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

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Message-ID: <ac3d5157-9251-f9fb-a973-f268ce58b4e0@amd.com>
Date:   Tue, 15 Feb 2022 16:35:56 -0500
From:   Felix Kuehling <felix.kuehling@....com>
To:     Jason Gunthorpe <jgg@...dia.com>, Christoph Hellwig <hch@....de>
Cc:     David Hildenbrand <david@...hat.com>,
        Alex Sierra <alex.sierra@....com>, akpm@...ux-foundation.org,
        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,
        jglisse@...hat.com, apopple@...dia.com, willy@...radead.org
Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support


On 2022-02-15 14:41, Jason Gunthorpe wrote:
> On Tue, Feb 15, 2022 at 07:32:09PM +0100, Christoph Hellwig wrote:
>> On Tue, Feb 15, 2022 at 10:45:24AM -0400, Jason Gunthorpe wrote:
>>>> Do you know if DEVICE_GENERIC pages would end up as PageAnon()? My
>>>> assumption was that they would be part of a special mapping.
>>> We need to stop using the special PTEs and VMAs for things that have a
>>> struct page. This is a mistake DAX created that must be undone.
>> Yes, we'll get to it.  Maybe we can do it for the non-DAX devmap
>> ptes first given that DAX is more complicated.
> Probably, I think we can check the page->pgmap type to tell the
> difference.
>
> I'm not sure how the DEVICE_GENERIC can work without this, as DAX was
> made safe by using the unmap_mapping_range(), which won't work
> here. Is there some other trick being used to keep track of references
> inside the AMD driver?

Not sure I'm following all the discussion about VMAs and DAX. So I may 
be answering the wrong question: We treat each ZONE_DEVICE page as a 
reference to the BO (buffer object) that backs the page. We increment 
the BO refcount for each page we migrate into it. In the 
dev_pagemap_ops.page_free callback we drop that reference. Once all 
pages backed by a BO are freed, the BO refcount reaches 0 [*] and we can 
free the BO allocation.

Regards,
   Felix


[*] That's a somewhat simplified view. There may be other references to 
the BO, which allows us to reuse the same BO for the same virtual 
address range.

>
> Jason