linux-kernel - Re: [PATCH v5 04/13] mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT

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Message-ID: <2c39702b-2a71-cda2-685-93908763912@google.com>
Date:   Mon, 11 Apr 2022 22:14:00 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
cc:     Chao Peng <chao.p.peng@...ux.intel.com>,
        Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...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@...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>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Shakeel Butt <shakeelb@...gle.com>, luto@...nel.org,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [PATCH v5 04/13] mm/shmem: Restrict MFD_INACCESSIBLE memory
 against RLIMIT_MEMLOCK

On Mon, 11 Apr 2022, Kirill A. Shutemov wrote:
> On Fri, Apr 08, 2022 at 09:02:54PM +0800, Chao Peng wrote:
> > > I think the correct approach is to not do the locking automatically for SHM_F_INACCESSIBLE,
> > > and instead require userspace to do shmctl(.., SHM_LOCK, ...) if userspace knows the
> > > consumers don't support migrate/swap.  That'd require wrapping migrate_page() and then
> > > wiring up notifier hooks for migrate/swap, but IMO that's a good thing to get sorted
> > > out sooner than later.  KVM isn't planning on support migrate/swap for TDX or SNP,
> > > but supporting at least migrate for a software-only implementation a la pKVM should
> > > be relatively straightforward.  On the notifiee side, KVM can terminate the VM if it
> > > gets an unexpected migrate/swap, e.g. so that TDX/SEV VMs don't die later with
> > > exceptions and/or data corruption (pre-SNP SEV guests) in the guest.
> > 
> > SHM_LOCK sounds like a good match.
> 
> Emm, no. shmctl(2) and SHM_LOCK are SysV IPC thing. I don't see how they
> fit here.

I am still struggling to formulate a constructive response on
MFD_INACCESSIBLE in general: but before doing so, let me jump in here
to say that I'm firmly on the side of SHM_LOCK being the right model -
but admittedly not through userspace calling shmctl(2).

Please refer to our last year's posting "[PATCH 10/16] tmpfs: fcntl(fd,
F_MEM_LOCK) to memlock a tmpfs file" for the example of how Shakeel did
it then (though only a small part of that would be needed for this case):
https://lore.kernel.org/linux-mm/54e03798-d836-ae64-f41-4a1d46bc115b@google.com/

And until such time as swapping is enabled, this memlock accounting would
be necessarily entailed by "MFD_INACCESSIBLE", or however that turns out
to be implemented: not something that we could trust userspace to call
separately.

Hugh