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Date:   Wed, 29 Jan 2020 09:51:24 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, elver@google.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH -next] mm/page_counter: mark intentional data races
Message-ID: <20200129085124.GF24244@dhcp22.suse.cz>
References: <20200129042019.3632-1-cai@lca.pw>
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On Tue 28-01-20 23:20:19, Qian Cai wrote:
> The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
> had memcg->memsw->failcnt and ->watermark could be 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
>   __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
>   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 the failcnt and watermark are tolerant of some inaccuracy, a data
> race will not be harmful, thus mark them as intentional data races with
> the data_race() macro.

I am not familiar with KCSAN and git grep for data_race on the current
linux-next doesn't really show any users of this macro. Is there a
general consensus that data_race is going to be used to silence all
KCSAN false positives?

> Signed-off-by: Qian Cai <cai@lca.pw>
> ---
>  mm/page_counter.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index de31470655f6..13934636eafd 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 (data_race(new > c->watermark))
> +			data_race(c->watermark = new);
>  	}
>  }
>  
> @@ -126,7 +126,7 @@ bool page_counter_try_charge(struct page_counter *counter,
>  			 * This is racy, but we can live with some
>  			 * inaccuracy in the failcnt.
>  			 */
> -			c->failcnt++;
> +			data_race(c->failcnt++);
>  			*fail = c;
>  			goto failed;
>  		}
> @@ -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 (data_race(new > c->watermark))
> +			data_race(c->watermark = new);
>  	}
>  	return true;
>  
> -- 
> 2.21.0 (Apple Git-122.2)

-- 
Michal Hocko
SUSE Labs