The patch titled
Subject: mm: ignore data-race in __swap_writepage
has been added to the -mm mm-unstable branch. Its filename is
mm-ignore-data-race-in-__swap_writepage.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-ignore-data-race-in-__swap_writepage.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Pei Li <peili.dev@xxxxxxxxx>
Subject: mm: ignore data-race in __swap_writepage
Date: Thu, 11 Jul 2024 09:32:30 -0700
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@xxxxxxxxx
Fixes: 3222d8c2a7f8 ("block: remove ->rw_page")
Signed-off-by: Pei Li <peili.dev@xxxxxxxxx>
Reported-by: syzbot+da25887cc13da6bf3b8c@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=da25887cc13da6bf3b8c
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page_io.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
--- a/mm/page_io.c~mm-ignore-data-race-in-__swap_writepage
+++ a/mm/page_io.c
@@ -382,7 +382,12 @@ void __swap_writepage(struct folio *foli
*/
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);
_
Patches currently in -mm which might be from peili.dev@xxxxxxxxx are
mm-ignore-data-race-in-__swap_writepage.patch
