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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>