dm cache: fix write path cache coherency in passthrough mode

In passthrough mode, dm-cache defers write bio submission until cache
invalidation completes to maintain existing coherency, requiring the
target map function to return DM_MAPIO_SUBMITTED. The current map_bio()
returns DM_MAPIO_REMAPPED, violating the required ordering constraint.

Reproduce steps:

1. Create a cache device

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0"
dmsetup create cdata --table "0 131072 linear /dev/sdc 8192"
dmsetup create corig --table "0 262144 linear /dev/sdc 262144"
dd if=/dev/zero of=/dev/mapper/cmeta bs=4k count=1 oflag=direct
dmsetup create cache --table "0 262144 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writethrough smq 0"

2. Promote the first data block into the cache

fio --filename=/dev/mapper/cache --name=populate --rw=write --bs=4k \
--direct=1 --size=64k

3. Reload the cache into passthrough mode

dmsetup suspend cache
dmsetup reload cache --table "0 262144 cache /dev/mapper/cmeta \
/dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 passthrough smq 0"
dmsetup resume cache

4. Write to the first data block, and check io ordering using ftrace

echo 1 > /sys/kernel/debug/tracing/events/block/block_bio_queue/enable
echo 1 > /sys/kernel/debug/tracing/events/block/block_bio_complete/enable
echo 1 > /sys/kernel/debug/tracing/events/block/block_rq_complete/enable
fio --filename=/dev/mapper/cache --name=test --rw=write --bs=64k \
--direct=1 --size 64k

5. ftrace logs show that write operations to the cache origin (252:2)
   and metadata operations (252:0) are unsynchronized: the origin write
   occurs before metadata commit.

 <snip>
       fio-146  [000] .....  420.139562: block_bio_queue: 252,3 WS 0 + 128 [fio]
       fio-146  [000] .....  420.149395: block_bio_queue: 252,2 WS 0 + 128 [fio]
       fio-146  [000] .....  420.149763: block_bio_queue: 8,32 WS 262144 + 128 [fio]
       fio-146  [000] dNh1.  420.151446: block_rq_complete: 8,32 WS () 262144 + 128 be,0,4 [0]
       fio-146  [000] dNh1.  420.152731: block_bio_complete: 252,2 WS 0 + 128 [0]
       fio-146  [000] dNh1.  420.154229: block_bio_complete: 252,3 WS 0 + 128 [0]
 kworker/0:0-9  [000] .....  420.160530: block_bio_queue: 252,0 W 408 + 8 [kworker/0:0]
 kworker/0:0-9  [000] .....  420.161641: block_bio_queue: 8,32 W 408 + 8 [kworker/0:0]
 kworker/0:0-9  [000] .....  420.162533: block_bio_queue: 252,0 W 416 + 8 [kworker/0:0]
 kworker/0:0-9  [000] .....  420.162821: block_bio_queue: 8,32 W 416 + 8 [kworker/0:0]
 <snip>

Fixes: b29d4986d0da ("dm cache: significant rework to leverage dm-bio-prison-v2")
Signed-off-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
index ae1edffd14eaadd0a717b80602fa1fe1dea222af..b608e88acd5118ebc62dcc702b8cf91e324207d0 100644 (file)
--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -1703,6 +1703,7 @@ static int map_bio(struct cache *cache, struct bio *bio, dm_oblock_t block,
                                bio_drop_shared_lock(cache, bio);
                                atomic_inc(&cache->stats.demotion);
                                invalidate_start(cache, cblock, block, bio);
+                               return DM_MAPIO_SUBMITTED;
                        } else
                                remap_to_origin_clear_discard(cache, bio, block);
                } else {