linux-ext4 - Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

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Date:   Fri, 11 Feb 2022 12:56:24 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alex Sierra <alex.sierra@....com>, akpm@...ux-foundation.org,
        Felix.Kuehling@....com, linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jglisse@...hat.com, apopple@...dia.com,
        willy@...radead.org
Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support

On Fri, Feb 11, 2022 at 05:49:08PM +0100, David Hildenbrand wrote:
> On 11.02.22 17:45, Jason Gunthorpe wrote:
> > On Fri, Feb 11, 2022 at 05:15:25PM +0100, David Hildenbrand wrote:
> > 
> >> ... I'm pretty sure we cannot FOLL_PIN DEVICE_PRIVATE pages
> > 
> > Currently the only way to get a DEVICE_PRIVATE page out of the page
> > tables is via hmm_range_fault() and that doesn't manipulate any ref
> > counts.
> 
> Thanks for clarifying Jason! ... and AFAIU, device exclusive entries are
> essentially just pointers at ordinary PageAnon() pages. So with DEVICE
> COHERENT we'll have the first PageAnon() ZONE_DEVICE pages mapped as
> present in the page tables where GUP could FOLL_PIN them.

This is my understanding

Though you probably understand what PageAnon means alot better than I
do.. I wonder if it really makes sense to talk about that together
with ZONE_DEVICE which has alot in common with filesystem originated
pages too.

I'm not sure what AMDs plan is here, is there an expecation that a GPU
driver will somehow stuff these pages into an existing anonymous
memory VMA or do they always come from a driver originated VMA?

Jason