Re: fs/buffer.c: WARNING: alloc_page_buffers while mke2fs

[Yang Shi <shy828301@gmail.com>]

On Tue, Mar 3, 2020 at 10:15 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Tue, Mar 3, 2020 at 9:47 AM Yang Shi <shy828301@gmail.com> wrote:
> >
> > On Tue, Mar 3, 2020 at 2:53 AM Tetsuo Handa
> > <penguin-kernel@i-love.sakura.ne.jp> wrote:
> > >
> > > Hello, Naresh.
> > >
> > > > [   98.003346] WARNING: CPU: 2 PID: 340 at
> > > > include/linux/sched/mm.h:323 alloc_page_buffers+0x210/0x288
> > >
> > > This is
> > >
> > > /**
> > >  * memalloc_use_memcg - Starts the remote memcg charging scope.
> > >  * @memcg: memcg to charge.
> > >  *
> > >  * This function marks the beginning of the remote memcg charging scope. All the
> > >  * __GFP_ACCOUNT allocations till the end of the scope will be charged to the
> > >  * given memcg.
> > >  *
> > >  * NOTE: This function is not nesting safe.
> > >  */
> > > static inline void memalloc_use_memcg(struct mem_cgroup *memcg)
> > > {
> > >         WARN_ON_ONCE(current->active_memcg);
> > >         current->active_memcg = memcg;
> > > }
> > >
> > > which is about memcg. Redirecting to linux-mm.
> >
> > Isn't this triggered by ("loop: use worker per cgroup instead of
> > kworker") in linux-next, which converted loop driver to use worker per
> > cgroup, so it may have multiple workers work at the mean time?
> >
> > So they may share the same "current", then it may cause kind of nested
> > call to memalloc_use_memcg().
> >
> > Could you please try the below debug patch? This is not the proper
> > fix, but it may help us narrow down the problem.
> >
> > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> > index c49257a..1cc1cdc 100644
> > --- a/include/linux/sched/mm.h
> > +++ b/include/linux/sched/mm.h
> > @@ -320,6 +320,10 @@ static inline void
> > memalloc_nocma_restore(unsigned int flags)
> >   */
> >  static inline void memalloc_use_memcg(struct mem_cgroup *memcg)
> >  {
> > +       if ((current->flags & PF_KTHREAD) &&
> > +            current->active_memcg)
> > +               return;
> > +
> >         WARN_ON_ONCE(current->active_memcg);
> >         current->active_memcg = memcg;
> >  }
> >
>
> Maybe it's time to make memalloc_use_memcg() nesting safe.

Need handle the below case:

            CPU A                                          CPU B
memalloc_use_memcg
                                                     memalloc_use_memcg
memalloc_unuse_memcg
                                                     memalloc_unuse_memcg


They may manipulate the same task->active_memcg, so CPU B may still
see wrong memcg, and the last call to memalloc_unuse_memcg() on CPU B
may not restore active_memcg to NULL. And, some code depends on
correct active_memcg.