linux-kernel - Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM guest private memory

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Message-ID: <CALCETrWw=Q=1AKW0Jcj3ZGscjyjDJXAjuxOnQx_sabQ6ZtS-wg@mail.gmail.com>
Date:   Tue, 14 Jun 2022 10:37:37 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Marc Orr <marcorr@...gle.com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86 <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>, aarcange@...hat.com,
        ddutile@...hat.com, dhildenb@...hat.com,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>, mhocko@...e.com
Subject: Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM
 guest private memory

On Tue, Jun 14, 2022 at 12:32 AM Chao Peng <chao.p.peng@...ux.intel.com> wrote:
>
> On Thu, Jun 09, 2022 at 08:29:06PM +0000, Sean Christopherson wrote:
> > On Wed, Jun 08, 2022, Vishal Annapurve wrote:
> >
> > One argument is that userspace can simply rely on cgroups to detect misbehaving
> > guests, but (a) those types of OOMs will be a nightmare to debug and (b) an OOM
> > kill from the host is typically considered a _host_ issue and will be treated as
> > a missed SLO.
> >
> > An idea for handling this in the kernel without too much complexity would be to
> > add F_SEAL_FAULT_ALLOCATIONS (terrible name) that would prevent page faults from
> > allocating pages, i.e. holes can only be filled by an explicit fallocate().  Minor
> > faults, e.g. due to NUMA balancing stupidity, and major faults due to swap would
> > still work, but writes to previously unreserved/unallocated memory would get a
> > SIGSEGV on something it has mapped.  That would allow the userspace VMM to prevent
> > unintentional allocations without having to coordinate unmapping/remapping across
> > multiple processes.
>
> Since this is mainly for shared memory and the motivation is catching
> misbehaved access, can we use mprotect(PROT_NONE) for this? We can mark
> those range backed by private fd as PROT_NONE during the conversion so
> subsequence misbehaved accesses will be blocked instead of causing double
> allocation silently.

This patch series is fairly close to implementing a rather more
efficient solution.  I'm not familiar enough with hypervisor userspace
to really know if this would work, but:

What if shared guest memory could also be file-backed, either in the
same fd or with a second fd covering the shared portion of a memslot?
This would allow changes to the backing store (punching holes, etc) to
be some without mmap_lock or host-userspace TLB flushes?  Depending on
what the guest is doing with its shared memory, userspace might need
the memory mapped or it might not.

--Andy