linux-kernel - Re: Will the recent memory leak fixes be backported to longterm kernels?

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Message-ID: <20181102165147.GG28039@dhcp22.suse.cz>
Date:   Fri, 2 Nov 2018 17:51:47 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Dexuan Cui <decui@...rosoft.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tejun Heo <tj@...nel.org>, Rik van Riel <riel@...riel.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Stable@...r.kernel.org" <Stable@...r.kernel.org>
Subject: Re: Will the recent memory leak fixes be backported to longterm
 kernels?

On Fri 02-11-18 16:22:41, Roman Gushchin wrote:
> On Fri, Nov 02, 2018 at 05:13:14PM +0100, Michal Hocko wrote:
> > On Fri 02-11-18 15:48:57, Roman Gushchin wrote:
> > > On Fri, Nov 02, 2018 at 09:03:55AM +0100, Michal Hocko wrote:
> > > > On Fri 02-11-18 02:45:42, Dexuan Cui wrote:
> > > > [...]
> > > > > I totally agree. I'm now just wondering if there is any temporary workaround,
> > > > > even if that means we have to run the kernel with some features disabled or
> > > > > with a suboptimal performance?
> > > > 
> > > > One way would be to disable kmem accounting (cgroup.memory=nokmem kernel
> > > > option). That would reduce the memory isolation because quite a lot of
> > > > memory will not be accounted for but the primary source of in-flight and
> > > > hard to reclaim memory will be gone.
> > > 
> > > In my experience disabling the kmem accounting doesn't really solve the issue
> > > (without patches), but can lower the rate of the leak.
> > 
> > This is unexpected. 90cbc2508827e was introduced to address offline
> > memcgs to be reclaim even when they are small. But maybe you mean that
> > we still leak in an absence of the memory pressure. Or what does prevent
> > memcg from going down?
> 
> There are 3 independent issues which are contributing to this leak:
> 1) Kernel stack accounting weirdness: processes can reuse stack accounted to
> different cgroups. So basically any running process can take a reference to any
> cgroup.

yes, but kmem accounting should rule that out, right? If not then this
is a clear bug and easy to backport because that would mean to add a
missing memcg_kmem_enabled check.

> 2) We do forget to scan the last page in the LRU list. So if we ended up with
> 1-page long LRU, it can stay there basically forever.

Why 
		/*
		 * If the cgroup's already been deleted, make sure to
		 * scrape out the remaining cache.
		 */
		if (!scan && !mem_cgroup_online(memcg))
			scan = min(size, SWAP_CLUSTER_MAX);

in get_scan_count doesn't work for that case?

> 3) We don't apply enough pressure on slab objects.

again kmem accounting disabled should make this moot

> Because one reference is enough to keep the entire memcg structure in place,
> we really have to close all three to eliminate the leak. Disabling kmem
> accounting mitigates only the last one.
-- 
Michal Hocko
SUSE Labs