Revert "MD: fix lock contention for flush bios"

This reverts commit 5a409b4f56d50b212334f338cb8465d65550cd85.

This patch has two problems.

1/ it make multiple calls to submit_bio() from inside a make_request_fn.
 The bios thus submitted will be queued on current->bio_list and not
 submitted immediately.  As the bios are allocated from a mempool,
 this can theoretically result in a deadlock - all the pool of requests
 could be in various ->bio_list queues and a subsequent mempool_alloc
 could block waiting for one of them to be released.

2/ It aims to handle a case when there are many concurrent flush requests.
  It handles this by submitting many requests in parallel - all of which
  are identical and so most of which do nothing useful.
  It would be more efficient to just send one lower-level request, but
  allow that to satisfy multiple upper-level requests.

Fixes: 5a409b4f56d5 ("MD: fix lock contention for flush bios")
Cc: <stable@vger.kernel.org> # v4.19+
Tested-by: Xiao Ni <xni@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 7 insertions, 15 deletions

diff --git a/drivers/md/md.h b/drivers/md/md.h
index c52afb52c776..2deb84fa93f9 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -252,19 +252,6 @@ enum mddev_sb_flags {
 	MD_SB_NEED_REWRITE,	/* metadata write needs to be repeated */
 };
 
-#define NR_FLUSH_INFOS 8
-#define NR_FLUSH_BIOS 64
-struct flush_info {
-	struct bio			*bio;
-	struct mddev			*mddev;
-	struct work_struct		flush_work;
-	atomic_t			flush_pending;
-};
-struct flush_bio {
-	struct flush_info *fi;
-	struct md_rdev *rdev;
-};
-
 struct mddev {
 	void				*private;
 	struct md_personality		*pers;
@@ -470,8 +457,13 @@ struct mddev {
 						   * metadata and bitmap writes
 						   */
 
-	mempool_t			*flush_pool;
-	mempool_t			*flush_bio_pool;
+	/* Generic flush handling.
+	 * The last to finish preflush schedules a worker to submit
+	 * the rest of the request (without the REQ_PREFLUSH flag).
+	 */
+	struct bio *flush_bio;
+	atomic_t flush_pending;
+	struct work_struct flush_work;
 	struct work_struct event_work;	/* used by dm to report failure event */
 	void (*sync_super)(struct mddev *mddev, struct md_rdev *rdev);
 	struct md_cluster_info		*cluster_info;