linux-kernel - Re: Suspicious error for CMA stress test

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Message-ID: <56DE2DE8.6070509@huawei.com>
Date:	Tue, 8 Mar 2016 09:42:00 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Hanjun Guo <guohanjun@...wei.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>
CC:	Laura Abbott <labbott@...hat.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Laura Abbott <lauraa@...eaurora.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	"thunder.leizhen@...wei.com" <thunder.leizhen@...wei.com>,
	dingtinahong <dingtianhong@...wei.com>, <chenjie6@...wei.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: Suspicious error for CMA stress test

On 2016/3/4 13:33, Hanjun Guo wrote:

> Hi Joonsoo,
> 
> On 2016/3/4 10:02, Joonsoo Kim wrote:
>> On Thu, Mar 03, 2016 at 08:49:01PM +0800, Hanjun Guo wrote:
>>> On 2016/3/3 15:42, Joonsoo Kim wrote:
>>>> 2016-03-03 10:25 GMT+09:00 Laura Abbott <labbott@...hat.com>:
>>>>> (cc -mm and Joonsoo Kim)
>>>>>
>>>>>
>>>>> On 03/02/2016 05:52 AM, Hanjun Guo wrote:
>>>>>> Hi,
>>>>>>
>>>>>> I came across a suspicious error for CMA stress test:
>>>>>>
>>>>>> 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
>>> [...]
>>>>> I played with this a bit and can see the same problem. The sanity
>>>>> check of CmaFree < CmaTotal generally triggers in
>>>>> __move_zone_freepage_state in unset_migratetype_isolate.
>>>>> This also seems to be present as far back as v4.0 which was the
>>>>> first version to have the updated accounting from Joonsoo.
>>>>> Were there known limitations with the new freepage accounting,
>>>>> Joonsoo?
>>>> I don't know. I also played with this and looks like there is
>>>> accounting problem, however, for my case, number of free page is slightly less
>>>> than total. I will take a look.
>>>>
>>>> Hanjun, could you tell me your malloc_size? I tested with 1 and it doesn't
>>>> look like your case.
>>> I tested with malloc_size with 2M, and it grows much bigger than 1M, also I
>>> did some other test:
>> Thanks! Now, I can re-generate erronous situation you mentioned.
>>
>>>  - run with single thread with 100000 times, everything is fine.
>>>
>>>  - I hack the cam_alloc() and free as below [1] to see if it's lock issue, with
>>>    the same test with 100 multi-thread, then I got:
>> [1] would not be sufficient to close this race.
>>
>> Try following things [A]. And, for more accurate test, I changed code a bit more
>> to prevent kernel page allocation from cma area [B]. This will prevent kernel
>> page allocation from cma area completely so we can focus cma_alloc/release race.
>>
>> Although, this is not correct fix, it could help that we can guess
>> where the problem is.
>>
>> Thanks.
>>
>> [A]
> 
> I tested this solution [A], it can fix the problem, as you are posting a new patch, I will
> test that one and leave [B] alone :)
> 

Hi Joonsoo,

How does this problem happen? Why the count is larger than total?

Patch A prevent the cma page free to pcp, right?

...
-               if (unlikely(is_migrate_isolate(migratetype))) {
+               if (is_migrate_cma(migratetype) ||
+                       unlikely(is_migrate_isolate(migratetype))) {
...

Thanks,
Xishi Qiu

> Thanks
> Hanjun
> 
> 
> 
> .
>