linux-kernel - Re: [PATCH v5 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.

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Message-ID: <YrnVxM/5KjVhkOnn@google.com>
Date:   Mon, 27 Jun 2022 16:07:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yosry Ahmed <yosryahmed@...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>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, kvm@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v5 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary
 page table uses.

On Mon, Jun 06, 2022, Yosry Ahmed wrote:
> Add NR_SECONDARY_PAGETABLE stat to count secondary page table uses, e.g.
> KVM mmu. This provides more insights on the kernel memory used
> by a workload.

Please provide more justification for NR_SECONDARY_PAGETABLE in the changelog.
Specially, answer the questions that were asked in the previous version:

  1. Why not piggyback NR_PAGETABLE?
  2. Why a "generic" NR_SECONDARY_PAGETABLE instead of NR_VIRT_PAGETABLE?

It doesn't have to be super long, but provide enough info so that reviewers and
future readers don't need to go spelunking to understand the motivation for the
new counter type.

And it's probably worth an explicit Link to Marc's question that prompted the long
discussion in the previous version, that way if someone does want the gory details
they have a link readily available.

Link: https://lore.kernel.org/all/87ilqoi77b.wl-maz@kernel.org