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 >
