From: Darrick J. Wong The dm-cache writethrough strategy introduced by commit e2e74d617eadc15 ("dm cache: fix race in writethrough implementation") issues a bio to the origin device, remaps and then issues the bio to the cache device. This more conservative in-series approach was selected to favor correctness over performance (of the previous parallel writethrough). However, this in-series implementation that reuses the same bio to write both the origin and cache device didn't take into account that the block layer's req_bio_endio() modifies a completing bio's bi_sector and bi_size. So the new writethrough strategy needs to preserve these bio fields, and restore them before submission to the cache device, otherwise nothing gets written to the cache (because bi_size is 0). This patch adds a struct dm_bio_details field to struct per_bio_data, and uses dm_bio_record() and dm_bio_restore() to ensure the bio is restored before reissuing to the cache device. Adding such a large structure to the per_bio_data is not ideal but we can improve this later, for now correctness is the important thing. This problem initially went unnoticed because the dm-cache test-suite uses a linear DM device for the dm-cache device's origin device. Writethrough worked as expected because DM submits a *clone* of the original bio, so the original bio which was reused for the cache was never touched. Signed-off-by: Darrick J. Wong Signed-off-by: Joe Thornber Signed-off-by: Mike Snitzer Signed-off-by: Alasdair G Kergon --- drivers/md/dm-cache-target.c | 4 ++++ 1 file changed, 4 insertions(+) Index: linux/drivers/md/dm-cache-target.c =================================================================== --- linux.orig/drivers/md/dm-cache-target.c +++ linux/drivers/md/dm-cache-target.c @@ -6,6 +6,7 @@ #include "dm.h" #include "dm-bio-prison.h" +#include "dm-bio-record.h" #include "dm-cache-metadata.h" #include @@ -205,6 +206,7 @@ struct per_bio_data { struct cache *cache; dm_cblock_t cblock; bio_end_io_t *saved_bi_end_io; + struct dm_bio_details bio_details; }; struct dm_cache_migration { @@ -643,6 +645,7 @@ static void writethrough_endio(struct bi return; } + dm_bio_restore(&pb->bio_details, bio); remap_to_cache(pb->cache, bio, pb->cblock); /* @@ -667,6 +670,7 @@ static void remap_to_origin_then_cache(s pb->cache = cache; pb->cblock = cblock; pb->saved_bi_end_io = bio->bi_end_io; + dm_bio_record(&pb->bio_details, bio); bio->bi_end_io = writethrough_endio; remap_to_origin_clear_discard(pb->cache, bio, oblock);