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

[Hanjun Guo]

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@xxxxxxxxxx>

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@xxxxxxxxxx>

Thanks for debugging!
Hanjun