From: Sean Christopherson <seanjc@google.com>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Yosry Ahmed <yosryahmed@google.com>,
Marc Zyngier <maz@kernel.org>, Tejun Heo <tj@kernel.org>,
Zefan Li <lizefan.x@bytedance.com>,
James Morse <james.morse@arm.com>,
Alexandru Elisei <alexandru.elisei@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
Andrew Morton <akpm@linux-foundation.org>,
Michal Hocko <mhocko@kernel.org>,
Roman Gushchin <roman.gushchin@linux.dev>,
Shakeel Butt <shakeelb@google.com>,
Oliver Upton <oupton@google.com>,
cgroups@vger.kernel.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses.
Date: Fri, 13 May 2022 16:12:40 +0000 [thread overview]
Message-ID: <Yn6DeEGLyR4Q0cDp@google.com> (raw)
In-Reply-To: <Yn5+OtZSSUZZgTQj@cmpxchg.org>
On Fri, May 13, 2022, Johannes Weiner wrote:
> On Thu, May 12, 2022 at 11:29:38PM +0000, Sean Christopherson wrote:
> > On Thu, May 12, 2022, Johannes Weiner wrote:
> > > On Mon, May 02, 2022 at 11:46:26AM -0700, Yosry Ahmed wrote:
> > > > On Mon, May 2, 2022 at 3:01 AM Marc Zyngier <maz@kernel.org> wrote:
> > > > > 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?).
> > > >
> > > > The reason this was named NR_SECONDARY_PAGTABLE instead of
> > > > NR_KVM_PAGETABLE is exactly that. To leave room to incrementally
> > > > account other types of secondary page tables to this stat. It is just
> > > > that we are currently interested in the KVM MMU usage.
> > >
> > > Do you actually care at the supervisor level that this memory is used
> > > for guest page tables?
> >
> > Hmm, yes? KVM does have a decent number of large-ish allocations that aren't
> > for page tables, but except for page tables, the number/size of those allocations
> > scales linearly with either the number of vCPUs or the amount of memory assigned
> > to the VM (with no room for improvement barring KVM changes).
> >
> > Off the top of my head, KVM's secondary page tables are the only allocations that
> > don't scale linearly, especially when nested virtualization is in use.
>
> Thanks, that's useful information.
>
> Are these other allocations accounted somewhere? If not, are they
> potential containment holes that will need fixing eventually?
All allocations that are tied to specific VM/vCPU are tagged GFP_KERNEL_ACCOUNT,
so we should be good on that front.
> > > It seems to me you primarily care that it is reported *somewhere*
> > > (hence the piggybacking off of NR_PAGETABLE at first). And whether
> > > it's page tables or iommu tables or whatever else allocated for the
> > > purpose of virtualization, it doesn't make much of a difference to the
> > > host/cgroup that is tracking it, right?
> > >
> > > (The proximity to nr_pagetable could also be confusing. A high page
> > > table count can be a hint to userspace to enable THP. It seems
> > > actionable in a different way than a high number of kvm page tables or
> > > iommu page tables.)
> >
> > I don't know about iommu page tables, but on the KVM side a high count can also
> > be a good signal that enabling THP would be beneficial.
>
> Well, maybe.
>
> It might help, but ultimately it's the process that's in control in
> all cases: it's unmovable kernel memory allocated to manage virtual
> address space inside the task.
>
> So I'm still a bit at a loss whether these things should all be lumped
> in together or kept separately. meminfo and memory.stat are permanent
> ABI, so we should try to establish in advance whether the new itme is
> really a first-class consumer or part of something bigger.
>
> The patch initially piggybacked on NR_PAGETABLE. I found an email of
> you asking why it couldn't be a separate item, but it didn't provide a
> reasoning for that decision. Could you share your thoughts on that?
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?
next prev parent reply other threads:[~2022-05-13 16:12 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-29 20:11 [PATCH v4 0/4] KVM: mm: count KVM mmu usage in memory stats Yosry Ahmed
2022-04-29 20:11 ` [PATCH v4 1/4] mm: add NR_SECONDARY_PAGETABLE to count secondary page table uses Yosry Ahmed
2022-05-02 10:01 ` Marc Zyngier
2022-05-02 18:46 ` Yosry Ahmed
2022-05-09 16:38 ` Yosry Ahmed
2022-05-12 20:36 ` Shakeel Butt
2022-05-12 23:07 ` Johannes Weiner
2022-05-12 23:29 ` Sean Christopherson
2022-05-13 15:50 ` Johannes Weiner
2022-05-13 16:12 ` Sean Christopherson [this message]
2022-05-13 16:22 ` Yosry Ahmed
2022-05-13 17:13 ` Shakeel Butt
2022-05-20 1:56 ` Yosry Ahmed
2022-05-20 14:39 ` Johannes Weiner
2022-05-24 22:31 ` Yosry Ahmed
2022-05-25 11:56 ` Johannes Weiner
2022-05-26 0:38 ` Sean Christopherson
2022-05-27 18:33 ` Yosry Ahmed
2022-06-03 16:42 ` Johannes Weiner
2022-04-29 20:11 ` [PATCH v4 2/4] KVM: mmu: add a helper to account memory used by KVM mmu Yosry Ahmed
2022-04-29 20:11 ` [PATCH v4 3/4] KVM: x86/mmu: count KVM mmu usage in secondary pagetable stats Yosry Ahmed
2022-07-22 20:58 ` Mingwei Zhang
2022-07-26 18:03 ` Sean Christopherson
2022-04-29 20:11 ` [PATCH v4 4/4] KVM: arm64/mmu: count KVM s2 " Yosry Ahmed
2022-05-02 7:24 ` Oliver Upton
2022-05-02 9:49 ` Marc Zyngier
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