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

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Message-ID: <20220215194107.GZ4160@nvidia.com>
Date:   Tue, 15 Feb 2022 15:41:07 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@....de>
Cc:     David Hildenbrand <david@...hat.com>,
        Felix Kuehling <felix.kuehling@....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 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?

Jason