Re: [Regression] 3.19-rc3 : memcg: Hang in mount memcg

[Will Deacon <will.deacon@arm.com>]

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@gmail.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.

Will

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