On Mon 23-07-18 09:17:28, Shakeel Butt wrote:

On Sun, Jul 22, 2018 at 11:44 PM Michal Hocko [off-list ref] wrote:

quoted

On Thu 19-07-18 09:23:10, Shakeel Butt wrote:

quoted

On Thu, Jul 19, 2018 at 3:43 AM Michal Hocko [off-list ref] wrote:

quoted

[CC Andrew]

On Thu 19-07-18 18:06:47, Jing Xia wrote:

quoted

It was reported that a kernel crash happened in mem_cgroup_iter(),
which can be triggered if the legacy cgroup-v1 non-hierarchical
mode is used.

Unable to handle kernel paging request at virtual address 6b6b6b6b6b6b8f
......
Call trace:
  mem_cgroup_iter+0x2e0/0x6d4
  shrink_zone+0x8c/0x324
  balance_pgdat+0x450/0x640
  kswapd+0x130/0x4b8
  kthread+0xe8/0xfc
  ret_from_fork+0x10/0x20

  mem_cgroup_iter():
      ......
      if (css_tryget(css))    <-- crash here
          break;
      ......

The crashing reason is that mem_cgroup_iter() uses the memcg object
whose pointer is stored in iter->position, which has been freed before
and filled with POISON_FREE(0x6b).

And the root cause of the use-after-free issue is that
invalidate_reclaim_iterators() fails to reset the value of
iter->position to NULL when the css of the memcg is released in non-
hierarchical mode.

Well, spotted!

I suspect
Fixes: 6df38689e0e9 ("mm: memcontrol: fix possible memcg leak due to interrupted reclaim")

but maybe it goes further into past. I also suggest
Cc: stable

even though the non-hierarchical mode is strongly discouraged.

Why not set root_mem_cgroup's use_hierarchy to true by default on
init? If someone wants non-hierarchical mode, they can explicitly set
it to false.

We do not change defaults under users feet usually.

Then how non-hierarchical mode is being discouraged currently? I don't
see any comments in the docs.

css_create warns about non-hierarchical hierarchies. We've been running
with a similar warning in (even older) SLES kernels for years now and
quite some tools have been updated because they simply didn't know they
are doing something wrong.
-- 
Michal Hocko
SUSE Labs