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

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Date:   Wed, 13 Apr 2022 12:36:11 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.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 12.04.22 18:08, Dave Hansen wrote:
> 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.

IIRC, the hypervisor will usually effectively populate all guest RAM
either way right now. So yes, for hypervisors that might optimize for
that, that statement would be true. But I lost track how helpful it
would be in the near future e.g., with the fd-based private guest memory
-- maybe they already optimize for delayed acceptance of memory, turning
it into delayed population.

> 
> 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.

That could also be an option, yes. At least being able to chose would be
good. But IIRC, DEFERRED_STRUCT_PAGE_INIT will still make the system get
stuck during boot and wait until everything was accepted.

I see the following variants:

1) Slow boot; after boot, all memory is already accepted.
2) Fast boot; after boot, all memory will slowly but steadily get
   accepted in the background. After a while, all memory is accepted and
   can be signaled to user space.
3) Fast boot; after boot, memory gets accepted on demand. This is what
   we have in this series.

I somehow don't quite like 3), but with deferred population in the
hypervisor, it might just make sense.

-- 
Thanks,

David / dhildenb