On Wed, Feb 16, 2022 at 09:31:03AM +0100, David Hildenbrand wrote: > On 16.02.22 03:36, Alistair Popple wrote: > > 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'm curious, how does the refcount of a PageAnon() DEVICE_COHERENT page > look like when mapped? I'd assume it's also (currently) still offset by > one, meaning, if it's mapped into a single page table it's always at > least 2. Christoph fixed this offset by one and updated the DEVICE_COHERENT patchset, I hope we will see that version merged. > >> 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. > > We really should have the same pinning rules for GUP vs. GUP-fast. > is_pinnable_page() should be the right place for such checks (similarly > as indicated in my reply to the migration series). Yes, I think this is a bug too. The other place that needs careful audit is all the callers using vm_normal_page() - they must all be able to accept a ZONE_DEVICE page if we don't set pte_devmap. Jason
