mm: do not reclaim private data from pinned page

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/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jan Kara <[email protected]>
Reviewed-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Lorenzo Stoakes <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: John Hubbard <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Acked-by: Peter Xu <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: jankara

mm/vmscan.c

@@ -1967,6 +1967,16 @@ 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)) {
 			/*