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Date:   Thu, 27 Jul 2023 12:12:11 +0530
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Subject: Re: [RFC PATCH v11 00/29] KVM: guest_memfd() and per-page attributes
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To:     Sean Christopherson <seanjc@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paul Moore <paul@paul-moore.com>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
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        Chao Peng <chao.p.peng@linux.intel.com>,
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        Jarkko Sakkinen <jarkko@kernel.org>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
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        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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From:   "Nikunj A. Dadhania" <nikunj@amd.com>
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On 7/26/2023 7:54 PM, Sean Christopherson wrote:
> On Wed, Jul 26, 2023, Nikunj A. Dadhania wrote:
>> On 7/24/2023 10:30 PM, Sean Christopherson wrote:

>>>>   /proc/<qemu pid>/smaps
>>>>   7f528be00000-7f5c8be00000 rw-p 00000000 00:01 26629                      /memfd:memory-backend-memfd-shared (deleted)
>>>>   7f5c90200000-7f5c90220000 rw-s 00000000 00:01 44033                      /memfd:rom-backend-memfd-shared (deleted)
>>>>   7f5c90400000-7f5c90420000 rw-s 00000000 00:01 44032                      /memfd:rom-backend-memfd-shared (deleted)
>>>>   7f5c90800000-7f5c90b7c000 rw-s 00000000 00:01 1025                       /memfd:rom-backend-memfd-shared (deleted)
>>>
>>> This is all expected, and IMO correct.  There are no userspace mappings, and so
>>> not accounting anything is working as intended.
>> Doesn't sound that correct, if 10 SNP guests are running each using 10GB, how
>> would we know who is using 100GB of memory?
> 
> It's correct with respect to what the interfaces show, which is how much memory
> is *mapped* into userspace.
> 
> As I said (or at least tried to say) in my first reply, I am not against exposing
> memory usage to userspace via stats, only that it's not obvious to me that the
> existing VMA-based stats are the most appropriate way to surface this information.

Right, then should we think in the line of creating a VM IOCTL for querying current memory
usage for guest-memfd ?

We could use memcg for statistics, but then memory cgroup can be disabled and so memcg 
isn't really a dependable option.

Do you have some ideas on how to expose the memory usage to the user space other than
VMA-based stats ?

Regards,
Nikunj