Re: [PATCH 0.14] oom detection rework v6

[Vlastimil Babka]

On 05/04/2016 07:45 AM, Joonsoo Kim wrote:
> I still don't agree with some part of this patchset that deal with
> !costly order. As you know, there was two regression reports from Hugh
> and Aaron and you fixed them by ensuring to trigger compaction. I
> think that these show the problem of this patchset. Previous kernel
> doesn't need to ensure to trigger compaction and just works fine in
> any case.

IIRC previous kernel somehow subtly never OOM'd for !costly orders. So 
anything that introduces the possibility of OOM may look like regression 
for some corner case workloads. But I don't think that it's OK to not 
OOM for e.g. kernel stack allocations?

> Your series make compaction necessary for all. OOM handling
> is essential part in MM but compaction isn't. OOM handling should not
> depend on compaction. I tested my own benchmark without
> CONFIG_COMPACTION and found that premature OOM happens.
>
> I hope that you try to test something without CONFIG_COMPACTION.

Hmm a valid point, !CONFIG_COMPACTION should be considered. But reclaim 
cannot guarantee forming an order>0 page. But neither does OOM. So would 
you suggest we keep reclaiming without OOM as before, to prevent these 
regressions? Or where to draw the line here?

> Thanks.