linux-kernel - Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

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Message-ID: <ebf3ccef-e6fe-62d5-74e8-91e30e7c2642@intel.com>
Date:   Tue, 12 Apr 2022 09:08:43 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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>,
        Ard Biesheuvel <ardb@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        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>,
        Brijesh Singh <brijesh.singh@....com>,
        Mike Rapoport <rppt@...nel.org>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

On 4/12/22 01:15, David Hildenbrand wrote:
> Can we simply automate this using a kthread or smth like that, which
> just traverses the free page lists and accepts pages (similar, but
> different to free page reporting)?

That's definitely doable.

The downside is that this will force premature consumption of physical
memory resources that the guest may never use.  That's a particular
problem on TDX systems since there is no way for a VMM to reclaim guest
memory short of killing the guest.

In other words, I can see a good argument either way:
1. The kernel should accept everything to avoid the perf nastiness
2. The kernel should accept only what it needs in order to reduce memory
   use

I'm kinda partial to #1 though, if I had to pick only one.

The other option might be to tie this all to DEFERRED_STRUCT_PAGE_INIT.
 Have the rule that everything that gets a 'struct page' must be
accepted.  If you want to do delayed acceptance, you do it via
DEFERRED_STRUCT_PAGE_INIT.