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

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Message-ID: <7b830dc4-37bc-fb7b-c094-16595bd2a128@redhat.com>
Date:   Tue, 15 Feb 2022 13:15:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.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 11.02.22 17:56, Jason Gunthorpe wrote:
> 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.

For me, PageAnon() means that modifications are visible only to the
modifying process. On actual CoW, the underlying page will get replaced
-- in the world of DEVICE_COHERENT that would mean that once you write
to a DEVICE_COHERENT you could suddenly have a !DEVICE_COHERENT page.

PageAnon() pages don't have a mapping, thus they can only be found in
MAP_ANON VMAs or in MAP_SHARED VMAs with MAP_PRIVATE. They can only be
found via a page table, and not looked up via the page cache (excluding
the swap cache).

So if we have PageAnon() pages on ZONE_DEVICE, they generally have the
exact same semantics as !ZONE_DEVICE pages, but the way they "appear" in
the page tables the allocation/freeing path differs -- I guess :)

... and as we want pinning semantics to be different we have to touch GUP.

> 
> 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?

My understanding is that a driver can just decide to replace "ordinary"
PageAnon() pages e.g., in a MAP_ANON VMA by these pages. Hopefully AMD
can clarify.


-- 
Thanks,

David / dhildenb