dm thin: fix discard support to a previously shared block

If a snapshot is created and later deleted the origin dm_thin_device's
snapshotted_time will have been updated to reflect the snapshot's
creation time.  The 'shared' flag in the dm_thin_lookup_result struct
returned from dm_thin_find_block() is an approximation based on
snapshotted_time -- this is done to avoid 0(n), or worse, time
complexity.  In this case, the shared flag would be true.

But because the 'shared' flag reflects an approximation a block can be
incorrectly assumed to be shared (e.g. false positive for 'shared'
because the snapshot no longer exists).  This could result in discards
issued to a thin device not being passed down to the pool's underlying
data device.

To fix this we double check that a thin block is really still in-use
after a mapping is removed using dm_pool_block_is_used().  If the
reference count for a block is now zero the discard is allowed to be
passed down.

Also add a 'definitely_not_shared' member to the dm_thin_new_mapping
structure -- reflects that the 'shared' flag in the response from
dm_thin_find_block() can only be held as definitive if false is
returned.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1043527

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

1 files changed, 12 insertions, 2 deletions

diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index da65febdb6c4..51e656a3002c 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -512,6 +512,7 @@ struct dm_thin_new_mapping {
 	unsigned quiesced:1;
 	unsigned prepared:1;
 	unsigned pass_discard:1;
+	unsigned definitely_not_shared:1;
 
 	struct thin_c *tc;
 	dm_block_t virt_block;
@@ -683,7 +684,15 @@ static void process_prepared_discard_passdown(struct dm_thin_new_mapping *m)
 	cell_defer_no_holder(tc, m->cell2);
 
 	if (m->pass_discard)
-		remap_and_issue(tc, m->bio, m->data_block);
+		if (m->definitely_not_shared)
+			remap_and_issue(tc, m->bio, m->data_block);
+		else {
+			bool used = false;
+			if (dm_pool_block_is_used(tc->pool->pmd, m->data_block, &used) || used)
+				bio_endio(m->bio, 0);
+			else
+				remap_and_issue(tc, m->bio, m->data_block);
+		}
 	else
 		bio_endio(m->bio, 0);
 
@@ -1036,7 +1045,8 @@ static void process_discard(struct thin_c *tc, struct bio *bio)
 			 */
 			m = get_next_mapping(pool);
 			m->tc = tc;
-			m->pass_discard = (!lookup_result.shared) && pool->pf.discard_passdown;
+			m->pass_discard = pool->pf.discard_passdown;
+			m->definitely_not_shared = !lookup_result.shared;
 			m->virt_block = block;
 			m->data_block = lookup_result.block;
 			m->cell = cell;