btrfs: avoid blocking when allocating context for nowait dio read/write

When doing a NOWAIT direct IO read/write, we allocate a context object
(struct btrfs_dio_data) with GFP_NOFS, which can result in blocking
waiting for memory allocation (GFP_NOFS is __GFP_RECLAIM | __GFP_IO).
This is undesirable for the NOWAIT semantics, so do the allocation with
GFP_NOWAIT if we are serving a NOWAIT request and if the allocation fails
return -EAGAIN, so that the caller can fallback to a blocking context and
retry with a non-blocking write.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -7588,9 +7588,15 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
 		}
 	}
 
+	if (flags & IOMAP_NOWAIT) {
+		dio_data = kzalloc(sizeof(*dio_data), GFP_NOWAIT);
+		if (!dio_data)
+			return -EAGAIN;
+	} else {
 		dio_data = kzalloc(sizeof(*dio_data), GFP_NOFS);
 		if (!dio_data)
 			return -ENOMEM;
+	}
 
 	iomap->private = dio_data;