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Date: Wed, 16 Feb 2022 13:36:30 +1100 From: Alistair Popple <apopple@...dia.com> To: Jason Gunthorpe <jgg@...dia.com> CC: Felix Kuehling <felix.kuehling@....com>, Christoph Hellwig <hch@....de>, 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>, <willy@...radead.org> Subject: Re: [PATCH v6 01/10] mm: add zone device coherent type memory support On Wednesday, 16 February 2022 1:03:57 PM AEDT Jason Gunthorpe wrote: > On Wed, Feb 16, 2022 at 12:23:44PM +1100, Alistair Popple wrote: > > > Device private and device coherent pages are not marked with pte_devmap and they > > are backed by a struct page. The only way of inserting them is via migrate_vma. > > The refcount is decremented in zap_pte_range() on munmap() with special handling > > for device private pages. Looking at it again though I wonder if there is any > > special treatment required in zap_pte_range() for device coherent pages given > > they count as present pages. > > This is what I guessed, but we shouldn't be able to just drop > pte_devmap on these pages without any other work?? Granted it does > very little already.. Yes, I agree we need to check this more closely. For device private pages not having pte_devmap is fine, because they are non-present swap entries so they always get special handling in the swap entry paths but the same isn't true for coherent device pages. > I thought at least gup_fast needed to be touched or did this get > handled by scanning the page list after the fact? Right, for gup I think the only special handling required is to prevent pinning. I had assumed that check_and_migrate_movable_pages() would still get called for gup_fast but unless I've missed something I don't think it does. That means gup_fast could still pin movable and coherent pages. Technically that is ok for coherent pages, but it's undesirable. - Alistair > Jason >
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