1 files changed, 13 insertions, 1 deletions
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 05e0c4a5affd..0172f75e051a 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7681,7 +7681,19 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
 	if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags) ||
 	    em->block_start == EXTENT_MAP_INLINE) {
 		free_extent_map(em);
-		ret = -ENOTBLK;
+		/*
+		 * If we are in a NOWAIT context, return -EAGAIN in order to
+		 * fallback to buffered IO. This is not only because we can
+		 * block with buffered IO (no support for NOWAIT semantics at
+		 * the moment) but also to avoid returning short reads to user
+		 * space - this happens if we were able to read some data from
+		 * previous non-compressed extents and then when we fallback to
+		 * buffered IO, at btrfs_file_read_iter() by calling
+		 * filemap_read(), we fail to fault in pages for the read buffer,
+		 * in which case filemap_read() returns a short read (the number
+		 * of bytes previously read is > 0, so it does not return -EFAULT).
+		 */
+		ret = (flags & IOMAP_NOWAIT) ? -EAGAIN : -ENOTBLK;
 		goto unlock_err;
 	}