Re: [PATCH v6 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.

[Sean Christopherson <seanjc-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

On Tue, Jul 12, 2022, Yosry Ahmed wrote:
> Thanks for taking another look at this!
> 
> On Thu, Jul 7, 2022 at 1:59 PM Sean Christopherson <seanjc-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
> >
> > On Tue, Jun 28, 2022, Yosry Ahmed wrote:
> > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > > index aab70355d64f3..13190d298c986 100644
> > > --- a/include/linux/mmzone.h
> > > +++ b/include/linux/mmzone.h
> > > @@ -216,6 +216,7 @@ enum node_stat_item {
> > >       NR_KERNEL_SCS_KB,       /* measured in KiB */
> > >  #endif
> > >       NR_PAGETABLE,           /* used for pagetables */
> > > +     NR_SECONDARY_PAGETABLE, /* secondary pagetables, e.g. kvm shadow pagetables */
> >
> > Nit, s/kvm/KVM, and drop the "shadow", which might be misinterpreted as saying KVM
> > pagetables are only accounted when KVM is using shadow paging.  KVM's usage of "shadow"
> > is messy, so I totally understand why you included it, but in this case it's unnecessary
> > and potentially confusing.
> >
> > And finally, something that's not a nit.  Should this be wrapped with CONFIG_KVM
> > (using IS_ENABLED() because KVM can be built as a module)?  That could be removed
> > if another non-KVM secondary MMU user comes along, but until then, #ifdeffery for
> > stats the depend on a single feature seems to be the status quo for this code.
> >
> 
> I will #ifdef the stat, but I will emphasize in the docs that is
> currently *only* used for KVM so that it makes sense if users without
> KVM don't see the stat at all. I will also remove the stat from
> show_free_areas() in mm/page_alloc.c as it seems like none of the
> #ifdefed stats show up there.

It's might be worth getting someone from mm/ to weigh in before going through the
trouble, my suggestion/question is based purely on the existing code.