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Date:	Tue, 6 Oct 2015 16:16:37 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, kent.overstreet@...il.com,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	"Alasdair G. Kergon" <agk@...hat.com>
Subject: [PATCH v2] block: flush queued bios when the process blocks

To give others context for why I'm caring about this issue again, this
recent BZ against 4.3-rc served as a reminder that we _need_ a fix:
https://bugzilla.redhat.com/show_bug.cgi?id=1267650

FYI, I cleaned up the plug-based approach a bit further, here is the
incremental patch:
http://git.kernel.org/cgit/linux/kernel/git/snitzer/linux.git/commit/?h=wip&id=f73d001ec692125308accbb5ca26f892f949c1b6

And here is a new version of the overall combined patch (sharing now
before I transition to looking at alternatives, though my gut is the use
of a plug in generic_make_request really wouldn't hurt us.. famous last
words):

 block/bio.c            | 82 +++++++++++++-------------------------------------
 block/blk-core.c       | 21 ++++++++-----
 drivers/md/dm-bufio.c  |  2 +-
 drivers/md/raid1.c     |  6 ++--
 drivers/md/raid10.c    |  6 ++--
 include/linux/blkdev.h | 11 +++++--
 include/linux/sched.h  |  4 ---
 7 files changed, 51 insertions(+), 81 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index ad3f276..3d03668 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -354,35 +354,31 @@ static void bio_alloc_rescue(struct work_struct *work)
 	}
 }
 
-static void punt_bios_to_rescuer(struct bio_set *bs)
+/**
+ * blk_flush_bio_list
+ * @plug: the blk_plug that may have collected bios
+ *
+ * Pop bios queued on plug->bio_list and submit each of them to
+ * their rescue workqueue.
+ *
+ * If the bio doesn't have a bio_set, we use the default fs_bio_set.
+ * However, stacking drivers should use bio_set, so this shouldn't be
+ * an issue.
+ */
+void blk_flush_bio_list(struct blk_plug *plug)
 {
-	struct bio_list punt, nopunt;
 	struct bio *bio;
 
-	/*
-	 * In order to guarantee forward progress we must punt only bios that
-	 * were allocated from this bio_set; otherwise, if there was a bio on
-	 * there for a stacking driver higher up in the stack, processing it
-	 * could require allocating bios from this bio_set, and doing that from
-	 * our own rescuer would be bad.
-	 *
-	 * Since bio lists are singly linked, pop them all instead of trying to
-	 * remove from the middle of the list:
-	 */
-
-	bio_list_init(&punt);
-	bio_list_init(&nopunt);
-
-	while ((bio = bio_list_pop(current->bio_list)))
-		bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
-
-	*current->bio_list = nopunt;
-
-	spin_lock(&bs->rescue_lock);
-	bio_list_merge(&bs->rescue_list, &punt);
-	spin_unlock(&bs->rescue_lock);
+	while ((bio = bio_list_pop(&plug->bio_list))) {
+		struct bio_set *bs = bio->bi_pool;
+		if (!bs)
+			bs = fs_bio_set;
 
-	queue_work(bs->rescue_workqueue, &bs->rescue_work);
+		spin_lock(&bs->rescue_lock);
+		bio_list_add(&bs->rescue_list, bio);
+		queue_work(bs->rescue_workqueue, &bs->rescue_work);
+		spin_unlock(&bs->rescue_lock);
+	}
 }
 
 /**
@@ -422,7 +418,6 @@ static void punt_bios_to_rescuer(struct bio_set *bs)
  */
 struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
 {
-	gfp_t saved_gfp = gfp_mask;
 	unsigned front_pad;
 	unsigned inline_vecs;
 	unsigned long idx = BIO_POOL_NONE;
@@ -443,37 +438,8 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
 		/* should not use nobvec bioset for nr_iovecs > 0 */
 		if (WARN_ON_ONCE(!bs->bvec_pool && nr_iovecs > 0))
 			return NULL;
-		/*
-		 * generic_make_request() converts recursion to iteration; this
-		 * means if we're running beneath it, any bios we allocate and
-		 * submit will not be submitted (and thus freed) until after we
-		 * return.
-		 *
-		 * This exposes us to a potential deadlock if we allocate
-		 * multiple bios from the same bio_set() while running
-		 * underneath generic_make_request(). If we were to allocate
-		 * multiple bios (say a stacking block driver that was splitting
-		 * bios), we would deadlock if we exhausted the mempool's
-		 * reserve.
-		 *
-		 * We solve this, and guarantee forward progress, with a rescuer
-		 * workqueue per bio_set. If we go to allocate and there are
-		 * bios on current->bio_list, we first try the allocation
-		 * without __GFP_WAIT; if that fails, we punt those bios we
-		 * would be blocking to the rescuer workqueue before we retry
-		 * with the original gfp_flags.
-		 */
-
-		if (current->bio_list && !bio_list_empty(current->bio_list))
-			gfp_mask &= ~__GFP_WAIT;
 
