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

[Muchun Song <songmuchun@bytedance.com>]

On Tue, Mar 2, 2021 at 4:44 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 02-03-21 15:37:33, Muchun Song wrote:
> > The alloc_thread_stack_node() cannot guarantee that allocated stack pages
> > are in the same node when CONFIG_VMAP_STACK. Because we do not specify
> > __GFP_THISNODE to __vmalloc_node_range(). Fix it by caling
> > mod_lruvec_page_state() for each page one by one.
>
> What is the actual problem you are trying to address by this patch?
> 991e7673859e has deliberately dropped the per page accounting. Can you
> explain why that was incorrect? There surely is some imprecision
> involved but does it matter and is it even observable?

When I read the code of account_kernel_stack(), I see a comment that
says "All stack pages are in the same node". I am confused about this.
IIUC, there is no guarantee about this. Right? Yeah, imprecision may
not be a problem. But if this is what we did deliberately, I think that
it is better to add a comment there. Thanks.

>
> > Fixes: 991e7673859e ("mm: memcontrol: account kernel stack per node")
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  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);
> > +
> > +             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