dm thin: always return -ENOSPC if no_free_space is set

If pool has 'no_free_space' set it means a previous allocation already
determined the pool has no free space (and failed that allocation with
-ENOSPC).  By always returning -ENOSPC if 'no_free_space' is set, we do
not allow the pool to oscillate between allocating blocks and then not.

But a side-effect of this determinism is that if a user wants to be able
to allocate new blocks they'll need to reload the pool's table (to clear
the 'no_free_space' flag).  This reload will happen automatically if the
pool's data volume is resized.  But if the user takes action to free a
lot of space by deleting snapshot volumes, etc the pool will no longer
allow data allocations to continue without an intervening table reload.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-thin.c

@@ -918,6 +918,13 @@ static int alloc_data_block(struct thin_c *tc, dm_block_t *result)
 	unsigned long flags;
 	struct pool *pool = tc->pool;
 
+	/*
+	 * Once no_free_space is set we must not allow allocation to succeed.
+	 * Otherwise it is difficult to explain, debug, test and support.
+	 */
+	if (pool->no_free_space)
+		return -ENOSPC;
+
 	r = dm_pool_get_free_block_count(pool->pmd, &free_blocks);
 	if (r)
 		return r;
@@ -932,9 +939,6 @@ static int alloc_data_block(struct thin_c *tc, dm_block_t *result)
 	}
 
 	if (!free_blocks) {
-		if (pool->no_free_space)
-			return -ENOSPC;
-		else {
 		/*
 		 * Try to commit to see if that will free up some
 		 * more space.
@@ -947,7 +951,10 @@ static int alloc_data_block(struct thin_c *tc, dm_block_t *result)
 
 		/*
 		 * If we still have no space we set a flag to avoid
-			 * doing all this checking and return -ENOSPC.
+		 * doing all this checking and return -ENOSPC.  This
+		 * flag serves as a latch that disallows allocations from
+		 * this pool until the admin takes action (e.g. resize or
+		 * table reload).
 		 */
 		if (!free_blocks) {
 			DMWARN("%s: no free space available.",
@@ -958,7 +965,6 @@ static int alloc_data_block(struct thin_c *tc, dm_block_t *result)
 			return -ENOSPC;
 		}
 	}
-	}
 
 	r = dm_pool_alloc_data_block(pool->pmd, result);
 	if (r)
@@ -2695,7 +2701,7 @@ static struct target_type pool_target = {
 	.name = "thin-pool",
 	.features = DM_TARGET_SINGLETON | DM_TARGET_ALWAYS_WRITEABLE |
 		    DM_TARGET_IMMUTABLE,
-	.version = {1, 8, 0},
+	.version = {1, 9, 0},
 	.module = THIS_MODULE,
 	.ctr = pool_ctr,
 	.dtr = pool_dtr,
@@ -2982,7 +2988,7 @@ static int thin_iterate_devices(struct dm_target *ti,
 
 static struct target_type thin_target = {
 	.name = "thin",
-	.version = {1, 8, 0},
+	.version = {1, 9, 0},
 	.module	= THIS_MODULE,
 	.ctr = thin_ctr,
 	.dtr = thin_dtr,