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

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Date: Thu, 20 Jun 2024 20:18:14 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Elliot Berman <quic_eberman@...cinc.com>
Cc: David Hildenbrand <david@...hat.com>, Fuad Tabba <tabba@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	John Hubbard <jhubbard@...dia.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 Thu, Jun 20, 2024 at 03:47:23PM -0700, Elliot Berman wrote:
> On Thu, Jun 20, 2024 at 11:29:56AM -0300, Jason Gunthorpe wrote:
> > On Thu, Jun 20, 2024 at 04:01:08PM +0200, David Hildenbrand wrote:
> > > Regarding huge pages: assume the huge page (e.g., 1 GiB hugetlb) is shared,
> > > now the VM requests to make one subpage private. 
> > 
> > I think the general CC model has the shared/private setup earlier on
> > the VM lifecycle with large runs of contiguous pages. It would only
> > become a problem if you intend to to high rate fine granual
> > shared/private switching. Which is why I am asking what the actual
> > "why" is here.
> > 
> 
> I'd let Fuad comment if he's aware of any specific/concrete Anrdoid
> usecases about converting between shared and private. One usecase I can
> think about is host providing large multimedia blobs (e.g. video) to the
> guest. Rather than using swiotlb, the CC guest can share pages back with
> the host so host can copy the blob in, possibly using H/W accel. I
> mention this example because we may not need to support shared/private
> conversions at granularity finer than huge pages. 

I suspect the more useful thing would be to be able to allocate actual
shared memory and use that to shuffle data without a copy, setup much
less frequently. Ie you could allocate a large shared buffer for video
sharing and stream the video frames through that memory without copy.

This is slightly different from converting arbitary memory in-place
into shared memory. The VM may be able to do a better job at
clustering the shared memory allocation requests, ie locate them all
within a 1GB region to further optimize the host side.

> Jason, do you have scenario in mind? I couldn't tell if we now had a
> usecase or are brainstorming a solution to have a solution.

No, I'm interested in what pKVM is doing that needs this to be so much
different than the CC case..

Jason