linux-kernel - Re: [PATCH -mm] memcg: do not trigger OOM from add_to_page_cache

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Date:	Mon, 26 Nov 2012 23:06:40 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	azurIt <azurit@...ox.sk>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, cgroups mailinglist <cgroups@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH -mm] memcg: do not trigger OOM from
 add_to_page_cache_locked

On Mon 26-11-12 15:19:18, Johannes Weiner wrote:
> On Mon, Nov 26, 2012 at 09:08:48PM +0100, Michal Hocko wrote:
[...]
> > OK, I guess I am getting what you are trying to say. So what you are
> > suggesting is to just let mem_cgroup_out_of_memory send the signal and
> > move on without retry (or with few charge retries without further OOM
> > killing) and fail the charge with your new FAULT_OOM_HANDLED (resp.
> > something like FAULT_RETRY) error code resp. ENOMEM depending on the
> > caller.  OOM disabled case would be "you are on your own" because this
> > has been dangerous anyway. Correct?
> 
> Yes.
> 
> > I do agree that the current endless retry loop is far from being ideal
> > and can see some updates but I am quite nervous about any potential
> > regressions in this area (e.g. too aggressive OOM etc...). I have to
> > think about it some more.
> 
> Agreed on all points.  Maybe we can keep a couple of the oom retry
> iterations or something like that, which is still much more than what
> global does and I don't think the global OOM killer is overly eager.

Yes we can offer less blood and more confort

> 
> Testing will show more.
> 
> > Anyway if you have some more specific ideas I would be happy to review
> > patches.
> 
> Okay, I just wanted to check back with you before going down this
> path.  What are we going to do short term, though?  Do you want to
> push the disable-oom-for-pagecache for now or should we put the
> VM_FAULT_OOM_HANDLED fix in the next version and do stable backports?
> 
> This issue has been around for a while so frankly I don't think it's
> urgent enough to rush things.

Yes, but now we have a real usecase where this hurts AFAIU. Unless
we come up with a fix/reasonable workaround I would rather go with
something simpler for starter and more sofisticated later.

I have to double check other places where we do charging but the last
time I've checked we don't hold page locks on already visible pages (we
do precharge in __do_fault f.e.), mem_map for reading in the page fault
path is also safe (with oom enabled) and I guess that tmpfs is ok as
well. Then we have a page cache and that one should be covered by my
patch. So we should be covered.

But I like your idea long term.

Thanks!
-- 
Michal Hocko
SUSE Labs
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