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

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Date:   Wed, 24 May 2017 13:52:44 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Xishi Qiu <qiuxishi@...wei.com>
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 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.

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
>