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

[Kirill A. Shutemov]

On Mon, Feb 10, 2020 at 10:55:45PM -0500, Qian Cai wrote:
> 
> 
> > 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.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov