linux-kernel - Re: [patch v3] mm, oom: fix unnecessary killing of additional processes

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Message-ID: <alpine.DEB.2.21.1807091323570.101462@chino.kir.corp.google.com>
Date:   Mon, 9 Jul 2018 13:30:10 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        kbuild test robot <fengguang.wu@...el.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch v3] mm, oom: fix unnecessary killing of additional
 processes

On Mon, 9 Jul 2018, Michal Hocko wrote:

> > Blockable mmu notifiers and mlocked memory is not the extent of the 
> > problem, if a process has a lot of virtual memory we must wait until 
> > free_pgtables() completes in exit_mmap() to prevent unnecessary oom 
> > killing.  For implementations such as tcmalloc, which does not release 
> > virtual memory, this is important because, well, it releases this only at 
> > exit_mmap().  Of course we cannot do that with only the protection of 
> > mm->mmap_sem for read.
> 
> And how exactly a timeout helps to prevent from "unnecessary killing" in
> that case?

As my patch does, it becomes mandatory to move MMF_OOM_SKIP to after 
free_pgtables() in exit_mmap() and then repurpose MMF_UNSTABLE to 
indicate that the oom reaper should not operate on a given mm.  In the 
event we cannot reach MMF_OOM_SKIP, we need to ensure forward progress and 
that is possible with a timeout period in the very rare instance where 
additional memory freeing is needed, and without unnecessary oom killing 
when it is not needed.