linux-kernel - Re: [RFC 0/4] mm, oom: get rid of TIF

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Message-ID: <20160915144118.GB25519@cmpxchg.org>
Date:   Thu, 15 Sep 2016 10:41:18 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Michal Hocko <mhocko@...e.com>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC 0/4] mm, oom: get rid of TIF_MEMDIE

Hi Michal,

On Thu, Sep 01, 2016 at 11:51:00AM +0200, Michal Hocko wrote:
> Hi,
> this is an early RFC to see whether the approach I've taken is acceptable.
> The series is on top of the current mmotm tree (2016-08-31-16-06). I didn't
> get to test it so it might be completely broken.
> 
> The primary point of this series is to get rid of TIF_MEMDIE finally.
> Recent changes in the oom proper allows for that finally, I believe. Now
> that all the oom victims are reapable we are no longer depending on
> ALLOC_NO_WATERMARKS because the memory held by the victim is reclaimed
> asynchronously. A partial access to memory reserves should be sufficient
> just to guarantee that the oom victim is not starved due to other
> memory consumers. This also means that we do not have to pretend to be
> conservative and give access to memory reserves only to one thread from
> the process at the time. This is patch 1.
>
> Patch 2 is a simple cleanup which turns TIF_MEMDIE users to tsk_is_oom_victim
> which is process rather than thread centric. None of those callers really
> requires to be thread aware AFAICS.
> 
> The tricky part then is exit_oom_victim vs. oom_killer_disable because
> TIF_MEMDIE acted as a token there so we had a way to count threads from
> the process. It didn't work 100% reliably and had it own issues but we
> have to replace it with something which doesn't rely on counting threads
> but rather find a moment when all threads have reached steady state in
> do_exit. This is what patch 3 does and I would really appreciate if Oleg
> could double check my thinking there. I am also CCing Al on that one
> because I am moving exit_io_context up in do_exit right before exit_notify.

You're explaining the mechanical thing you are doing, but I'm having
trouble understanding why you want to get rid of TIF_MEMDIE. For one,
it's more code. And apparently, it's also more complicated than what
we have right now.

Can you please explain in the cover letter what's broken/undesirable?

Thanks