Date:	Fri, 23 Jan 2015 15:00:00 +0000
From:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
To:	Johannes Weiner <hannes@...xchg.org>,
	Will Deacon <Will.Deacon@....com>
CC:	Vladimir Davydov <vdavydov@...allels.com>,
	Tejun Heo <tj@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mhocko@...e.cz" <mhocko@...e.cz>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [Regression] 3.19-rc3 : memcg: Hang in mount memcg

On 22/01/15 13:45, Johannes Weiner wrote:
> On Wed, Jan 21, 2015 at 04:39:55PM +0000, Will Deacon wrote:
>> On Mon, Jan 19, 2015 at 12:51:27PM +0000, Suzuki K. Poulose wrote:
>>> On 10/01/15 08:55, Vladimir Davydov wrote:
>>>> The problem is that the memory cgroup controller takes a css reference
>>>> per each charged page and does not reparent charged pages on css
>>>> offline, while cgroup_mount/cgroup_kill_sb expect all css references to
>>>> offline cgroups to be gone soon, restarting the syscall if the ref count
>>>> != 0. As a result, if you create a memory cgroup, charge some page cache
>>>> to it, and then remove it, unmount/mount will hang forever.
>>>>
>>>> May be, we should kill the ref counter to the memory controller root in
>>>> cgroup_kill_sb only if there is no children at all, neither online nor
>>>> offline.
>>>>
>>>
>>> Still reproducible on 3.19-rc5 with the same setup.
>>
>> Yeah, I'm seeing the same failure on my setup too.
>>
>>>  From git bisect, the last good commit is :
>>>
>>> commit 8df0c2dcf61781d2efa8e6e5b06870f6c6785735
>>> Author: Pranith Kumar <bobby.prani@...il.com>
>>> Date:   Wed Dec 10 15:42:28 2014 -0800
>>>
>>>       slab: replace smp_read_barrier_depends() with lockless_dereference()
>>
>> So that points at 3e32cb2e0a12 ("mm: memcontrol: lockless page counters")
>> as the offending commit.
>
> With b2052564e66d ("mm: memcontrol: continue cache reclaim from
> offlined groups"), page cache can pin an old css and its ancestors
> indefinitely, making that hang in a second mount() very likely.
>
> However, swap entries have also been doing that for quite a while now,
> and as Vladimir pointed out, the same is true for kernel memory.  This
> latest change just makes this existing bug easier to trigger.
>
> I think we have to update the lifetime rules to reflect reality here:
> memory and swap lifetime is indefinite, so once the memory controller
> is used, it has state that is independent from whether its mounted or
> not.  We can support an identical remount, but have to fail mounting
> with new parameters that would change the behavior of the controller.
>
> Suzuki, Will, could you give the following patch a shot?


>
> Tejun, would that route be acceptable to you?
>
> Thanks
>
> ---
>  From c5e88d02d185c52748df664aa30a2c5f8949b0f7 Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <hannes@...xchg.org>
> Date: Thu, 22 Jan 2015 08:16:31 -0500
> Subject: [patch] kernel: cgroup: prevent mount hang due to memory controller
>   lifetime
>

>
> Don't offline the controller root as long as there are any children,
> dead or alive.  A remount will no longer wait for these old references
> to drain, it will simply mount the persistent controller state again.
>
> Reported-by: "Suzuki K. Poulose" <Suzuki.Poulose@....com>
> Reported-by: Will Deacon <will.deacon@....com>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
This one fixes the issue.

Tested-by : Suzuki K. Poulose <suzuki.poulose@....com>

Thanks
Suzuki



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