linux-kernel - Re: [PATCH v2 5/8] mm/isolation: change pageblock isolation logic to fix freepage counting bugs

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Date:	Thu, 7 Aug 2014 21:26:15 +0900
From:	Joonsoo Kim <js1304@...il.com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan@...nel.org>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Tang Chen <tangchen@...fujitsu.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
	Wen Congyang <wency@...fujitsu.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Heesub Shin <heesub.shin@...sung.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Ritesh Harjani <ritesh.list@...il.com>,
	t.stanislaws@...sung.com, Gioh Kim <gioh.kim@....com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/8] mm/isolation: change pageblock isolation logic to
 fix freepage counting bugs

2014-08-07 17:53 GMT+09:00 Vlastimil Babka <vbabka@...e.cz>:
> On 08/07/2014 10:19 AM, Joonsoo Kim wrote:
>>>
>>> Is it needed to disable the pcp list? Shouldn't drain be enough?
>>> After the drain you already are sure that future freeing will see
>>> MIGRATE_ISOLATE and skip pcp list anyway, so why disable it
>>> completely?
>>
>>
>> Yes, it is needed. Until we move freepages from normal buddy list
>> to isolate buddy list, freepages could be allocated by others. In this
>> case, they could be moved to pcp list. When it is flushed from pcp list
>> to buddy list, we need to check whether it is on isolate migratetype
>> pageblock or not. But, we don't want that hook in free_pcppages_bulk()
>> because it is page allocator's normal freepath. To remove it, we shoule
>> disable the pcp list here.
>
>
> Ah, right. I thought that everything going to pcp lists would be through
> freeing which would already observe the isolate migratetype and skip
> pcplist. I forgot about the direct filling of pcplists from buddy list.
> You're right that we don't want extra hooks there.
>
> Still, couldn't this be solved in a simpler way via another pcplist drain
> after the pages are moved from normal to isolate buddy list? Should be even
> faster because instead of disable - drain - enable (5 all-cpu kicks, since
> each pageset_update does 2 kicks) you have drain - drain (2 kicks). While
> it's true that pageset_update is single-zone operation, I guess we would
> easily benefit from having a single-zone drain operation as well.

I hope so, but, it's not possible. Consider following situation.

Page A: on pcplist of CPU2 and it is on isolate pageblock.

CPU 1                   CPU 2
drain pcplist
wait IPI finished     move A to normal buddy list
finish IPI
                            A is moved to pcplist by allocation request

move doesn't catch A,
because it is on pcplist.

drain pcplist
wait IPI finished     move A to normal buddy list
finish IPI
                            A is moved to pcplist by allocation request

repeat!!

It could happen infinitely, though, low possibility.

Thanks.
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