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

[Vladimir Davydov]

On Fri, Jan 09, 2015 at 05:43:17PM +0000, Suzuki K. Poulose wrote:
> Hi
> 
> We have hit a hang on ARM64 defconfig, while running LTP tests on
> 3.19-rc3. We are
> in the process of a git bisect and will update the results as and
> when we find the commit.
> 
> During the ksm ltp run, the test hangs trying to mount memcg with
> the following strace
> output:
> 
> mount("memcg", "/dev/cgroup", "cgroup", 0, "memory") = ?
> ERESTARTNOINTR (To be restarted)
> mount("memcg", "/dev/cgroup", "cgroup", 0, "memory") = ?
> ERESTARTNOINTR (To be restarted)
> [ ... repeated forever ... ]
> 
> At this point, one can try mounting the memcg to verify the problem.
> # mount -t cgroup -o memory memcg memcg_dir
> --hangs--
> 
> Strangely, if we run the mount command from a cold boot (i.e.
> without running LTP first),
> then it succeeds.
> 
> Upon a quick look we are hitting the following code :
> kernel/cgroup.c: cgroup_mount() :
> 
> 1779         for_each_subsys(ss, i) {
> 1780                 if (!(opts.subsys_mask & (1 << i)) ||
> 1781                     ss->root == &cgrp_dfl_root)
> 1782                         continue;
> 1783
> 1784                 if
> (!percpu_ref_tryget_live(&ss->root->cgrp.self.refcnt)) {
> 1785                         mutex_unlock(&cgroup_mutex);
> 1786                         msleep(10);
> 1787                         ret = restart_syscall(); <=====
> 1788                         goto out_free;
> 1789                 }
> 1790                 cgroup_put(&ss->root->cgrp);
> 1791         }
> 
> with ss->root->cgrp.self.refct.percpu_count_ptr == __PERCPU_REF_ATOMIC_DEAD
> 
> Any ideas?

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.

Thanks,
Vladimir
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