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