linux-kernel - Re: [PATCH] mm/page_alloc: prevent merging between isolated and other pageblocks

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Message-ID: <56F2803E.70100@huawei.com>
Date:	Wed, 23 Mar 2016 19:38:38 +0800
From:	Hanjun Guo <guohanjun@...wei.com>
To:	Vlastimil Babka <vbabka@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	Leizhen <thunder.leizhen@...wei.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	qiuxishi <qiuxishi@...wei.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	dingtinahong <dingtianhong@...wei.com>, <chenjie6@...wei.com>,
	Lucas Stach <l.stach@...gutronix.de>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Laura Abbott <labbott@...hat.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	"Michal Nazarewicz" <mina86@...a86.com>,
	Minchan Kim <minchan@...nel.org>,
	"Naoya Horiguchi" <n-horiguchi@...jp.nec.com>,
	<stable@...r.kernel.org>,
	"Yasuaki Ishimatsu" <isimatu.yasuaki@...fujitsu.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>
Subject: Re: [PATCH] mm/page_alloc: prevent merging between isolated and other
 pageblocks

On 2016/3/23 17:40, Vlastimil Babka wrote:
> Hanjun Guo has reported that a CMA stress test causes broken accounting of
> CMA and free pages:
>
>> Before the test, I got:
>> -bash-4.3# cat /proc/meminfo | grep Cma
>> CmaTotal:         204800 kB
>> CmaFree:          195044 kB
>>
>>
>> After running the test:
>> -bash-4.3# cat /proc/meminfo | grep Cma
>> CmaTotal:         204800 kB
>> CmaFree:         6602584 kB
>>
>> So the freed CMA memory is more than total..
>>
>> Also the the MemFree is more than mem total:
>>
>> -bash-4.3# cat /proc/meminfo
>> MemTotal:       16342016 kB
>> MemFree:        22367268 kB
>> MemAvailable:   22370528 kB
> Laura Abbott has confirmed the issue and suspected the freepage accounting
> rewrite around 3.18/4.0 by Joonsoo Kim. Joonsoo had a theory that this is
> caused by unexpected merging between MIGRATE_ISOLATE and MIGRATE_CMA
> pageblocks:
>
>> CMA isolates MAX_ORDER aligned blocks, but, during the process,
>> partialy isolated block exists. If MAX_ORDER is 11 and
>> pageblock_order is 9, two pageblocks make up MAX_ORDER
>> aligned block and I can think following scenario because pageblock
>> (un)isolation would be done one by one.
>>
>> (each character means one pageblock. 'C', 'I' means MIGRATE_CMA,
>> MIGRATE_ISOLATE, respectively.
>>
>> CC -> IC -> II (Isolation)
>> II -> CI -> CC (Un-isolation)
>>
>> If some pages are freed at this intermediate state such as IC or CI,
>> that page could be merged to the other page that is resident on
>> different type of pageblock and it will cause wrong freepage count.
> This was supposed to be prevented by CMA operating on MAX_ORDER blocks, but
> since it doesn't hold the zone->lock between pageblocks, a race window does
> exist.
>
> It's also likely that unexpected merging can occur between MIGRATE_ISOLATE
> and non-CMA pageblocks. This should be prevented in __free_one_page() since
> commit 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated
> pageblock"). However, we only check the migratetype of the pageblock where
> buddy merging has been initiated, not the migratetype of the buddy pageblock
> (or group of pageblocks) which can be MIGRATE_ISOLATE.
>
> Joonsoo has suggested checking for buddy migratetype as part of
> page_is_buddy(), but that would add extra checks in allocator hotpath and
> bloat-o-meter has shown significant code bloat (the function is inline).
>
> This patch reduces the bloat at some expense of more complicated code. The
> buddy-merging while-loop in __free_one_page() is initially bounded to
> pageblock_border and without any migratetype checks. The checks are placed
> outside, bumping the max_order if merging is allowed, and returning to the
> while-loop with a statement which can't be possibly considered harmful.
>
> This fixes the accounting bug and also removes the arguably weird state in the
> original commit 3c605096d315 where buddies could be left unmerged.
>
> Fixes: 3c605096d315 ("mm/page_alloc: restrict max order of merging on isolated pageblock")
> Link: https://lkml.org/lkml/2016/3/2/280
> Reported-by: Hanjun Guo <guohanjun@...wei.com>

With the same stress test case (alloc/free cma) running for more than
one hour, the bug I reported is gone.

Tested-by: Hanjun Guo <guohanjun@...wei.com>

Thanks for debugging!
Hanjun