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 3222d8c2a7f8 ("block: remove
->rw_page"), where this if branch is introduced.

Link: https://lkml.kernel.org/r/20240711-bug13-v1-1-cea2b8ae8d76@gmail.com
Fixes: 3222d8c2a7f8 ("block: remove ->rw_page")
Signed-off-by: Pei Li <peili.dev@gmail.com>
Reported-by: syzbot+da25887cc13da6bf3b8c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=da25887cc13da6bf3b8c
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_io.c b/mm/page_io.c
index 6c1c1828bb8871b3ef3a12881b60da0902051e09..ff8c99ee3af7ec477fe3d36c207dd6e097d32ca0 100644 (file)
--- a/mm/page_io.c
+++ b/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);