linux-kernel - Re: [PATCH 2/5] oom reaper: handle mlocked pages

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Date:	Tue, 8 Mar 2016 14:40:32 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrea Argangeli <andrea@...nel.org>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/5] oom reaper: handle mlocked pages

On Mon 29-02-16 14:41:39, Michal Hocko wrote:
> On Sun 28-02-16 19:19:11, Hugh Dickins wrote:
> > On Tue, 23 Feb 2016, Michal Hocko wrote:
> > > On Mon 22-02-16 17:36:07, David Rientjes wrote:
> > > > 
> > > > Are we concerned about munlock_vma_pages_all() taking lock_page() and 
> > > > perhaps stalling forever, the same way it would stall in exit_mmap() for 
> > > > VM_LOCKED vmas, if another thread has locked the same page and is doing an 
> > > > allocation?
> > > 
> > > This is a good question. I have checked for that particular case
> > > previously and managed to convinced myself that this is OK(ish).
> > > munlock_vma_pages_range locks only THP pages to prevent from the
> > > parallel split-up AFAICS.
> > 
> > I think you're mistaken on that: there is also the lock_page()
> > on every page in Phase 2 of __munlock_pagevec().
> 
> Ohh, I have missed that one. Thanks for pointing it out!
> 
> [...]
> > > Just for the reference this is what I came up with (just compile tested).
> > 
> > I tried something similar internally (on an earlier kernel).  Like
> > you I've set that work aside for now, there were quicker ways to fix
> > the issue at hand.  But it does continue to offend me that munlock
> > demands all those page locks: so if you don't get back to it before me,
> > I shall eventually.
> > 
> > I didn't understand why you complicated yours with the "enforce"
> > arg to munlock_vma_pages_range(): why not just trylock in all cases?
> 
> Well, I have to confess that I am not really sure I understand all the
> consequences of the locking here. It has always been subtle and weird
> issues popping up from time to time. So I only wanted to have that
> change limitted to the oom_reaper. So I would really appreciate if
> somebody more knowledgeable had a look. We can drop the mlock patch for
> now.

According to the rc7 announcement it seems we are approaching the merge
window. Should we drop the patch for now or the risk of the lockup is
too low to care about and keep it in for now as it might be already
useful and change the munlock path to not depend on page locks later on?

I am OK with both ways.
-- 
Michal Hocko
SUSE Labs