mm/vmscan: add a comment about MADV_FREE pages check in folio_check_dirty_writeback

Patch series "A few cleanup and fixup patches for vmscan

This series contains a few patches to remove obsolete comment, introduce
helper to remove duplicated code and so no.  Also we take all base pages
of THP into account in rare race condition.  More details can be found in
the respective changelogs.


This patch (of 6):

The MADV_FREE pages check in folio_check_dirty_writeback is a bit hard to
follow.  Add a comment to make the code clear.

Link: https://lkml.kernel.org/r/20220425111232.23182-2-linmiaohe@huawei.comSuggested-by: Huang, Ying <ying.huang@intel.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -1435,7 +1435,10 @@ static void folio_check_dirty_writeback(struct folio *folio,
 
 	/*
 	 * Anonymous pages are not handled by flushers and must be written
-	 * from reclaim context. Do not stall reclaim based on them
+	 * from reclaim context. Do not stall reclaim based on them.
+	 * MADV_FREE anonymous pages are put into inactive file list too.
+	 * They could be mistakenly treated as file lru. So further anon
+	 * test is needed.
 	 */
 	if (!folio_is_file_lru(folio) ||
 	    (folio_test_anon(folio) && !folio_test_swapbacked(folio))) {