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

From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: Dexuan Cui <decui@microsoft.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>,
	Shakeel Butt <shakeelb@google.com>,
	Johannes Weiner <hannes@cmpxchg.org>, Tejun Heo <tj@kernel.org>,
	Rik van Riel <riel@surriel.com>,
	Konstantin Khlebnikov <koct9i@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	"Stable@vger.kernel.org" <Stable@vger.kernel.org>
Subject: Re: Will the recent memory leak fixes be backported to longterm kernels?
Date: Fri, 2 Nov 2018 17:51:47 +0100	[thread overview]
Message-ID: <20181102165147.GG28039@dhcp22.suse.cz> (raw)
In-Reply-To: <20181102162237.GB17619@tower.DHCP.thefacebook.com>

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