Subject: Re: [PATCH] mm, oom: protect !costly allocations some more
To: Michal Hocko <mhocko@kernel.org>
References: <20160203132718.GI6757@dhcp22.suse.cz>
 <alpine.LSU.2.11.1602241832160.15564@eggly.anvils>
 <20160225092315.GD17573@dhcp22.suse.cz>
 <20160229210213.GX16930@dhcp22.suse.cz>
 <20160307160838.GB5028@dhcp22.suse.cz> <56DE9A68.2010301@suse.cz>
 <20160308094612.GB13542@dhcp22.suse.cz> <56DEA0CF.2070902@suse.cz>
 <20160308101016.GC13542@dhcp22.suse.cz> <56DEB394.40602@suse.cz>
 <20160308122241.GD13542@dhcp22.suse.cz>
Cc: Hugh Dickins <hughd@google.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>, Mel Gorman <mgorman@suse.de>,
        David Rientjes <rientjes@google.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Hillf Danton <hillf.zj@alibaba-inc.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, linux-mm@kvack.org,
        LKML <linux-kernel@vger.kernel.org>, Joonsoo Kim <js1304@gmail.com>
From: Vlastimil Babka <vbabka@suse.cz>
Message-ID: <56DEC5BE.6040209@suse.cz>
Date: Tue, 8 Mar 2016 13:29:50 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.6.0
MIME-Version: 1.0
In-Reply-To: <20160308122241.GD13542@dhcp22.suse.cz>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1186
Lines: 26

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) :)