linux-kernel - Re: [Question] Mlocked count will not be decreased

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Message-ID: <5925784E.802@huawei.com>
Date:   Wed, 24 May 2017 20:10:54 +0800
From:   Xishi Qiu <qiuxishi@...wei.com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Yisheng Xie <xieyisheng1@...wei.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, zhongjiang <zhongjiang@...wei.com>
Subject: Re: [Question] Mlocked count will not be decreased

On 2017/5/24 19:52, Vlastimil Babka wrote:

> On 05/24/2017 01:38 PM, Xishi Qiu wrote:
>>>
>>> Race condition with what? Who else would isolate our pages?
>>>
>>
>> Hi Vlastimil,
>>
>> I find the root cause, if the page was not cached on the current cpu,
>> lru_add_drain() will not push it to LRU. So we should handle fail
>> case in mlock_vma_page().
> 
> Yeah that would explain it.
> 
>> follow_page_pte()
>> 		...
>> 		if (page->mapping && trylock_page(page)) {
>> 			lru_add_drain();  /* push cached pages to LRU */
>> 			/*
>> 			 * Because we lock page here, and migration is
>> 			 * blocked by the pte's page reference, and we
>> 			 * know the page is still mapped, we don't even
>> 			 * need to check for file-cache page truncation.
>> 			 */
>> 			mlock_vma_page(page);
>> 			unlock_page(page);
>> 		}
>> 		...
>>
>> I think we should add yisheng's patch, also we should add the following change.
>> I think it is better than use lru_add_drain_all().
> 
> I agree about yisheng's fix (but v2 didn't address my comments). I don't
> think we should add the hunk below, as that deviates from the rest of
> the design.

Hi Vlastimil,

The rest of the design is that mlock should always success here, right?

If we don't handle the fail case, the page will be in anon/file lru list
later when call __pagevec_lru_add(), but NR_MLOCK increased,
this is wrong, right?

Thanks,
Xishi Qiu

> 
> Thanks,
> Vlastimil
> 
>> diff --git a/mm/mlock.c b/mm/mlock.c
>> index 3d3ee6c..ca2aeb9 100644
>> --- a/mm/mlock.c
>> +++ b/mm/mlock.c
>> @@ -88,6 +88,11 @@ void mlock_vma_page(struct page *page)
>>  		count_vm_event(UNEVICTABLE_PGMLOCKED);
>>  		if (!isolate_lru_page(page))
>>  			putback_lru_page(page);
>> +		else {
>> +			ClearPageMlocked(page);
>> +			mod_zone_page_state(page_zone(page), NR_MLOCK,
>> +					-hpage_nr_pages(page));
>> +		}
>>  	}
>>  }
>>
>> Thanks,
>> Xishi Qiu
>>
> 
> 
> .
>