linux-kernel - Re: [PATCH v2] vmscan: fix increasing nr_isolated incurred by putback unevictable pages

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Date:	Wed, 05 Aug 2015 09:52:07 +0900
From:	Jaewon Kim <jaewon31.kim@...sung.com>
To:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mgorman@...e.de, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	jaewon31.kim@...il.com
Subject: Re: [PATCH v2] vmscan: fix increasing nr_isolated incurred by putback
 unevictable pages



On 2015년 08월 05일 08:31, Minchan Kim wrote:
> Hello,
> 
> On Tue, Aug 04, 2015 at 03:09:37PM -0700, Andrew Morton wrote:
>> On Tue, 04 Aug 2015 19:40:08 +0900 Jaewon Kim <jaewon31.kim@...sung.com> wrote:
>>
>>> reclaim_clean_pages_from_list() assumes that shrink_page_list() returns
>>> number of pages removed from the candidate list. But shrink_page_list()
>>> puts back mlocked pages without passing it to caller and without
>>> counting as nr_reclaimed. This incurrs increasing nr_isolated.
>>> To fix this, this patch changes shrink_page_list() to pass unevictable
>>> pages back to caller. Caller will take care those pages.
>>>
>>> ..
>>>
>>> --- a/mm/vmscan.c
>>> +++ b/mm/vmscan.c
>>> @@ -1157,7 +1157,7 @@ cull_mlocked:
>>>  		if (PageSwapCache(page))
>>>  			try_to_free_swap(page);
>>>  		unlock_page(page);
>>> -		putback_lru_page(page);
>>> +		list_add(&page->lru, &ret_pages);
>>>  		continue;
>>>  
>>>  activate_locked:
>>
>> Is this going to cause a whole bunch of mlocked pages to be migrated
>> whereas in current kernels they stay where they are?
>>
> 
> It fixes two issues.
> 
> 1. With unevictable page, cma_alloc will be successful.
> 
> Exactly speaking, cma_alloc of current kernel will fail due to unevictable pages.
> 
> 2. fix leaking of NR_ISOLATED counter of vmstat
> 
> With it, too_many_isolated works. Otherwise, it could make hang until
> the process get SIGKILL.
> 
> So, I think it's stable material.
> 
> Acked-by: Minchan Kim <minchan@...nel.org>
> 
> 
> 
Hello

Traditional shrink_inactive_list will put back the unevictable pages as it does through putback_inactive_pages.
However as Minchan Kim said, cma_alloc will be more successful by migrating unevictable pages.
In current kernel, I think, cma_alloc is already trying to migrate unevictable pages except clean page cache.
This patch will allow clean page cache also to be migrated in cma_alloc.

Thank you
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