+ /*
+ * No buffer_head is allocated.
+ * (1) bmap: It's enough to fill bh_result without I/O.
+ * (2) read: The unwritten part should be filled with 0
+ * If a folio does not have any buffers,
+ * let's returns -EAGAIN to fallback to
+ * per-bh IO like block_read_full_folio().
+ */
+ if (!folio_buffers(bh_result->b_folio)) {
+ err = -EAGAIN;
+ goto done;
+ }
bh_result is set as mapped by map_bh(), should we need to clear it if return an error?
+
+ BUG_ON(size > sb->s_blocksize);
This check is equivalent to the following condition and is not necessary.
} else if (iblock == valid_blks &&
(ei->valid_size & (sb->s_blocksize - 1))) {
