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

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Date:   Mon, 15 Aug 2022 11:13:51 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Oliver Upton <oupton@...gle.com>, Huang@...gle.com,
        Shaoqin <shaoqin.huang@...el.com>,
        Cgroups <cgroups@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v6 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary
 page table uses.

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@...gle.com> wrote:
> >
> > On Tue, Jul 12, 2022 at 4:06 PM Sean Christopherson <seanjc@...gle.com> 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@...gle.com> 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.