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Date:   Fri, 14 Feb 2020 09:30:26 -0500
From:   Qian Cai <cai@....pw>
To:     paulmck@...nel.org
Cc:     akpm@...ux-foundation.org, elver@...gle.com, david@...hat.com,
        jack@...e.cz, jhubbard@...dia.com, ira.weiny@...el.com,
        dan.j.williams@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH -next v2] mm: annotate a data race in page_zonenum()

On Thu, 2020-02-13 at 11:26 -0800, Paul E. McKenney wrote:
> On Thu, Feb 13, 2020 at 01:38:09PM -0500, Qian Cai wrote:
> >  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
> > 
> > A page never changes its zone number. The zone number happens to be
> > stored in the same word as other bits which are modified, but the zone
> > number bits will never be modified by any other write, so it can accept
> > a reload of the zone bits after an intervening write and it don't need
> > to use READ_ONCE(). Thus, annotate this data race using
> > ASSERT_EXCLUSIVE_BITS() to also assert that there are no concurrent
> > writes to it.
> > 
> > Suggested-by: Marco Elver <elver@...gle.com>
> > Signed-off-by: Qian Cai <cai@....pw>
> > ---
> > 
> > v2: use ASSERT_EXCLUSIVE_BITS().
> > 
> > BTW, not sure if it is easier for Andrew with Paul to pick this up (with
> > Andrew's ACK), since ASSERT_EXCLUSIVE_BITS() is in -rcu tree only (or likely
> > tomorrow's -next tree).
> 
> Here are the options I know of, any of which work for me:
> 
> 1.	I take the patch given appropriate acks/reviews.
> 
> 2.	Someone hangs onto the patch until the KCSAN infrastructure
> 	hits mainline and then sends it up via whatever path.
> 
> 3.	One way to do #2 is to merge the -rcu tree's "kcsan" branch and
> 	then queue this patch on top of that, again sending this patch
> 	along once KCSAN hits mainline.  Unusually for the -rcu tree,
> 	the "kcsan" branch is not supposed to be rebased.
> 
> Either way, just let me know!

I suppose it is up to Andrew what he prefers. I may have another pile of patches
depends on some of those KCSAN things (except the data_race() macro) are coming,
so I'd like to find a way to get into -next first rather than waiting to send
them in April (once KCSAN hit the mainline) if possible.

> 
> 							Thanx, Paul
> 
> >  include/linux/mm.h | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 52269e56c514..0d70fafd055c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -920,6 +920,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
> >  
> >  static inline enum zone_type page_zonenum(const struct page *page)
> >  {
> > +	ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);
> >  	return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
> >  }
> >  
> > -- 
> > 1.8.3.1
> > 

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