_bdev = bdev;
    bio->bi_private = &wait;

    bio_get(bio);
    submit_bio(WRITE_FLUSH, bio);
    wait_for_completion(&wait);

    /*
     * 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_sector;

    if (!bio_flagged(bio, BIO_UPTODATE))
        ret = -EIO;

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