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dm-writecache: fix a quirk in statistics

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writecache_bio_copy_ssd() unconditionally adds the number of extra blocks
in the merged bio to stats.writes_allocate. That is only correct when the
entries were taken off the freelist.

When the function is called with search_used == true -- i.e. from the
write_hits_uncommitted / write_hits_committed paths in
writecache_map_write() -- the first entry was found in the rbtree and
every following entry is picked up with rb_next(), so nothing is
allocated at all. Accordingly the writes_allocate++ that accompanies a
successful writecache_pop_from_freelist() is skipped for that case, but
the batch accounting below still adds n - 1.

An in-place overwrite of n contiguous cached blocks therefore reports n -
1 allocations that never happened. Only the dmsetup status output is
affected; there is no functional change.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-5
Cc: stable@vger.kernel.org
Fixes: b2676e1482 ("dm writecache: count number of blocks written, not number of write bios")
This commit is contained in:
Mikulas Patocka 2026-09-30 21:39:44 +02:00
parent 78a6afc299
commit c1ab80af0d
1 changed files with 2 additions and 1 deletions
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3
drivers/md/dm-writecache.c
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@ -1425,7 +1425,8 @@ static void writecache_bio_copy_ssd(struct dm_writecache *wc, struct bio *bio,
dm_accept_partial_bio(bio, bio_size >> SECTOR_SHIFT);
wc->stats.writes += bio->bi_iter.bi_size >> wc->block_size_bits;
wc->stats.writes_allocate += (bio->bi_iter.bi_size - wc->block_size) >> wc->block_size_bits;
if (!search_used)
wc->stats.writes_allocate += (bio->bi_iter.bi_size - wc->block_size) >> wc->block_size_bits;
if (unlikely(wc->uncommitted_blocks >= wc->autocommit_blocks)) {
wc->uncommitted_blocks = 0;