Date: Mon, 10 Jul 2017 17:01:01 -0700 (PDT)
From: David Rientjes <rientjes@google.com>
To: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
cc: mhocko@kernel.org, linux-mm@kvack.org, oleg@redhat.com,
        andrea@kernel.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, mhocko@suse.com
Subject: Re: [RFC PATCH] mm, oom: allow oom reaper to race with exit_mmap
In-Reply-To: <201706271952.FEB21375.SFJFHOQLOtVOMF@I-love.SAKURA.ne.jp>
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References: <20170626130346.26314-1-mhocko@kernel.org> <201706271952.FEB21375.SFJFHOQLOtVOMF@I-love.SAKURA.ne.jp>
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On Tue, 27 Jun 2017, Tetsuo Handa wrote:

> I wonder why you prefer timeout based approach. Your patch will after all
> set MMF_OOM_SKIP if operations between down_write() and up_write() took
> more than one second. lock_anon_vma_root() from unlink_anon_vmas() from
> free_pgtables() for example calls down_write()/up_write(). unlink_file_vma()
>  from free_pgtables() for another example calls down_write()/up_write().
> This means that it might happen that exit_mmap() takes more than one second
> with mm->mmap_sem held for write, doesn't this?
> 

I certainly have no objection to increasing the timeout period or 
increasing MAX_OOM_REAP_RETRIES to be substantially higher.  All threads 
holding mm->mmap_sem should be oom killed and be able to access memory 
reserves to make forward progress if they fail to reclaim.  If we are 
truly blocked on mm->mmap_sem, waiting longer than one second to declare 
that seems justifiable to prevent the exact situation you describe.