linux-kernel - [PATCH AUTOSEL 5.12 062/160] dm writecache: don't split bios when overwriting contiguous cache content

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Date:   Tue,  6 Jul 2021 07:16:48 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Sasha Levin <sashal@...nel.org>, dm-devel@...hat.com
Subject: [PATCH AUTOSEL 5.12 062/160] dm writecache: don't split bios when overwriting contiguous cache content

From: Mikulas Patocka <mpatocka@...hat.com>

[ Upstream commit ee50cc19d80e9b9a8283d1fb517a778faf2f6899 ]

If dm-writecache overwrites existing cached data, it splits the
incoming bio into many block-sized bios. The I/O scheduler does merge
these bios into one large request but this needless splitting and
merging causes performance degradation.

Fix this by avoiding bio splitting if the cache target area that is
being overwritten is contiguous.

Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
Signed-off-by: Mike Snitzer <snitzer@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/md/dm-writecache.c | 38 ++++++++++++++++++++++++++++++--------
 1 file changed, 30 insertions(+), 8 deletions(-)

diff --git a/drivers/md/dm-writecache.c b/drivers/md/dm-writecache.c
index 4f72b6f66c3a..7bb4d83e90cc 100644
--- a/drivers/md/dm-writecache.c
+++ b/drivers/md/dm-writecache.c
@@ -1360,14 +1360,18 @@ static int writecache_map(struct dm_target *ti, struct bio *bio)
 	} else {
 		do {
 			bool found_entry = false;
+			bool search_used = false;
 			if (writecache_has_error(wc))
 				goto unlock_error;
 			e = writecache_find_entry(wc, bio->bi_iter.bi_sector, 0);
 			if (e) {
-				if (!writecache_entry_is_committed(wc, e))
+				if (!writecache_entry_is_committed(wc, e)) {
+					search_used = true;
 					goto bio_copy;
+				}
 				if (!WC_MODE_PMEM(wc) && !e->write_in_progress) {
 					wc->overwrote_committed = true;
+					search_used = true;
 					goto bio_copy;
 				}
 				found_entry = true;
@@ -1404,13 +1408,31 @@ static int writecache_map(struct dm_target *ti, struct bio *bio)
 				sector_t current_cache_sec = start_cache_sec + (bio_size >> SECTOR_SHIFT);
 
 				while (bio_size < bio->bi_iter.bi_size) {
-					struct wc_entry *f = writecache_pop_from_freelist(wc, current_cache_sec);
-					if (!f)
-						break;
-					write_original_sector_seq_count(wc, f, bio->bi_iter.bi_sector +
-									(bio_size >> SECTOR_SHIFT), wc->seq_count);
-					writecache_insert_entry(wc, f);
-					wc->uncommitted_blocks++;
+					if (!search_used) {
+						struct wc_entry *f = writecache_pop_from_freelist(wc, current_cache_sec);
+						if (!f)
+							break;
+						write_original_sector_seq_count(wc, f, bio->bi_iter.bi_sector +
+										(bio_size >> SECTOR_SHIFT), wc->seq_count);
+						writecache_insert_entry(wc, f);
+						wc->uncommitted_blocks++;
+					} else {
+						struct wc_entry *f;
+						struct rb_node *next = rb_next(&e->rb_node);
+						if (!next)
+							break;
+						f = container_of(next, struct wc_entry, rb_node);
+						if (f != e + 1)
+							break;
+						if (read_original_sector(wc, f) !=
+						    read_original_sector(wc, e) + (wc->block_size >> SECTOR_SHIFT))
+							break;
+						if (unlikely(f->write_in_progress))
+							break;
+						if (writecache_entry_is_committed(wc, f))
+							wc->overwrote_committed = true;
+						e = f;
+					}
 					bio_size += wc->block_size;
 					current_cache_sec += wc->block_size >> SECTOR_SHIFT;
 				}
-- 
2.30.2