linux-kernel - Re: [PATCH v3 4/6] KVM: arm64/mmu: count KVM page table pages in pagetable stats

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Message-ID: <YmrSywSU1ezREvT6@google.com>
Date:   Thu, 28 Apr 2022 17:45:47 +0000
From:   Oliver Upton <oupton@...gle.com>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Huacai Chen <chenhuacai@...nel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@...il.com>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        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>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        James Morse <james.morse@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        Marc Zyngier <maz@...nel.org>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-riscv@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        cgroups@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v3 4/6] KVM: arm64/mmu: count KVM page table pages in
 pagetable stats

On Tue, Apr 26, 2022 at 12:27:57PM -0700, Yosry Ahmed wrote:
> > What page tables do we want to account? KVM on ARM manages several page
> > tables.
> >
> > For regular KVM, the host kernel manages allocations for the hyp stage 1
> > tables in addition to the stage 2 tables used for a particular VM. The
> > former is system overhead whereas the latter could be attributed to a
> > guest VM.
> 
> Honestly I would love to get your input on this. The main motivation
> here is to give users insights on the kernel memory usage on their
> system (or in a cgroup). We currently have NR_PAGETABLE stats for
> normal kernel page tables (allocated using
> __pte_alloc_one()/pte_free()), this shows up in /proc/meminfo,
> /path/to/cgroup/memory.stat, and node stats. The idea is to add
> NR_SECONDARY_PAGETABLE that should include the memory used for kvm
> pagetables, which should be a separate category (no overlap). What
> gets included or not depends on the semantics of KVM and what exactly
> falls under the category of secondary pagetables from the user's pov.
> 
> Currently it looks like s2 page table allocations get accounted to
> kmem of memory control groups (GFP_KERNEL_ACCOUNT), while hyp page
> table allocations do not (GFP_KERNEL). So we could either follow this
> and only account s2 page table allocations in the stats, or make hyp
> allocations use GFP_KERNEL_ACCOUNT as well and add them to the stats.
> Let me know what you think.

I think it is reasonable to just focus on stage 2 table allocations and
ignore all else. As Marc pointed out it isn't workable in other
contexts anyway (pKVM), and keeps the patch tidy too.

GFP_KERNEL_ACCOUNT for hyp allocations wouldn't make sense, as it is
done at init to build out the system page tables for EL2.

--
Thanks,
Oliver