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

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Message-ID: <CAJD7tkY91oiDWTj5FY2Upc5vabsjLk+CBMNzAepXLUdF_GS11w@mail.gmail.com>
Date:   Mon, 18 Jul 2022 11:26:01 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.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 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?