From: Mikulas Patocka When process_discard receives a partial discard that doesn't cover a full block, it sends this discard down to that block. Unfortunately, the block can be shared and the discard would corrupt the other snapshots sharing this block. This patch detects block sharing and ends the discard with success when sending it to the shared block. The above change means that if the device supports discard it can't be guaranteed that a discard request zeroes data. Therefore, we set ti->discard_zeroes_data_unsupported. Thin target discard support with this bug arrived in commit 104655fd4dcebd50068ef30253a001da72e3a081 (dm thin: support discards). Signed-off-by: Mikulas Patocka Cc: stable@vger.kernel.org Signed-off-by: Mike Snitzer Signed-off-by: Alasdair G Kergon --- drivers/md/dm-thin.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) Index: linux-3.5-rc7/drivers/md/dm-thin.c =================================================================== --- linux-3.5-rc7.orig/drivers/md/dm-thin.c +++ linux-3.5-rc7/drivers/md/dm-thin.c @@ -1245,7 +1245,10 @@ static void process_discard(struct thin_ cell_release_singleton(cell, bio); cell_release_singleton(cell2, bio); - remap_and_issue(tc, bio, lookup_result.block); + if ((!lookup_result.shared) && pool->pf.discard_passdown) + remap_and_issue(tc, bio, lookup_result.block); + else + bio_endio(bio, 0); } break; @@ -2628,6 +2631,7 @@ static int thin_ctr(struct dm_target *ti if (tc->pool->pf.discard_enabled) { ti->discards_supported = 1; ti->num_discard_requests = 1; + ti->discard_zeroes_data_unsupported = 1; } dm_put(pool_md);