linux-kernel - Re: [PATCH 3/3] mm, oom: protect !costly allocations some more

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Date:	Tue, 8 Mar 2016 16:03:37 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Joonsoo Kim <js1304@...il.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm, oom: protect !costly allocations some more

On 03/08/2016 03:48 PM, Michal Hocko wrote:
> On Tue 08-03-16 15:34:37, Vlastimil Babka wrote:
>>> --- a/include/linux/compaction.h
>>> +++ b/include/linux/compaction.h
>>> @@ -14,6 +14,11 @@ enum compact_result {
>>>  	/* compaction should continue to another pageblock */
>>>  	COMPACT_CONTINUE,
>>>  	/*
>>> +	 * whoever is calling compaction should retry because it was either
>>> +	 * not active or it tells us there is more work to be done.
>>> +	 */
>>> +	COMPACT_SHOULD_RETRY = COMPACT_CONTINUE,
>>
>> Hmm, I'm not sure about this. AFAIK compact_zone() doesn't ever return
>> COMPACT_CONTINUE, and thus try_to_compact_pages() also doesn't. This
>> overloading of CONTINUE only applies to compaction_suitable(). But the
>> value that should_compact_retry() is testing comes only from
>> try_to_compact_pages(). So this is not wrong, but perhaps a bit misleading?
> 
> Well the idea was that I wanted to cover all the _possible_ cases where
> compaction might want to tell us "please try again even when the last
> round wasn't really successful". COMPACT_CONTINUE might not be returned
> right now but we can come up with that in the future. It sounds like a
> sensible feedback to me. But maybe there would be a better name for such
> a feedback. I confess this is a bit oom-rework centric name...

Hmm, I see. But it doesn't really tell use to please try again. That
interpretation is indeed oom-specific. What it's actually telling us is
either a) reclaim and then try again (COMPACT_SKIPPED), b) try again
just to overcome the deferred state (COMPACT_DEFERRED). COMPACT_CONTINUE
says "go ahead", but only from compaction_suitable().
So the attempt a generic name doesn't really work here I'm afraid :/

> Also I find it better to hide details behind a more generic name.
> 
> I am open to suggestions here, of course.
>