linux-kernel - Re: [PATCH] fs, mm: account filp and names caches to kmemcg

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Date:   Thu, 26 Oct 2017 21:45:56 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>, Greg Thelen <gthelen@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>, linux-fsdevel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs, mm: account filp and names caches to kmemcg

On 2017/10/26 16:49, Michal Hocko wrote:
> On Wed 25-10-17 15:49:21, Greg Thelen wrote:
>> Johannes Weiner <hannes@...xchg.org> wrote:
>>
>>> On Wed, Oct 25, 2017 at 09:00:57PM +0200, Michal Hocko wrote:
> [...]
>>>> So just to make it clear you would be OK with the retry on successful
>>>> OOM killer invocation and force charge on oom failure, right?
>>>
>>> Yeah, that sounds reasonable to me.
>>
>> Assuming we're talking about retrying within try_charge(), then there's
>> a detail to iron out...
>>
>> If there is a pending oom victim blocked on a lock held by try_charge() caller
>> (the "#2 Locks" case), then I think repeated calls to out_of_memory() will
>> return true until the victim either gets MMF_OOM_SKIP or disappears.
> 
> true. And oom_reaper guarantees that MMF_OOM_SKIP gets set in the finit
> amount of time.

Just a confirmation. You are talking about kmemcg, aren't you? And kmemcg
depends on CONFIG_MMU=y, doesn't it? If no, there is no such guarantee.

> 
>> So a force
>> charge fallback might be a needed even with oom killer successful invocations.
>> Or we'll need to teach out_of_memory() to return three values (e.g. NO_VICTIM,
>> NEW_VICTIM, PENDING_VICTIM) and try_charge() can loop on NEW_VICTIM.
> 
> No we, really want to wait for the oom victim to do its job. The only
> thing we should be worried about is when out_of_memory doesn't invoke
> the reaper. There is only one case like that AFAIK - GFP_NOFS request. I
> have to think about this case some more. We currently fail in that case
> the request.
> 

Do we really need to apply

	/*
	 * The OOM killer does not compensate for IO-less reclaim.
	 * pagefault_out_of_memory lost its gfp context so we have to
	 * make sure exclude 0 mask - all other users should have at least
	 * ___GFP_DIRECT_RECLAIM to get here.
	 */
	if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS))
		return true;

unconditionally?

We can encourage !__GFP_FS allocations to use __GFP_NORETRY or
__GFP_RETRY_MAYFAIL if their allocations are not important.
Then, only important !__GFP_FS allocations will be checked here.
I think that we can allow such important allocations to invoke the OOM
killer (i.e. remove this check) because situation is already hopeless
if important !__GFP_FS allocations cannot make progress.