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

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Date:   Mon, 02 May 2022 11:01:44 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.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>,
        Sean Christopherson <seanjc@...gle.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 v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.

On Fri, 29 Apr 2022 21:11:28 +0100,
Yosry Ahmed <yosryahmed@...gle.com> 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.
> 
> This stat will be used by subsequent patches to count KVM mmu
> memory usage.
> 
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> ---
>  Documentation/admin-guide/cgroup-v2.rst | 5 +++++
>  Documentation/filesystems/proc.rst      | 4 ++++
>  drivers/base/node.c                     | 2 ++
>  fs/proc/meminfo.c                       | 2 ++
>  include/linux/mmzone.h                  | 1 +
>  mm/memcontrol.c                         | 1 +
>  mm/page_alloc.c                         | 6 +++++-
>  mm/vmstat.c                             | 1 +
>  8 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 69d7a6983f78..828cb6b6f918 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -1312,6 +1312,11 @@ PAGE_SIZE multiple when read back.
>  	  pagetables
>                  Amount of memory allocated for page tables.
>  
> +	  secondary_pagetables
> +		Amount of memory allocated for secondary page tables,
> +		this currently includes KVM mmu allocations on x86
> +		and arm64.

Can you please explain what the rationale is for this? We already
account for the (arm64) S2 PTs as a userspace allocation (see
115bae923ac8bb29ee635). You are saying that this is related to a
'workload', but given that the accounting is global, I fail to see how
you can attribute these allocations on a particular VM.

What do you plan to do for IOMMU page tables? After all, they serve
the exact same purpose, and I'd expect these to be handled the same
way (i.e. why is this KVM specific?).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.