linux-kernel - Re: [PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.

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Date:   Fri, 13 May 2022 10:13:54 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        Marc Zyngier <maz@...nel.org>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        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>,
        Oliver Upton <oupton@...gle.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 v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary
 page table uses.

On Fri, May 13, 2022 at 9:12 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
[...]
>
> It was mostly an honest question, I too am trying to understand what userspace
> wants to do with this information.  I was/am also trying to understand the benefits
> of doing the tracking through page_state and not a dedicated KVM stat.  E.g. KVM
> already has specific stats for the number of leaf pages mapped into a VM, why not
> do the same for non-leaf pages?

Let me answer why a more general stat is useful and the potential
userspace reaction:

For a memory type which is significant enough, it is useful to expose
it in the general interfaces, so that the general data/stat collection
infra can collect them instead of having workload dependent stat
collectors. In addition, not necessarily that stat has to have a
userspace reaction in an online fashion. We do collect stats for
offline analysis which greatly influence the priority order of
optimization workitems.

Next the question is do we really need a separate stat item
(secondary_pagetable instead of just plain pagetable) exposed in the
stable API? To me secondary_pagetable is general (not kvm specific)
enough and can be significant, so having a separate dedicated stat
should be ok. Though I am ok with lump it with pagetable stat for now
but we do want it to be accounted somewhere.