Re: [PATCH v2] sched/core: Don't use dying mm as active_mm of kthreads

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 29, 2019 at 10:52:35AM +0200, Peter Zijlstra wrote:
> On Sat, Jul 27, 2019 at 01:10:47PM -0400, Waiman Long wrote:
> > It was found that a dying mm_struct where the owning task has exited
> > can stay on as active_mm of kernel threads as long as no other user
> > tasks run on those CPUs that use it as active_mm. This prolongs the
> > life time of dying mm holding up memory and other resources like swap
> > space that cannot be freed.
> 
> Sure, but this has been so 'forever', why is it a problem now?
> 
> > Fix that by forcing the kernel threads to use init_mm as the active_mm
> > if the previous active_mm is dying.
> > 
> > The determination of a dying mm is based on the absence of an owning
> > task. The selection of the owning task only happens with the CONFIG_MEMCG
> > option. Without that, there is no simple way to determine the life span
> > of a given mm. So it falls back to the old behavior.
> > 
> > Signed-off-by: Waiman Long <longman@redhat.com>
> > ---
> >  include/linux/mm_types.h | 15 +++++++++++++++
> >  kernel/sched/core.c      | 13 +++++++++++--
> >  mm/init-mm.c             |  4 ++++
> >  3 files changed, 30 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 3a37a89eb7a7..32712e78763c 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -623,6 +623,21 @@ static inline bool mm_tlb_flush_nested(struct mm_struct *mm)
> >  	return atomic_read(&mm->tlb_flush_pending) > 1;
> >  }
> >  
> > +#ifdef CONFIG_MEMCG
> > +/*
> > + * A mm is considered dying if there is no owning task.
> > + */
> > +static inline bool mm_dying(struct mm_struct *mm)
> > +{
> > +	return !mm->owner;
> > +}
> > +#else
> > +static inline bool mm_dying(struct mm_struct *mm)
> > +{
> > +	return false;
> > +}
> > +#endif
> > +
> >  struct vm_fault;
> 
> Yuck. So people without memcg will still suffer the terrible 'whatever
> it is this patch fixes'.

Also; why then not key off that owner tracking to free the resources
(and leave the struct mm around) and avoid touching this scheduling
hot-path ?