linux-kernel - Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

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Date: Thu, 20 Jun 2024 16:01:08 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>, Fuad Tabba <tabba@...gle.com>
Cc: Christoph Hellwig <hch@...radead.org>, John Hubbard
 <jhubbard@...dia.com>, Elliot Berman <quic_eberman@...cinc.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Matthew Wilcox <willy@...radead.org>, maz@...nel.org, kvm@...r.kernel.org,
 linux-arm-msm@...r.kernel.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
 pbonzini@...hat.com
Subject: Re: [PATCH RFC 0/5] mm/gup: Introduce exclusive GUP pinning

On 20.06.24 15:55, Jason Gunthorpe wrote:
> On Thu, Jun 20, 2024 at 09:32:11AM +0100, Fuad Tabba wrote:
>> Hi,
>>
>> On Thu, Jun 20, 2024 at 5:11 AM Christoph Hellwig <hch@...radead.org> wrote:
>>>
>>> On Wed, Jun 19, 2024 at 08:51:35AM -0300, Jason Gunthorpe wrote:
>>>> If you can't agree with the guest_memfd people on how to get there
>>>> then maybe you need a guest_memfd2 for this slightly different special
>>>> stuff instead of intruding on the core mm so much. (though that would
>>>> be sad)
>>>
>>> Or we're just not going to support it at all.  It's not like supporting
>>> this weird usage model is a must-have for Linux to start with.
>>
>> Sorry, but could you please clarify to me what usage model you're
>> referring to exactly, and why you think it's weird? It's just that we
>> have covered a few things in this thread, and to me it's not clear if
>> you're referring to protected VMs sharing memory, or being able to
>> (conditionally) map a VM's memory that's backed by guest_memfd(), or
>> if it's the Exclusive pin.
> 
> Personally I think mapping memory under guest_memfd is pretty weird.
> 
> I don't really understand why you end up with something different than
> normal CC. Normal CC has memory that the VMM can access and memory it
> cannot access. guest_memory is supposed to hold the memory the VMM cannot
> reach, right?
> 
> So how does normal CC handle memory switching between private and
> shared and why doesn't that work for pKVM? I think the normal CC path
> effectively discards the memory content on these switches and is
> slow. Are you trying to make the switch content preserving and faster?
> 
> If yes, why? What is wrong with the normal CC model of slow and
> non-preserving shared memory?

I'll leave the !huge page part to Fuad.

Regarding huge pages: assume the huge page (e.g., 1 GiB hugetlb) is 
shared, now the VM requests to make one subpage private. How to handle 
that without eventually running into a double memory-allocation? (in the 
worst case, allocating a 1GiB huge page for shared and for private memory).

In the world of RT, you want your VM to be consistently backed by 
huge/gigantic mappings, not some weird mixture -- so I've been told by 
our RT team.

(there are more issues with huge pages in the style hugetlb, where we 
actually want to preallocate all pages and not rely on dynamic 
allocation at runtime when we convert back and forth between shared and 
private)

-- 
Cheers,

David / dhildenb