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

[Qian Cai <cai@lca.pw>]

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@lca.pw>
> > ---
> >   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.