Re: [PATCH -next] mm: mark a intentional data race in page_zonenum()

[Marco Elver]

On Thu, 6 Feb 2020 at 15:01, Qian Cai <cai@xxxxxx> wrote:
>
> On Wed, 2020-02-05 at 20:50 -0800, John Hubbard wrote:
> > On 2/5/20 7:52 PM, Qian Cai wrote:
> > > The commit 07d802699528 ("mm: devmap: refactor 1-based refcounting for
> > > ZONE_DEVICE pages") introduced a data race as page->flags could be
> >
> > Hi,
> >
> > I really don't think so. This "race" was there long before that commit.
> > Anyway, more below:
> >
> > > accessed concurrently as noticied by KCSAN,
> > >
> > >   BUG: KCSAN: data-race in page_cpupid_xchg_last / put_page
> > >
> > >   write (marked) to 0xfffffc0d48ec1a00 of 8 bytes by task 91442 on cpu 3:
> > >    page_cpupid_xchg_last+0x51/0x80
> > >    page_cpupid_xchg_last at mm/mmzone.c:109 (discriminator 11)
> > >    wp_page_reuse+0x3e/0xc0
> > >    wp_page_reuse at mm/memory.c:2453
> > >    do_wp_page+0x472/0x7b0
> > >    do_wp_page at mm/memory.c:2798
> > >    __handle_mm_fault+0xcb0/0xd00
> > >    handle_pte_fault at mm/memory.c:4049
> > >    (inlined by) __handle_mm_fault at mm/memory.c:4163
> > >    handle_mm_fault+0xfc/0x2f0
> > >    handle_mm_fault at mm/memory.c:4200
> > >    do_page_fault+0x263/0x6f9
> > >    do_user_addr_fault at arch/x86/mm/fault.c:1465
> > >    (inlined by) do_page_fault at arch/x86/mm/fault.c:1539
> > >    page_fault+0x34/0x40
> > >
> > >   read to 0xfffffc0d48ec1a00 of 8 bytes by task 94817 on cpu 69:
> > >    put_page+0x15a/0x1f0
> > >    page_zonenum at include/linux/mm.h:923
> > >    (inlined by) is_zone_device_page at include/linux/mm.h:929
> > >    (inlined by) page_is_devmap_managed at include/linux/mm.h:948
> > >    (inlined by) put_page at include/linux/mm.h:1023
> > >    wp_page_copy+0x571/0x930
> > >    wp_page_copy at mm/memory.c:2615
> > >    do_wp_page+0x107/0x7b0
> > >    __handle_mm_fault+0xcb0/0xd00
> > >    handle_mm_fault+0xfc/0x2f0
> > >    do_page_fault+0x263/0x6f9
> > >    page_fault+0x34/0x40
> > >
> > >   Reported by Kernel Concurrency Sanitizer on:
> > >   CPU: 69 PID: 94817 Comm: systemd-udevd Tainted: G        W  O L 5.5.0-next-20200204+ #6
> > >   Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
> > >
> > > Both the read and write are done only with the non-exclusive mmap_sem
> > > held. Since the read only check for a specific bit in the flag, even if
> >
> >
> > Perhaps a clearer explanation is that the read of the page flags is always
> > looking at a bit range (zone number: up to 3 bits) that is not being written to by
> > the writer.
> >
> >
> > > load tearing happens, it will be harmless, so just mark it as an
> > > intentional data races using the data_race() macro.
> > >
> > > Signed-off-by: Qian Cai <cai@xxxxxx>
> > > ---
> > >   include/linux/mm.h | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > index 52269e56c514..cafccad584c2 100644
> > > --- a/include/linux/mm.h
> > > +++ b/include/linux/mm.h
> > > @@ -920,7 +920,7 @@ vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf);
> > >
> > >   static inline enum zone_type page_zonenum(const struct page *page)
> > >   {
> > > -   return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
> > > +   return data_race((page->flags >> ZONES_PGSHIFT) & ZONES_MASK);
> >
> >
> > I don't know about this. Lots of the kernel is written to do this sort
> > of thing, and adding a load of "data_race()" everywhere is...well, I'm not
> > sure if it's really the best way.  I wonder: could we maybe teach this
> > kcsan thing to understand a few of the key idioms, particularly about page
> > flags, instead of annotating all over the place?
>
> My understanding is that it is rather difficult to change the compilers, but it
> is a good question and I Cc Marco who is the maintainer for KCSAN that might
> give you a definite answer.

The problem is that there is no general idiom where we could say with
confidence that a data race is safe across the whole kernel. Here it
might not matter, but somewhere else it might matter a lot.

If you think that it turns out the entire file may be littered with
'data_race()', and you do not want to use annotations, you can
blacklist the file. I already had to do this for other files in mm/,
because concurrent flag modification/checking is pervasive and a lot
of them seem 'benign'. We decided to revisit those files later.

Feel free to add 'KCSAN_SANITIZE_memory.o := n' or whatever other
files you think are full of these to mm/Makefile.

The only problem I see with that is that it's not obvious what is
concurrently modified and what isn't. The annotations would have
helped document what is happening.

Thanks,
-- Marco