To: mhocko@kernel.org
Cc: linux-mm@kvack.org, rientjes@google.com, oleg@redhat.com,
        andrea@kernel.org, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH] mm, oom: allow oom reaper to race with exit_mmap
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
References: <20170626130346.26314-1-mhocko@kernel.org>
        <201706271952.FEB21375.SFJFHOQLOtVOMF@I-love.SAKURA.ne.jp>
        <20170627112650.GK28072@dhcp22.suse.cz>
        <201706272039.HGG51520.QOMHFVOFtOSJFL@I-love.SAKURA.ne.jp>
        <20170627120317.GL28072@dhcp22.suse.cz>
In-Reply-To: <20170627120317.GL28072@dhcp22.suse.cz>
Message-Id: <201706272231.ABH00025.FMOFOJSVLOQHFt@I-love.SAKURA.ne.jp>
Date: Tue, 27 Jun 2017 22:31:58 +0900
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Michal Hocko wrote:
> On Tue 27-06-17 20:39:28, Tetsuo Handa wrote:
> > Michal Hocko 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.
> > > 
> > > if we reach down_write then we have unmapped the address space in
> > > exit_mmap and oom reaper cannot do much more.
> > 
> > So, by the time down_write() is called, majority of memory is already released, isn't it?
> 
> In most cases yes. To be put it in other words. By the time exit_mmap
> takes down_write there is nothing more oom reaper could reclaim.
> 
Then, aren't there two exceptions which your patch cannot guarantee;
down_write(&mm->mmap_sem) in __ksm_exit() and __khugepaged_exit() ?

Since for some reason exit_mmap() cannot be brought to before
ksm_exit(mm)/khugepaged_exit(mm) calls,

	ksm_exit(mm);
	khugepaged_exit(mm); /* must run before exit_mmap */
	exit_mmap(mm);

shouldn't we try __oom_reap_task_mm() before calling these down_write()
if mm is OOM victim's?