iomap: move folio_unlock out of iomap_writeback_folio

JIRA: https://issues.redhat.com/browse/RHEL-121230

commit f8b6a94
Author: Joanne Koong <joannelkoong@gmail.com>
Date:   Thu Jul 10 15:33:33 2025 +0200

    iomap: move folio_unlock out of iomap_writeback_folio

    Move unlocking the folio out of iomap_writeback_folio into the caller.
    This means the end writeback machinery is now run with the folio locked
    when no writeback happened, or writeback completed extremely fast.

    Note that having the folio locked over the call to folio_end_writeback in
    iomap_writeback_folio means that the dropbehind handling there will never
    run because the trylock fails.  The only way this can happen is if the
    writepage either never wrote back any dirty data at all, in which case
    the dropbehind handling isn't needed, or if all writeback finished
    instantly, which is rather unlikely.  Even in the latter case the
    dropbehind handling is an optional optimization so skipping it will not
    cause correctness issues.

    This prepares for exporting iomap_writeback_folio for use in folio
    laundering.

    Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
    [hch: split from a larger patch]
    Signed-off-by: Christoph Hellwig <hch@lst.de>
    Link: https://lore.kernel.org/20250710133343.399917-10-hch@lst.de
    Reviewed-by: Darrick J. Wong <djwong@kernel.org>
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Brian Foster <bfoster@redhat.com>

Author: Brian Foster

fs/iomap/buffered-io.c

@@ -1651,10 +1651,8 @@ static int iomap_writeback_folio(struct iomap_writepage_ctx *wpc,
 
 	trace_iomap_writeback_folio(inode, pos, folio_size(folio));
 
-	if (!iomap_writeback_handle_eof(folio, inode, &end_pos)) {
-		folio_unlock(folio);
+	if (!iomap_writeback_handle_eof(folio, inode, &end_pos))
 		return 0;
-	}
 	WARN_ON_ONCE(end_pos <= pos);
 
 	if (i_blocks_per_folio(inode, folio) > 1) {
@@ -1708,7 +1706,6 @@ static int iomap_writeback_folio(struct iomap_writepage_ctx *wpc,
 	 * already at this point.  In that case we need to clear the writeback
 	 * bit ourselves right after unlocking the page.
 	 */
-	folio_unlock(folio);
 	if (ifs) {
 		if (atomic_dec_and_test(&ifs->write_bytes_pending))
 			folio_end_writeback(folio);
@@ -1735,8 +1732,10 @@ iomap_writepages(struct iomap_writepage_ctx *wpc)
 			PF_MEMALLOC))
 		return -EIO;
 
-	while ((folio = writeback_iter(mapping, wpc->wbc, folio, &error)))
+	while ((folio = writeback_iter(mapping, wpc->wbc, folio, &error))) {
 		error = iomap_writeback_folio(wpc, folio);
+		folio_unlock(folio);
+	}
 
 	/*
 	 * If @error is non-zero, it means that we have a situation where some