Re: [RFC] can we use vmalloc to alloc thread stack if compaction failed

[Xishi Qiu]

On 2016/7/28 17:43, Michal Hocko wrote:

> On Thu 28-07-16 16:45:06, Xishi Qiu wrote:
>> On 2016/7/28 15:58, Michal Hocko wrote:
>>
>>> On Thu 28-07-16 15:41:53, Xishi Qiu wrote:
>>>> On 2016/7/28 15:20, Michal Hocko wrote:
>>>>
>>>>> On Thu 28-07-16 15:08:26, Xishi Qiu wrote:
>>>>>> Usually THREAD_SIZE_ORDER is 2, it means we need to alloc 16kb continuous
>>>>>> physical memory during fork a new process.
>>>>>>
>>>>>> If the system's memory is very small, especially the smart phone, maybe there
>>>>>> is only 1G memory. So the free memory is very small and compaction is not
>>>>>> always success in slowpath(__alloc_pages_slowpath), then alloc thread stack
>>>>>> may be failed for memory fragment.
>>>>>
>>>>> Well, with the current implementation of the page allocator those
>>>>> requests will not fail in most cases. The oom killer would be invoked in
>>>>> order to free up some memory.
>>>>>
>>>>
>>>> Hi Michal,
>>>>
>>>> Yes, it success in most cases, but I did have seen this problem in some
>>>> stress-test.
>>>>
>>>> DMA free:470628kB, but alloc 2 order block failed during fork a new process.
>>>> There are so many memory fragments and the large block may be soon taken by
>>>> others after compact because of stress-test.
>>>>
>>>> --- dmesg messages ---
>>>> 07-13 08:41:51.341 <4>[309805.658142s][pid:1361,cpu5,sManagerService]sManagerService: page allocation failure: order:2, mode:0x2000d1
>>>
>>> Yes but this is __GFP_DMA allocation. I guess you have already reported
>>> this failure and you've been told that this is quite unexpected for the
>>> kernel stack allocation. It is your out-of-tree patch which just makes
>>> things worse because DMA restricted allocations are considered "lowmem"
>>> and so they do not invoke OOM killer and do not retry like regular
>>> GFP_KERNEL allocations.
>>
>> Hi Michal,
>>
>> Yes, we add GFP_DMA, but I don't think this is the key for the problem.
> 
> You are restricting the allocation request to a single zone which is
> definitely not good. Look at how many larger order pages are available
> in the Normal zone.
> 
>> If we do oom-killer, maybe we will get a large block later, but there
>> is enough free memory before oom(although most of them are fragments).
> 
> Killing a task is of course the last resort action. It would give you
> larger order blocks used for the victims thread.
> 
>> I wonder if we can alloc success without kill any process in this situation.
> 
> Sure it would be preferable to compact that memory but that might be
> hard with your restriction in place. Consider that DMA zone would tend
> to be less movable than normal zones as users would have to pin it for
> DMA. Your DMA is really large so this might turn out to just happen to
> work but note that the primary problem here is that you put a zone
> restriction for your allocations.
> 
>> Maybe use vmalloc is a good way, but I don't know the influence.
> 
> You can have a look at vmalloc patches posted by Andy. They are not that
> trivial.
> 

Hi Michal,

Thank you for your comment, could you give me the link?

Thanks,
Xishi Qiu