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

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Message-ID: <CAJD7tkYJcsSvCUCkNgcWvi2Xoa3GDZk81p5GUptZzkOkrhrTWQ@mail.gmail.com>
Date:   Mon, 15 Aug 2022 08:39:23 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Johannes Weiner <hannes@...xchg.org>
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 15, 2022 at 8:13 AM Johannes Weiner <hannes@...xchg.org> 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@...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.

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).