Jason Gunthorpe <jgg@xxxxxxxxxx> writes: > 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. Agreed, I will add a fix for it to my series as I was surprised the rules for PUP-fast were different. I can see how this happened though - check_and_migrate_cma_pages() (the precursor to check_and_migrate_movable_pages()) was added before PUP-fast and FOLL_LONGTERM so I guess we just never added this check there. - Alistair > 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
