blob: 8e364bda516618135dbd05e0b09d9f0565a99e6c [file] [log] [blame]
// SPDX-License-Identifier: GPL-2.0
* Functions to sequence PREFLUSH and FUA writes.
* Copyright (C) 2011 Max Planck Institute for Gravitational Physics
* Copyright (C) 2011 Tejun Heo <>
* 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
* 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),
* If flush has been pending longer than the following timeout,
* it's issued even if flush_data requests are still in flight.
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)
if (!(fflags & (1UL << QUEUE_FLAG_FUA)) &&
(rq->cmd_flags & REQ_FUA))
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);
static void blk_account_io_flush(struct request *rq)
struct block_device *part = rq->rq_disk->part0;
part_stat_inc(part, ios[STAT_FLUSH]);
part_stat_add(part, nsecs[STAT_FLUSH],
ktime_get_ns() - rq->start_time_ns);
* 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.
* spin_lock_irq(fq->mq_flush_lock)
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);
switch (seq) {
/* queue for flush */
if (list_empty(pending))
fq->flush_pending_since = jiffies;
list_move_tail(&rq->flush.list, pending);
list_move_tail(&rq->flush.list, &fq->flush_data_in_flight);
blk_flush_queue_rq(rq, true);
* @rq was previously adjusted by blk_insert_flush() for
* flush sequencing and may already have gone through the
* flush data request completion path. Restore @rq for
* normal completion and end it.
blk_mq_end_request(rq, error);
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);
/* release the tag's ownership to the req cloned from */
spin_lock_irqsave(&fq->mq_flush_lock, flags);
if (!refcount_dec_and_test(&flush_rq->ref)) {
fq->rq_status = error;
spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
* Flush request has to be marked as IDLE when it is really ended
* because its .end_io() is called from timeout code path too for
* avoiding use-after-free.
WRITE_ONCE(flush_rq->state, MQ_RQ_IDLE);
if (fq->rq_status != BLK_STS_OK)
error = fq->rq_status;
if (!q->elevator) {
flush_rq->tag = BLK_MQ_NO_TAG;
} else {
flush_rq->internal_tag = BLK_MQ_NO_TAG;
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);
blk_flush_complete_seq(rq, fq, seq, error);
spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
bool is_flush_rq(struct request *rq)
return rq->end_io == flush_end_io;
* 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.
* 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))
/* C2 and C3 */
if (!list_empty(&fq->flush_data_in_flight) &&
fq->flush_pending_since + FLUSH_PENDING_TIMEOUT))
* 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) {
flush_rq->tag = first_rq->tag;
* We borrow data request's driver tag, so have to mark
* this flush request as INFLIGHT for avoiding double
* account of this driver tag
flush_rq->rq_flags |= RQF_MQ_INFLIGHT;
} 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;
* Order WRITE ->end_io and WRITE rq->ref, and its pair is the one
* implied in refcount_inc_not_zero() called from
* blk_mq_find_and_get_req(), which orders WRITE/READ flush_rq->ref
* and READ flush_rq->end_io
refcount_set(&flush_rq->ref, 1);
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);
* 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_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.
bool 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 true;
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) &&
return false;
* @rq should go through flush machinery. Mark it part of flush
* sequence and submit for further processing.
memset(&rq->flush, 0, sizeof(rq->flush));
rq->rq_flags |= RQF_FLUSH_SEQ;
rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
rq->end_io = mq_flush_data_end_io;
blk_flush_complete_seq(rq, fq, REQ_FSEQ_ACTIONS & ~policy, 0);
return true;
* blkdev_issue_flush - queue a flush
* @bdev: blockdev to issue flush for
* Description:
* Issue a flush for the block device in question.
int blkdev_issue_flush(struct block_device *bdev)
struct bio bio;
bio_init(&bio, NULL, 0);
bio_set_dev(&bio, bdev);
return submit_bio_wait(&bio);
struct blk_flush_queue *blk_alloc_flush_queue(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;
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;
return fq;
return NULL;
void blk_free_flush_queue(struct blk_flush_queue *fq)
/* bio based request queue hasn't flush queue */
if (!fq)
* Allow driver to set its own lock class to fq->mq_flush_lock for
* avoiding lockdep complaint.
* flush_end_io() may be called recursively from some driver, such as
* nvme-loop, so lockdep may complain 'possible recursive locking' because
* all 'struct blk_flush_queue' instance share same mq_flush_lock lock class
* key. We need to assign different lock class for these driver's
* fq->mq_flush_lock for avoiding the lockdep warning.
* Use dynamically allocated lock class key for each 'blk_flush_queue'
* instance is over-kill, and more worse it introduces horrible boot delay
* issue because synchronize_rcu() is implied in lockdep_unregister_key which
* is called for each hctx release. SCSI probing may synchronously create and
* destroy lots of MQ request_queues for non-existent devices, and some robot
* test kernel always enable lockdep option. It is observed that more than half
* an hour is taken during SCSI MQ probe with per-fq lock class.
void blk_mq_hctx_set_fq_lock_class(struct blk_mq_hw_ctx *hctx,
struct lock_class_key *key)
lockdep_set_class(&hctx->fq->mq_flush_lock, key);