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. Just a note that if my assumption is correct and if we'd have such a page mapped R/O, do_wp_page() would always have to copy it unconditionally and would not be able to reuse it on write faults. (while I'm working on improving the reuse logic, I think there is also work in progress to avoid this additional reference on some ZONE_DEVICE stuff -- I'd assume that would include DEVICE_COHERENT ?) > >> 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). -- Thanks, David / dhildenb
