Re: [Question]page allocation failure: order:2, mode:0x2000d1

[Xishi Qiu]

On 2016/7/20 15:47, Michal Hocko wrote:

> On Wed 20-07-16 09:33:30, Yisheng Xie wrote:
>>
>>
>> On 2016/7/19 22:14, Vlastimil Babka wrote:
>>> On 07/19/2016 03:48 PM, Xishi Qiu wrote:
> [...]
>>>> mode:0x2000d1 means it expects to alloc from zone_dma, (on arm64 zone_dma is 0-4G)
>>>
>>> Yes, but I don't see where the __GFP_DMA comes from. The backtrace
>>> suggests it's alloc_thread_info_node() which uses THREADINFO_GFP
>>> which is GFP_KERNEL | __GFP_NOTRACK. There shouldn't be __GFP_DMA,
>>> even on arm64. Are there some local modifications to the kernel
>>> source?
>>>
>>>> The page cache is very small(active_file:292kB inactive_file:240kB),
>>>> so did_some_progress may be zero, and will not retry, right?
>>>
>>> Could be, and then __alloc_pages_may_oom() has this:
>>>
>>>         /* The OOM killer does not needlessly kill tasks for lowmem */
>>>         if (ac->high_zoneidx < ZONE_NORMAL)
>>>                 goto out;
>>>
>>> So no oom and no faking progress for non-costly order that would
>>> result in retry, because of that mysterious __GFP_DMA...
>>
>> hi Vlastimil,
>> We do make change and add __GFP_DMA flag here for our platform driver's problem.
> 
> Why would you want to force thread_info to the DMA zone?
> 

Hi Michal,

Because of our platform driver's problem, so we change the code(add GFP_DMA) to let
it alloc from zone_dma. (on arm64 zone_dma is 0-4G)

Thanks,
Xishi Qiu

>> Another question is why it will do retry here, for it will goto out
>> with did_some_progress=0 ?
>>
>>              if (!did_some_progress)
>>                  goto nopage;
> 
> Do you mean:
>                 /*
>                  * If we fail to make progress by freeing individual
>                  * pages, but the allocation wants us to keep going,
>                  * start OOM killing tasks.
>                  */
>                 if (!did_some_progress) {
>                         page = __alloc_pages_may_oom(gfp_mask, order, ac,
>                                                         &did_some_progress);
>                         if (page)
>                                 goto got_pg;
>                         if (!did_some_progress)
>                                 goto nopage;
>                 }
> 
> If yes then this code simply tells that if even oom path didn't make any
> progress then we should fail. As DMA request doesn't invoke OOM killer
> because it is effectively a lowmem request (see above check pointed
> by Vlastimil) then the OOM path couldn't make any progress and we are
> failing. If invoked the OOM killer then we would consider this as a
> forward progress and retry the allocation request.