Re: [PATCH] mm, oom: protect !costly allocations some more

[Vlastimil Babka]

On 03/08/2016 01:22 PM, Michal Hocko wrote:
>> Thanks.
>>
>>> A more important question is whether the criteria I have chosen are
>>> reasonable and reasonably independent on the particular implementation
>>> of the compaction. I still cannot convince myself about the convergence
>>> here. Is it possible that the compaction would keep returning 
>>> compact_result <= COMPACT_CONTINUE while not making any progress at all?
>>
>> Theoretically, if reclaim/compaction suitability decisions and
>> allocation attempts didn't match the watermark checks, including the
>> alloc_flags and classzone_idx parameters. Possible scenarios:
>>
>> - reclaim thinks compaction has enough to proceed, but compaction thinks
>> otherwise and returns COMPACT_SKIPPED
>> - compaction thinks it succeeded and returns COMPACT_PARTIAL, but
>> allocation attempt fails
>> - and perhaps some other combinations
> 
> But that might happen right now as well so it wouldn't be a regression,
> right?

Maybe, somehow, I didn't study closely how the retry decisions work.
Your patch adds another way to retry so it's theoretically more
dangerous. Just hinting at what to possibly check (the watermark checks) :)