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Subject: Re: [PATCH -next] mm: mark a intentional data race in page_zonenum()
To:     Qian Cai <cai@lca.pw>, <akpm@linux-foundation.org>
CC:     <ira.weiny@intel.com>, <dan.j.williams@intel.com>, <jack@suse.cz>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20200206035235.2537-1-cai@lca.pw>
From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <480a7dde-f678-c07b-2231-4da8e0a38753@nvidia.com>
Date:   Wed, 5 Feb 2020 20:50:34 -0800
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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?


thanks,
-- 
John Hubbard
NVIDIA


>   }
>   
>   #ifdef CONFIG_ZONE_DEVICE
>