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

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Date:	Thu, 6 Aug 2015 14:21:38 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Jaewon Kim <jaewon31.kim@...sung.com>,
	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 08/05/2015 02:52 AM, Jaewon Kim wrote:
>
>
> 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?

The only user that will see the change wrt migration is 
__alloc_contig_migrate_range() which is explicit about isolating mlocked 
page for migration (isolate_migratepages_range() calls 
isolate_migratepages_block() with ISOLATE_UNEVICTABLE). So this will 
make the migration work for clean page cache too.

>>
>>
>> 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.

This should be more explicit in the changelog. The first issue is not 
mentioned at all. The second is not clear from the description.

>>
>> So, I think it's stable material.
>>
>> Acked-by: Minchan Kim <minchan@...nel.org>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

>>
>>
> 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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