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Subject: Re: [PATCH] mm/page_counter: fix various data races
From:   David Hildenbrand <david@redhat.com>
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>
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Message-ID: <b384e7bb-9a45-a595-daed-57aeeb00f8d5@redhat.com>
Date:   Wed, 29 Jan 2020 11:58:25 +0100
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On 29.01.20 11:57, David Hildenbrand wrote:
> 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.

s/use/lose/ :)


-- 
Thanks,

David / dhildenb