Re: [External] Re: [PATCH v2] mm: memcontrol: fix kernel stack account

[Muchun Song]

On Wed, Mar 3, 2021 at 6:25 PM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Wed 03-03-21 17:39:56, Muchun Song wrote:
> > For simplification 991e7673859e ("mm: memcontrol: account kernel stack
> > per node") has changed the per zone vmalloc backed stack pages
> > accounting to per node. By doing that we have lost a certain precision
> > because those pages might live in different NUMA nodes. In the end
> > NR_KERNEL_STACK_KB exported to the userspace might be over estimated on
> > some nodes while underestimated on others.
> >
> > This doesn't impose any real problem to correctnes of the kernel
> > behavior as the counter is not used for any internal processing but it
> > can cause some confusion to the userspace.
>
> You have skipped over one part of the changelog I have proposed and that
> is to provide an actual data.

Because this is a problem I found by looking at the code, not a real world
problem. I do not have any actual data. :-(

>
> > Address the problem by accounting each vmalloc backing page to its own
> > node.
> >
> > Fixes: 991e7673859e ("mm: memcontrol: account kernel stack per node")
>
> Fixes tag might make somebody assume this is worth backporting but I
> highly doubt so.

OK. I can remove the Fixes tag.

>
> > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
>
> Anyway
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks for your review.

>
> as the patch is correct with one comment below
>
> > ---
> > Changelog in v2:
> >  - Rework commit log suggested by Michal.
> >
> >  Thanks to Michal and Shakeel for review.
> >
> >  kernel/fork.c | 15 ++++++++++-----
> >  1 file changed, 10 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index d66cd1014211..6e2201feb524 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -379,14 +379,19 @@ static void account_kernel_stack(struct task_struct *tsk, int account)
> >       void *stack = task_stack_page(tsk);
> >       struct vm_struct *vm = task_stack_vm_area(tsk);
> >
> > +     if (vm) {
> > +             int i;
> >
> > -     /* All stack pages are in the same node. */
> > -     if (vm)
> > -             mod_lruvec_page_state(vm->pages[0], NR_KERNEL_STACK_KB,
> > -                                   account * (THREAD_SIZE / 1024));
> > -     else
> > +             BUG_ON(vm->nr_pages != THREAD_SIZE / PAGE_SIZE);
>
> I do not think we need this BUG_ON. What kind of purpose does it serve?

vm->nr_pages should be always equal to THREAD_SIZE / PAGE_SIZE
if the system is not corrupted. It makes sense to remove the BUG_ON.
I will remove it in the next version. Thanks.

>
> > +
> > +             for (i = 0; i < THREAD_SIZE / PAGE_SIZE; i++)
> > +                     mod_lruvec_page_state(vm->pages[i], NR_KERNEL_STACK_KB,
> > +                                           account * (PAGE_SIZE / 1024));
> > +     } else {
> > +             /* All stack pages are in the same node. */
> >               mod_lruvec_kmem_state(stack, NR_KERNEL_STACK_KB,
> >                                     account * (THREAD_SIZE / 1024));
> > +     }
> >  }
> >
> >  static int memcg_charge_kernel_stack(struct task_struct *tsk)
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs