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

[Yosry Ahmed]

On Mon, Aug 15, 2022 at 8:13 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Mon, Aug 08, 2022 at 01:06:15PM -0700, Yosry Ahmed wrote:
> > On Mon, Jul 18, 2022 at 11:26 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Jul 12, 2022 at 4:06 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > >
> > > > 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@xxxxxxxxxx> 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.
> > >
> > > Any mm folks with an opinion about this?
> > >
> > > Any preference on whether we should wrap NR_SECONDARY_PAGETABLE stats
> > > with #ifdef CONFIG_KVM for now as it is currently the only source for
> > > this stat?
> >
> > Any input here?
> >
> > Johannes, you have been involved in discussions in earlier versions of
> > this series, any thoughts here?
>
> No super strong feelings here. Most major distros have CONFIG_KVM=y/n,
> so it'll be a common fixture anyway, and the ifdef is proooobably not
> worth it for hiding it from people. OTOH, the ifdef is useful for
> documenting the code.
>
> If you've already ifdeffed it now, I'd say go ahead with
> it. Otherwise, don't :) My 2c.

Thanks a lot, Johannes! I haven't ifdeffed it yet so I'll send a v7
with a few nits and collect ACKs. Andrew, would you prefer me to
rebase on top of mm-unstable? Or will this go in through the kvm tree?
(currently it's based on an old-ish kvm/queue).