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

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Date: Thu, 20 Jun 2024 15:47:23 -0700
From: Elliot Berman <quic_eberman@...cinc.com>
To: Jason Gunthorpe <jgg@...dia.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 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. The host and guest can
negotiate the minimum size that can be converted and you never run into
issue where subpages of a folio are differently shared. I can't think of
a usecase where we need such granularity for converting private/shared.

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.

Thanks,
Elliot