Re: [PATCH v2] mm/filemap: fix a data race in filemap_fault()

[Matthew Wilcox]

On Mon, Feb 10, 2020 at 10:01:34PM -0500, Qian Cai wrote:
> struct file_ra_state ra.mmap_miss could be accessed concurrently during
> page faults as noticed by KCSAN,
> 
>  BUG: KCSAN: data-race in filemap_fault / filemap_map_pages
> 
>  write to 0xffff9b1700a2c1b4 of 4 bytes by task 3292 on cpu 30:
>   filemap_fault+0x920/0xfc0
>   do_sync_mmap_readahead at mm/filemap.c:2384
>   (inlined by) filemap_fault at mm/filemap.c:2486
>   __xfs_filemap_fault+0x112/0x3e0 [xfs]
>   xfs_filemap_fault+0x74/0x90 [xfs]
>   __do_fault+0x9e/0x220
>   do_fault+0x4a0/0x920
>   __handle_mm_fault+0xc69/0xd00
>   handle_mm_fault+0xfc/0x2f0
>   do_page_fault+0x263/0x6f9
>   page_fault+0x34/0x40
> 
>  read to 0xffff9b1700a2c1b4 of 4 bytes by task 3313 on cpu 32:
>   filemap_map_pages+0xc2e/0xd80
>   filemap_map_pages at mm/filemap.c:2625
>   do_fault+0x3da/0x920
>   __handle_mm_fault+0xc69/0xd00
>   handle_mm_fault+0xfc/0x2f0
>   do_page_fault+0x263/0x6f9
>   page_fault+0x34/0x40
> 
>  Reported by Kernel Concurrency Sanitizer on:
>  CPU: 32 PID: 3313 Comm: systemd-udevd Tainted: G        W    L 5.5.0-next-20200210+ #1
>  Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
> 
> ra.mmap_miss is used to contribute the readahead decisions, a data race
> could be undesirable. Both the read and write is only under
> non-exclusive mmap_sem, two concurrent writers could even overflow the
> counter. Fixing the underflow by writing to a local variable before
> committing a final store to ra.mmap_miss given a small inaccuracy of the
> counter should be acceptable.
> 
> Suggested-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Signed-off-by: Qian Cai <cai@xxxxxx>

That's more than Suggested-by.  The correct way to submit this patch is:

From: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
(at the top of the patch, so it gets credited to Kirill)

then in this section:

Signed-off-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
Tested-by: Qian Cai <cai@xxxxxx>

And now you can add:

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>