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

[Peter Zijlstra <peterz@infradead.org>]

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'.

>  /**
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2b037f195473..923a63262dfd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3233,13 +3233,22 @@ context_switch(struct rq *rq, struct task_struct *prev,
>  	 * Both of these contain the full memory barrier required by
>  	 * membarrier after storing to rq->curr, before returning to
>  	 * user-space.
> +	 *
> +	 * If mm is NULL and oldmm is dying (!owner), we switch to
> +	 * init_mm instead to make sure that oldmm can be freed ASAP.
>  	 */
> -	if (!mm) {
> +	if (!mm && !mm_dying(oldmm)) {
>  		next->active_mm = oldmm;
>  		mmgrab(oldmm);
>  		enter_lazy_tlb(oldmm, next);
> -	} else
> +	} else {
> +		if (!mm) {
> +			mm = &init_mm;
> +			next->active_mm = mm;
> +			mmgrab(mm);
> +		}
>  		switch_mm_irqs_off(oldmm, mm, next);
> +	}
>  
>  	if (!prev->mm) {
>  		prev->active_mm = NULL;

Bah, I see we _still_ haven't 'fixed' that code. And you're making an
even bigger mess of it.

Let me go find where that cleanup went.