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

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Date:   Thu, 7 Jul 2022 20:59:41 +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>, Huang@...gle.com,
        Shaoqin <shaoqin.huang@...el.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 v6 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary
 page table uses.

On Tue, Jun 28, 2022, Yosry Ahmed wrote:
> We keep track of several kernel memory stats (total kernel memory, page
> tables, stack, vmalloc, etc) on multiple levels (global, per-node,
> per-memcg, etc). These stats give insights to users to how much memory
> is used by the kernel and for what purposes.
> 
> Currently, memory used by kvm mmu is not accounted in any of those

Nit, capitalize KVM (mainly to be consistent).

> @@ -1085,6 +1086,9 @@ KernelStack
>                Memory consumed by the kernel stacks of all tasks
>  PageTables
>                Memory consumed by userspace page tables
> +SecPageTables
> +              Memory consumed by secondary page tables, this currently
> +	      currently includes KVM mmu allocations on x86 and arm64.

Nit, this line has a tab instead of eight spaces.  Not sure if it actually matters,
there are plenty of tabs elsewhere in the file, but all the entries in this block
use only spaces.

> 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.

>  #ifdef CONFIG_SWAP
>  	NR_SWAPCACHE,
>  #endif