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;
}