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

[Qian Cai]

> On Feb 10, 2020, at 10:49 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> 
> 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)

Sure, if Kirill is going to provide his Signed-off-by in the first place, Iâll be happy to
submit it on his behalf.

> 
> 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>