 		p = mempool_alloc(bs->bio_pool, gfp_mask);
-		if (!p && gfp_mask != saved_gfp) {
-			punt_bios_to_rescuer(bs);
-			gfp_mask = saved_gfp;
-			p = mempool_alloc(bs->bio_pool, gfp_mask);
-		}
-
 		front_pad = bs->front_pad;
 		inline_vecs = BIO_INLINE_VECS;
 	}
@@ -486,12 +452,6 @@ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs)
 
 	if (nr_iovecs > inline_vecs) {
 		bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool);
-		if (!bvl && gfp_mask != saved_gfp) {
-			punt_bios_to_rescuer(bs);
-			gfp_mask = saved_gfp;
-			bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool);
-		}
-
 		if (unlikely(!bvl))
 			goto err_free;
 
diff --git a/block/blk-core.c b/block/blk-core.c
index 2eb722d..cf0706a 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1927,6 +1927,7 @@ end_io:
 void generic_make_request(struct bio *bio)
 {
 	struct bio_list bio_list_on_stack;
+	struct blk_plug plug;
 
 	if (!generic_make_request_checks(bio))
 		return;
@@ -1934,15 +1935,15 @@ void generic_make_request(struct bio *bio)
 	/*
 	 * We only want one ->make_request_fn to be active at a time, else
 	 * stack usage with stacked devices could be a problem.  So use
-	 * current->bio_list to keep a list of requests submited by a
-	 * make_request_fn function.  current->bio_list is also used as a
+	 * current->plug->bio_list to keep a list of requests submitted by a
+	 * make_request_fn function.  current->plug->bio_list is also used as a
 	 * flag to say if generic_make_request is currently active in this
 	 * task or not.  If it is NULL, then no make_request is active.  If
 	 * it is non-NULL, then a make_request is active, and new requests
 	 * should be added at the tail
 	 */
-	if (current->bio_list) {
-		bio_list_add(current->bio_list, bio);
+	if (current->plug && current->plug->bio_list) {
+		bio_list_add(&current->plug->bio_list, bio);
 		return;
 	}
 
@@ -1962,15 +1963,17 @@ void generic_make_request(struct bio *bio)
 	 */
 	BUG_ON(bio->bi_next);
 	bio_list_init(&bio_list_on_stack);
-	current->bio_list = &bio_list_on_stack;
+	blk_start_plug(&plug);
+	current->plug->bio_list = &bio_list_on_stack;
 	do {
 		struct request_queue *q = bdev_get_queue(bio->bi_bdev);
 
 		q->make_request_fn(q, bio);
 
-		bio = bio_list_pop(current->bio_list);
+		bio = bio_list_pop(current->plug->bio_list);
 	} while (bio);
-	current->bio_list = NULL; /* deactivate */
+	current->plug->bio_list = NULL; /* deactivate */
+	blk_finish_plug(&plug);
 }
 EXPORT_SYMBOL(generic_make_request);
 
@@ -3065,6 +3068,8 @@ void blk_start_plug(struct blk_plug *plug)
 	INIT_LIST_HEAD(&plug->list);
 	INIT_LIST_HEAD(&plug->mq_list);
 	INIT_LIST_HEAD(&plug->cb_list);
+	plug->bio_list = NULL;
+
 	/*
 	 * Store ordering should not be needed here, since a potential
 	 * preempt will imply a full memory barrier
@@ -3151,6 +3156,8 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
 	LIST_HEAD(list);
 	unsigned int depth;
 
+	blk_flush_bio_list(plug);
+
 	flush_plug_callbacks(plug, from_schedule);
 
 	if (!list_empty(&plug->mq_list))
diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 2dd3308..c2bff16 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -168,7 +168,7 @@ static inline int dm_bufio_cache_index(struct dm_bufio_client *c)
 #define DM_BUFIO_CACHE(c)	(dm_bufio_caches[dm_bufio_cache_index(c)])
 #define DM_BUFIO_CACHE_NAME(c)	(dm_bufio_cache_names[dm_bufio_cache_index(c)])
 
