dm thin: fix passdown_double_checking_shared_status()

Commit 00a0ea3 ("dm thin: do not queue freed thin mapping for next
stage processing") changed process_prepared_discard_passdown_pt1() to
increment all the blocks being discarded until after the passdown had
completed to avoid them being prematurely reused.

IO issued to a thin device that breaks sharing with a snapshot, followed
by a discard issued to snapshot(s) that previously shared the block(s),
results in passdown_double_checking_shared_status() being called to
iterate through the blocks double checking their reference count is zero
and issuing the passdown if so.  So a side effect of commit 00a0ea3
is passdown_double_checking_shared_status() was broken.

Fix this by checking if the block reference count is greater than 1.
Also, rename dm_pool_block_is_used() to dm_pool_block_is_shared().

Fixes: 00a0ea3 ("dm thin: do not queue freed thin mapping for next stage processing")
Cc: stable@vger.kernel.org # 4.9+
Reported-by: ryan.p.norwood@gmail.com
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin-metadata.c

@@ -1678,15 +1678,15 @@ int dm_thin_remove_range(struct dm_thin_device *td,
 	return r;
 }
 
-int dm_pool_block_is_used(struct dm_pool_metadata *pmd, dm_block_t b, bool *result)
+int dm_pool_block_is_shared(struct dm_pool_metadata *pmd, dm_block_t b, bool *result)
 {
 	int r;
 	uint32_t ref_count;
 
 	down_read(&pmd->root_lock);
 	r = dm_sm_get_count(pmd->data_sm, b, &ref_count);
 	if (!r)
-		*result = (ref_count != 0);
+		*result = (ref_count > 1);
 	up_read(&pmd->root_lock);
 
 	return r;

drivers/md/dm-thin-metadata.h

@@ -195,7 +195,7 @@ int dm_pool_get_metadata_dev_size(struct dm_pool_metadata *pmd,
 
 int dm_pool_get_data_dev_size(struct dm_pool_metadata *pmd, dm_block_t *result);
 
-int dm_pool_block_is_used(struct dm_pool_metadata *pmd, dm_block_t b, bool *result);
+int dm_pool_block_is_shared(struct dm_pool_metadata *pmd, dm_block_t b, bool *result);
 
 int dm_pool_inc_data_range(struct dm_pool_metadata *pmd, dm_block_t b, dm_block_t e);
 int dm_pool_dec_data_range(struct dm_pool_metadata *pmd, dm_block_t b, dm_block_t e);

drivers/md/dm-thin.c

@@ -1048,7 +1048,7 @@ static void passdown_double_checking_shared_status(struct dm_thin_new_mapping *m
 	 * passdown we have to check that these blocks are now unused.
 	 */
 	int r = 0;
-	bool used = true;
+	bool shared = true;
 	struct thin_c *tc = m->tc;
 	struct pool *pool = tc->pool;
 	dm_block_t b = m->data_block, e, end = m->data_block + m->virt_end - m->virt_begin;
@@ -1058,11 +1058,11 @@ static void passdown_double_checking_shared_status(struct dm_thin_new_mapping *m
 	while (b != end) {
 		/* find start of unmapped run */
 		for (; b < end; b++) {
-			r = dm_pool_block_is_used(pool->pmd, b, &used);
+			r = dm_pool_block_is_shared(pool->pmd, b, &shared);
 			if (r)
 				goto out;
 
-			if (!used)
+			if (!shared)
 				break;
 		}
 
@@ -1071,11 +1071,11 @@ static void passdown_double_checking_shared_status(struct dm_thin_new_mapping *m
 
 		/* find end of run */
 		for (e = b + 1; e != end; e++) {
-			r = dm_pool_block_is_used(pool->pmd, e, &used);
+			r = dm_pool_block_is_shared(pool->pmd, e, &shared);
 			if (r)
 				goto out;
 
-			if (used)
+			if (shared)
 				break;
 		}