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 <[email protected]> Assisted-by: Claude:claude-opus-5 Fixes: b2676e1482af ("dm writecache: count number of blocks written, not number of write bios") --- drivers/md/dm-writecache.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) Index: linux-2.6/drivers/md/dm-writecache.c =================================================================== --- linux-2.6.orig/drivers/md/dm-writecache.c +++ linux-2.6/drivers/md/dm-writecache.c @@ -1424,7 +1424,8 @@ static void writecache_bio_copy_ssd(stru 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;
