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

[Shakeel Butt]

On Wed, Mar 3, 2021 at 6:02 AM Michal Hocko  wrote:
>
[...]
> > > > + 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.
>
> BUG_ON is not an annotation for "this shouldn't happen". Even if the
> system was corrupted and nr_pages wouldn't match then this is not a
> reason to crash the kernel right away.
>
> In general there should be a very _strong_ reason to add a BUG_ON.
>

I agree with Michal. We should remove this BUG_ON or at least convert
it into VM_BUG_ON.

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

[Michal Hocko]

On Wed 03-03-21 21:27:24, Muchun Song wrote:
> On Wed, Mar 3, 2021 at 6:25 PM Michal Hocko  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. :-(

As I've mentioned several times already, this is all fine but it should
be made explicit in the changelog. There might be people spending their
time to evaluate this code to find out whether this is something that
somebody depend on.

[...]
> > > - /* 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.

BUG_ON is not an annotation for "this shouldn't happen". Even if the
system was corrupted and nr_pages wouldn't match then this is not a
reason to crash the kernel right away.

In general there should be a very _strong_ reason to add a BUG_ON.

-- 
Michal Hocko
SUSE Labs

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

[Muchun Song]

On Wed, Mar 3, 2021 at 6:25 PM Michal Hocko  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 
> > Reviewed-by: Shakeel Butt 
>
> Anyway
> Acked-by: Michal Hocko 

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