linux-kernel - Re: [PATCH 1/5] mm: Add support for unaccepted memory

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Date:   Tue, 10 Aug 2021 14:20:08 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 1/5] mm: Add support for unaccepted memory


> These boot blips are not the biggest issue in the world.  But, it is
> fully under the guest's control and I think the guest has some
> responsibility to provide *some* mitigation for it.

It sounds more like an exercise in preliminary optimization at this 
point. If it's a real problem we can still worry about it later.


> 1. Do background acceptance, as opposed to relying 100% on demand-driven
>     acceptance.  Guarantees a limited window in which blips can occur.

Like Kirill wrote this was abandoned because it always allocates all 
memory on the host even if the guest doesn't need it.


> 2. Do acceptance based on user input, like from sysfs.

You can easily do that by running "memhog" at boot. No need for anything 
in the kernel.

BTW I believe this is also configurable at the guest BIOS level.

> 3. Add a command-line argument to accept everything up front, or at
>     least before userspace runs.

Same.


> 4. Add some statistic for how much unaccepted memory remains.

Yes that makes sense. We should have statistic counters for all of this.

Also I agree with your suggestion that we should get the slow path out 
of the zone locks/interrupt disable region. That should be easy enough 
and is an obvious improvement.

-Andi