From 2ccb8878c149443c6acf628b438c9c942c20abb2 Mon Sep 17 00:00:00 2001 From: Ming-Hung Tsai Date: Tue, 18 Aug 2026 18:05:47 +0800 Subject: [PATCH] dm cache: fix demotion stats in passthrough mode The demotion counter is incremented per incoming write bio before the invalidation begins, causing the demotion count to exceed the actual number of cached blocks when multiple bios target the same cached block. Additionally, the counter is incremented unconditionally regardless of invalidation failure. Reproduce steps: 1. Create a cache device consisting of 512 cache entries modprobe brd rd_size=262144 dmsetup create cmeta --table "0 8192 linear /dev/ram0 0" dmsetup create cdata --table "0 65536 linear /dev/ram0 8192" dmsetup create corig --table "0 65536 linear /dev/ram0 262144" dd if=/dev/zero of=/dev/mapper/cmeta bs=4k count=1 oflag=direct dmsetup create cache --table "0 65536 cache /dev/mapper/cmeta \ /dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writethrough smq 0" 2. Populate the cache, and record the number of cached blocks fio --name=populate --filename=/dev/mapper/cache --rw=randwrite --bs=4k \ --direct=1 --ioengine=libaio --iodepth=32 --io_size=2048m nr_cached=$(dmsetup status cache | awk '{split($7, a, "/"); print a[1]}') 3. Reload the cache into passthrough mode dmsetup suspend cache dmsetup reload cache --table "0 65536 cache /dev/mapper/cmeta \ /dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 passthrough smq 0" dmsetup resume cache 4. Write to the passthrough cache with multiple jobs to trigger multiple bios hitting the same cached block. fio --filename=/dev/mapper/cache --name=test --rw=write --bs=4k \ --direct=1 --ioengine=libaio --iodepth=32 --numjobs=4 5. Check if demoted matches cached block count. These numbers should match but may differ due to overcounting per bio. nr_demoted=$(dmsetup status cache | awk '{print $12}') echo "$nr_cached, $nr_demoted" Fix by moving the demotion counter increment into invalidate_complete(), gated on the success flag. Reported-by: Ben Marzinski Fixes: b29d4986d0da ("dm cache: significant rework to leverage dm-bio-prison-v2") Cc: stable@vger.kernel.org Signed-off-by: Ming-Hung Tsai Reviewed-by: Benjamin Marzinski Signed-off-by: Mikulas Patocka --- drivers/md/dm-cache-target.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c index d8d9c63d2a67..1a5072425c4a 100644 --- a/drivers/md/dm-cache-target.c +++ b/drivers/md/dm-cache-target.c @@ -1463,6 +1463,9 @@ static void invalidate_complete(struct dm_cache_migration *mg, bool success) struct bio_list bios; struct cache *cache = mg->cache; + if (success) + atomic_inc(&cache->stats.demotion); + bio_list_init(&bios); if (mg->cell) { if (dm_cell_unlock_v2(cache->prison, mg->cell, &bios)) @@ -1734,7 +1737,6 @@ static int map_bio(struct cache *cache, struct bio *bio, dm_oblock_t block, if (passthrough_mode(cache)) { if (bio_data_dir(bio) == WRITE) { bio_drop_shared_lock(cache, bio); - atomic_inc(&cache->stats.demotion); invalidate_start(cache, cblock, block, bio); return DM_MAPIO_SUBMITTED; } else