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 // 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 <tj@kernel.org>
  *
  * 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 <linux/part_stat.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, blk_opf_t flags);
 
 static inline struct blk_flush_queue *
 blk_get_flush_queue(struct request_queue *q, struct blk_mq_ctx *ctx)
 {
     return blk_mq_map_queue(q, REQ_OP_FLUSH, ctx)->fq;
 }
 
 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);
 }
 
 static void blk_account_io_flush(struct request *rq)
 {
     struct block_device *part = rq->q->disk->part0;
 
     part_stat_lock();
     part_stat_inc(part, ios[STAT_FLUSH]);
     part_stat_add(part, nsecs[STAT_FLUSH],
               ktime_get_ns() - rq->start_time_ns);
     part_stat_unlock();
 }
 
 /**
  * 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)
  */
 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];
     blk_opf_t 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_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.
          */
         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);
 
     /* release the tag's ownership to the req cloned from */
     spin_lock_irqsave(&fq->mq_flush_lock, flags);
 
     if (!req_ref_put_and_test(flush_rq)) {
         fq->rq_status = error;
         spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
         return;
     }
 
     blk_account_io_flush(flush_rq);
     /*
      * 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;
         fq->rq_status = BLK_STS_OK;
     }
 
     if (!q->elevator) {
         flush_rq->tag = BLK_MQ_NO_TAG;
     } else {
         blk_mq_put_driver_tag(flush_rq);
         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);
 
         BUG_ON(seq != REQ_FSEQ_PREFLUSH && seq != REQ_FSEQ_POSTFLUSH);
         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.
  *
  * CONTEXT:
  * spin_lock_irq(fq->mq_flush_lock)
  *
  */
 static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
                blk_opf_t 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 */
     if (!list_empty(&fq->flush_data_in_flight) &&
         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) {
         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->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
      */
     smp_wmb();
     req_ref_set(flush_rq, 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);
         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, true);
         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
  *
  * Description:
  *    Issue a flush for the block device in question.
  */
 int blkdev_issue_flush(struct block_device *bdev)
 {
     struct bio bio;
 
     bio_init(&bio, bdev, NULL, 0, REQ_OP_WRITE | REQ_PREFLUSH);
     return submit_bio_wait(&bio);
 }
 EXPORT_SYMBOL(blkdev_issue_flush);
 
 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;
 
     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);
 }
 
 /*
  * 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);
 }
 EXPORT_SYMBOL_GPL(blk_mq_hctx_set_fq_lock_class);

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