mm: ignore data-race in __swap_writepage

Syzbot reported a possible data race:

BUG: KCSAN: data-race in __swap_writepage / scan_swap_map_slots

read-write to 0xffff888102fca610 of 8 bytes by task 7106 on cpu 1.
read to 0xffff888102fca610 of 8 bytes by task 7080 on cpu 0.

While we are in __swap_writepage to read sis->flags, scan_swap_map_slots
is trying to update it with SWP_SCANNING.

value changed: 0x0000000000008083 -> 0x0000000000004083.

While this can be updated non-atomicially, this won't affect
SWP_SYNCHRONOUS_IO, so we consider this data-race safe.

This is possibly introduced by commit 3222d8c ("block: remove
->rw_page"), where this if branch is introduced.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 3222d8c ("block: remove ->rw_page")
Signed-off-by: Pei Li <[email protected]>
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=da25887cc13da6bf3b8c
Cc: Dan Williams <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: peilii

mm/page_io.c

@@ -382,7 +382,12 @@ void __swap_writepage(struct folio *folio, struct writeback_control *wbc)
 	 */
 	if (data_race(sis->flags & SWP_FS_OPS))
 		swap_writepage_fs(folio, wbc);
-	else if (sis->flags & SWP_SYNCHRONOUS_IO)
+	/*
+	 * ->flags can be updated non-atomicially (scan_swap_map_slots),
+	 * but that will never affect SWP_SYNCHRONOUS_IO, so the data_race
+	 * is safe.
+	 */
+	else if (data_race(sis->flags & SWP_SYNCHRONOUS_IO))
 		swap_writepage_bdev_sync(folio, wbc, sis);
 	else
 		swap_writepage_bdev_async(folio, wbc, sis);