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

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Message-ID: <20220413175208.GI64706@ziepe.ca>
Date:   Wed, 13 Apr 2022 14:52:08 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     David Hildenbrand <david@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Linux API <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>,
        the arch/x86 maintainers <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>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v5 04/13] mm/shmem: Restrict MFD_INACCESSIBLE memory
 against RLIMIT_MEMLOCK

On Wed, Apr 13, 2022 at 06:24:56PM +0200, David Hildenbrand wrote:
> On 12.04.22 16:36, Jason Gunthorpe wrote:
> > On Fri, Apr 08, 2022 at 08:54:02PM +0200, David Hildenbrand wrote:
> > 
> >> RLIMIT_MEMLOCK was the obvious candidate, but as we discovered int he
> >> past already with secretmem, it's not 100% that good of a fit (unmovable
> >> is worth than mlocked). But it gets the job done for now at least.
> > 
> > No, it doesn't. There are too many different interpretations how
> > MELOCK is supposed to work
> > 
> > eg VFIO accounts per-process so hostile users can just fork to go past
> > it.
> > 
> > RDMA is per-process but uses a different counter, so you can double up
> > 
> > iouring is per-user and users a 3rd counter, so it can triple up on
> > the above two
> 
> Thanks for that summary, very helpful.

I kicked off a big discussion when I suggested to change vfio to use
the same as io_uring

We may still end up trying it, but the major concern is that libvirt
sets the RLIMIT_MEMLOCK and if we touch anything here - including
fixing RDMA, or anything really, it becomes a uAPI break for libvirt..

> >> So I'm open for alternative to limit the amount of unmovable memory we
> >> might allocate for user space, and then we could convert seretmem as well.
> > 
> > I think it has to be cgroup based considering where we are now :\
> 
> Most probably. I think the important lessons we learned are that
> 
> * mlocked != unmovable.
> * RLIMIT_MEMLOCK should most probably never have been abused for
>   unmovable memory (especially, long-term pinning)

The trouble is I'm not sure how anything can correctly/meaningfully
set a limit.

Consider qemu where we might have 3 different things all pinning the
same page (rdma, iouring, vfio) - should the cgroup give 3x the limit?
What use is that really?

IMHO there are only two meaningful scenarios - either you are unpriv
and limited to a very small number for your user/cgroup - or you are
priv and you can do whatever you want.

The idea we can fine tune this to exactly the right amount for a
workload does not seem realistic and ends up exporting internal kernel
decisions into a uAPI..

Jason