Re: [PATCH 1/2] mm: memcg: switch to css_tryget() in get_mem_cgroup_from_mm()

[Roman Gushchin]

On Wed, Nov 06, 2019 at 05:25:26PM -0800, Shakeel Butt wrote:
> On Wed, Nov 6, 2019 at 4:22 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >
> > On Wed, Nov 06, 2019 at 02:51:30PM -0800, Roman Gushchin wrote:
> > > We've encountered a rcu stall in get_mem_cgroup_from_mm():
> > >
> > >  rcu: INFO: rcu_sched self-detected stall on CPU
> > >  rcu: 33-....: (21000 ticks this GP) idle=6c6/1/0x4000000000000002 softirq=35441/35441 fqs=5017
> > >  (t=21031 jiffies g=324821 q=95837) NMI backtrace for cpu 33
> > >  <...>
> > >  RIP: 0010:get_mem_cgroup_from_mm+0x2f/0x90
> > >  <...>
> > >  __memcg_kmem_charge+0x55/0x140
> > >  __alloc_pages_nodemask+0x267/0x320
> > >  pipe_write+0x1ad/0x400
> > >  new_sync_write+0x127/0x1c0
> > >  __kernel_write+0x4f/0xf0
> > >  dump_emit+0x91/0xc0
> > >  writenote+0xa0/0xc0
> > >  elf_core_dump+0x11af/0x1430
> > >  do_coredump+0xc65/0xee0
> > >  ? unix_stream_sendmsg+0x37d/0x3b0
> > >  get_signal+0x132/0x7c0
> > >  do_signal+0x36/0x640
> > >  ? recalc_sigpending+0x17/0x50
> > >  exit_to_usermode_loop+0x61/0xd0
> > >  do_syscall_64+0xd4/0x100
> > >  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > >
> > > The problem is caused by an exiting task which is associated with
> > > an offline memcg. We're iterating over and over in the
> > > do {} while (!css_tryget_online()) loop, but obviously the memcg won't
> > > become online and the exiting task won't be migrated to a live memcg.
> > >
> > > Let's fix it by switching from css_tryget_online() to css_tryget().
> > >
> > > As css_tryget_online() cannot guarantee that the memcg won't go
> > > offline, the check is usually useless, except some rare cases
> > > when for example it determines if something should be presented
> > > to a user.
> > >
> > > A similar problem is described by commit 18fa84a2db0e ("cgroup: Use
> > > css_tryget() instead of css_tryget_online() in task_get_css()").
> > >
> > > Signed-off-by: Roman Gushchin <guro@xxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Cc: Tejun Heo <tj@xxxxxxxxxx>
> >
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> >
> > The bug aside, it doesn't matter whether the cgroup is online for the
> > callers. It used to matter when offlining needed to evacuate all
> > charges from the memcg, and so needed to prevent new ones from showing
> > up, but we don't care now.
> 
> Should get_mem_cgroup_from_current() and get_mem_cgroup_from_page() be
> switched to css_tryget() as well then?

In those case it can't cause a rcu stall, so it's not a so urgent.
But you are right, we should probably do the same here. I'll look
at all remaining callers and prepare the patchset.

I'll also probably rename it to css_tryget_if_online() to make obvious
that it doesn't hold the cgroup from being onlined.

Thanks!