Btrfs: avoid premature -ENOMEM in clear_extent_bit()

We try to allocate an extent state structure before acquiring the extent
state tree's spinlock as we might need a new one later and therefore avoid
doing later an atomic allocation while holding the tree's spinlock. However
we returned -ENOMEM if that initial non-atomic allocation failed, which is
a bit excessive since we might end up not needing the pre-allocated extent
state at all - for the case where the tree doesn't have any extent states
that cover the input range and cover too any other range. Therefore don't
return -ENOMEM if that pre-allocation fails.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent_io.c

@@ -595,9 +595,14 @@ int clear_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
 		clear = 1;
 again:
 	if (!prealloc && (mask & __GFP_WAIT)) {
+		/*
+		 * Don't care for allocation failure here because we might end
+		 * up not needing the pre-allocated extent state at all, which
+		 * is the case if we only have in the tree extent states that
+		 * cover our input range and don't cover too any other range.
+		 * If we end up needing a new extent state we allocate it later.
+		 */
 		prealloc = alloc_extent_state(mask);
-		if (!prealloc)
-			return -ENOMEM;
 	}
 
 	spin_lock(&tree->lock);