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Date:   Thu, 13 Feb 2020 11:26:59 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, elver@google.com, david@redhat.com,
        jack@suse.cz, jhubbard@nvidia.com, ira.weiny@intel.com,
        dan.j.williams@intel.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next v2] mm: annotate a data race in page_zonenum()
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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@google.com>
> Signed-off-by: Qian Cai <cai@lca.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!

							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
>