[PATCH] mm: Do not reclaim private data from pinned page

[Jan Kara <jack@suse.cz>]

If the page is pinned, there's no point in trying to reclaim it.
Furthermore if the page is from the page cache we don't want to reclaim
fs-private data from the page because the pinning process may be writing
to the page at any time and reclaiming fs private info on a dirty page
can upset the filesystem (see link below).

Link: https://lore.kernel.org/linux-mm/20180103100430.GE4911@quack2.suse.cz
Signed-off-by: Jan Kara <jack@suse.cz>
---
 mm/vmscan.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

This was the non-controversial part of my series [1] dealing with pinned pages
in filesystems. It is already a win as it avoids crashes in the filesystem and
we can drop workarounds for this in ext4. Can we merge it please?

[1] https://lore.kernel.org/all/20230209121046.25360-1-jack@suse.cz/

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bf3eedf0209c..401a379ea99a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1901,6 +1901,16 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
 			}
 		}
 
+		/*
+		 * Folio is unmapped now so it cannot be newly pinned anymore.
+		 * No point in trying to reclaim folio if it is pinned.
+		 * Furthermore we don't want to reclaim underlying fs metadata
+		 * if the folio is pinned and thus potentially modified by the
+		 * pinning process as that may upset the filesystem.
+		 */
+		if (folio_maybe_dma_pinned(folio))
+			goto activate_locked;
+
 		mapping = folio_mapping(folio);
 		if (folio_test_dirty(folio)) {
 			/*
-- 
2.35.3