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Subject: Re: [PATCH] mm/page_counter: fix various data races
To:     Qian Cai <cai@lca.pw>, akpm@linux-foundation.org
Cc:     hannes@cmpxchg.org, elver@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20200129105224.4016-1-cai@lca.pw>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <f5105746-2642-472a-e097-c9a427be5556@redhat.com>
Date:   Wed, 29 Jan 2020 11:57:39 +0100
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On 29.01.20 11:52, Qian Cai wrote:
> The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters") could
> had memcg->memsw->watermark been accessed concurrently as reported by
> KCSAN,
> 
>  Reported by Kernel Concurrency Sanitizer on:
>  BUG: KCSAN: data-race in page_counter_try_charge / page_counter_try_charge
> 
>  read to 0xffff8fb18c4cd190 of 8 bytes by task 1081 on cpu 59:
>   page_counter_try_charge+0x4d/0x150 mm/page_counter.c:138
>   try_charge+0x131/0xd50 mm/memcontrol.c:2405
>   __memcg_kmem_charge_memcg+0x58/0x140
>   __memcg_kmem_charge+0xcc/0x280
>   __alloc_pages_nodemask+0x1e1/0x450
>   alloc_pages_current+0xa6/0x120
>   pte_alloc_one+0x17/0xd0
>   __pte_alloc+0x3a/0x1f0
>   copy_p4d_range+0xc36/0x1990
>   copy_page_range+0x21d/0x360
>   dup_mmap+0x5f5/0x7a0
>   dup_mm+0xa2/0x240
>   copy_process+0x1b3f/0x3460
>   _do_fork+0xaa/0xa20
>   __x64_sys_clone+0x13b/0x170
>   do_syscall_64+0x91/0xb47
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
>  write to 0xffff8fb18c4cd190 of 8 bytes by task 1153 on cpu 120:
>   page_counter_try_charge+0x5b/0x150 mm/page_counter.c:139
>   try_charge+0x131/0xd50 mm/memcontrol.c:2405
>   mem_cgroup_try_charge+0x159/0x460
>   mem_cgroup_try_charge_delay+0x3d/0xa0
>   wp_page_copy+0x14d/0x930
>   do_wp_page+0x107/0x7b0
>   __handle_mm_fault+0xce6/0xd40
>   handle_mm_fault+0xfc/0x2f0
>   do_page_fault+0x263/0x6f9
>   page_fault+0x34/0x40
> 
> Since watermark could be compared or set to garbage due to load or
> store tearing which would change the code logic, fix it by adding a pair
> of READ_ONCE() and WRITE_ONCE() in those places.
> 
> Fixes: 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  mm/page_counter.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index de31470655f6..a17841150906 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -82,8 +82,8 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages)
>  		 * This is indeed racy, but we can live with some
>  		 * inaccuracy in the watermark.
>  		 */
> -		if (new > c->watermark)
> -			c->watermark = new;
> +		if (new > READ_ONCE(c->watermark))
> +			WRITE_ONCE(c->watermark, new);
>  	}
>  }
>  
> @@ -135,8 +135,8 @@ bool page_counter_try_charge(struct page_counter *counter,
>  		 * Just like with failcnt, we can live with some
>  		 * inaccuracy in the watermark.
>  		 */
> -		if (new > c->watermark)
> -			c->watermark = new;
> +		if (new > READ_ONCE(c->watermark))
> +			WRITE_ONCE(c->watermark, new);

So, if this is racy, isn't it a problem that that "new" could suddenly
be < c->watermark (concurrent writer). So you would use the "higher"
watermark.


-- 
Thanks,

David / dhildenb