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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
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