-#define dm_bufio_in_request()	(!!current->bio_list)
+#define dm_bufio_in_request()	(current->plug && !!current->plug->bio_list)
 
 static void dm_bufio_lock(struct dm_bufio_client *c)
 {
diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 4517f06..357782f 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -874,8 +874,8 @@ static sector_t wait_barrier(struct r1conf *conf, struct bio *bio)
 				    (!conf->barrier ||
 				     ((conf->start_next_window <
 				       conf->next_resync + RESYNC_SECTORS) &&
-				      current->bio_list &&
-				      !bio_list_empty(current->bio_list))),
+				      (current->plug && current->plug->bio_list &&
+				       !bio_list_empty(current->plug->bio_list)))),
 				    conf->resync_lock);
 		conf->nr_waiting--;
 	}
@@ -1013,7 +1013,7 @@ static void raid1_unplug(struct blk_plug_cb *cb, bool from_schedule)
 	struct r1conf *conf = mddev->private;
 	struct bio *bio;
 
-	if (from_schedule || current->bio_list) {
+	if (from_schedule || (current->plug && current->plug->bio_list)) {
 		spin_lock_irq(&conf->device_lock);
 		bio_list_merge(&conf->pending_bio_list, &plug->pending);
 		conf->pending_count += plug->pending_cnt;
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 0fc33eb..780681f 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -944,8 +944,8 @@ static void wait_barrier(struct r10conf *conf)
 		wait_event_lock_irq(conf->wait_barrier,
 				    !conf->barrier ||
 				    (conf->nr_pending &&
-				     current->bio_list &&
-				     !bio_list_empty(current->bio_list)),
+				     (current->plug && current->plug->bio_list &&
+				      !bio_list_empty(current->plug->bio_list))),
 				    conf->resync_lock);
 		conf->nr_waiting--;
 	}
@@ -1021,7 +1021,7 @@ static void raid10_unplug(struct blk_plug_cb *cb, bool from_schedule)
 	struct r10conf *conf = mddev->private;
 	struct bio *bio;
 
-	if (from_schedule || current->bio_list) {
+	if (from_schedule || (current->plug && current->plug->bio_list)) {
 		spin_lock_irq(&conf->device_lock);
 		bio_list_merge(&conf->pending_bio_list, &plug->pending);
 		conf->pending_count += plug->pending_cnt;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 99da9eb..9bdac70 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1040,6 +1040,7 @@ struct blk_plug {
 	struct list_head list; /* requests */
 	struct list_head mq_list; /* blk-mq requests */
 	struct list_head cb_list; /* md requires an unplug callback */
+	struct bio_list *bio_list; /* queued bios from stacked block device */
 };
 #define BLK_MAX_REQUEST_COUNT 16
 
@@ -1079,9 +1080,12 @@ static inline bool blk_needs_flush_plug(struct task_struct *tsk)
 	return plug &&
 		(!list_empty(&plug->list) ||
 		 !list_empty(&plug->mq_list) ||
-		 !list_empty(&plug->cb_list));
+		 !list_empty(&plug->cb_list) ||
+		 (plug->bio_list && !bio_list_empty(plug->bio_list)));
 }
 
+extern void blk_flush_bio_list(struct blk_plug *plug);
+
 /*
  * tag stuff
  */
@@ -1673,12 +1677,15 @@ static inline void blk_schedule_flush_plug(struct task_struct *task)
 {
 }
 
-
 static inline bool blk_needs_flush_plug(struct task_struct *tsk)
 {
 	return false;
 }
 
+static inline void blk_flush_bio_list(void)
+{
+}
+
 static inline int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask,
 				     sector_t *error_sector)
 {
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b7b9501..ca304f1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -128,7 +128,6 @@ struct sched_attr {
 
 struct futex_pi_state;
 struct robust_list_head;
-struct bio_list;
 struct fs_struct;
 struct perf_event_context;
 struct blk_plug;
@@ -1633,9 +1632,6 @@ struct task_struct {
 /* journalling filesystem info */
 	void *journal_info;
 
-/* stacked block device info */
-	struct bio_list *bio_list;
-
 #ifdef CONFIG_BLOCK
 /* stack plugging */
 	struct blk_plug *plug;
--
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