Re: [PATCH] mm: fix data race in PagePoisoned()

From: Will Deacon
Date: Tue Sep 14 2021 - 04:31:40 EST


On Mon, Sep 13, 2021 at 01:35:43PM +0200, Marco Elver wrote:
> PagePoisoned() accesses page->flags which can be updated concurrently:
>
> | BUG: KCSAN: data-race in next_uptodate_page / unlock_page
> |
> | write (marked) to 0xffffea00050f37c0 of 8 bytes by task 1872 on cpu 1:
> | instrument_atomic_write include/linux/instrumented.h:87 [inline]
> | clear_bit_unlock_is_negative_byte include/asm-generic/bitops/instrumented-lock.h:74 [inline]
> | unlock_page+0x102/0x1b0 mm/filemap.c:1465
> | filemap_map_pages+0x6c6/0x890 mm/filemap.c:3057
> | ...
> | read to 0xffffea00050f37c0 of 8 bytes by task 1873 on cpu 0:
> | PagePoisoned include/linux/page-flags.h:204 [inline]
> | PageReadahead include/linux/page-flags.h:382 [inline]
> | next_uptodate_page+0x456/0x830 mm/filemap.c:2975
> | ...
> | CPU: 0 PID: 1873 Comm: systemd-udevd Not tainted 5.11.0-rc4-00001-gf9ce0be71d1f #1
>
> To avoid the compiler tearing or otherwise optimizing the access, use
> READ_ONCE() to access flags.
>
> Link: https://lore.kernel.org/all/20210826144157.GA26950@xsang-OptiPlex-9020/
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
> include/linux/page-flags.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index a558d67ee86f..628ab237665e 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -206,7 +206,7 @@ static __always_inline int PageCompound(struct page *page)
> #define PAGE_POISON_PATTERN -1l
> static inline int PagePoisoned(const struct page *page)
> {
> - return page->flags == PAGE_POISON_PATTERN;
> + return READ_ONCE(page->flags) == PAGE_POISON_PATTERN;
> }

Acked-by: Will Deacon <will@xxxxxxxxxx>

Thanks for writing up the patch!

Will