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

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Date:   Thu, 12 Aug 2021 10:19:59 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     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>,
        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

On Tue, Aug 10, 2021 at 02:20:08PM -0700, Andi Kleen wrote:
> 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.

I also agree that the slow-path needs to be outside of the memory
allocator locks. But I think this conflicts with the concept of
accepting memory in 2MB chunks even if allocation size is smaller.

Given some kernel code allocated 2 pages and the allocator path starts
to validate the whole 2MB page the memory is on, then there are
potential races to take into account.

Either some other code path allocates memory from that page and returns
it before validation is finished or we end up with double validation.
Returning unvalidated memory is a guest-problem and double validation
will cause security issues for SNP guests.

Regards,

	Joerg