On Wed, Dec 07, 2022 at 08:53:36PM +0100, David Hildenbrand wrote: > Once user space successfully placed an uffd-wp marker, and e.g., verified > using pagemap that it is indeed placed, the system should not silently drop > it. Note that the anon path doesn't use pte markers. We won't lose a pte marker, hopefully, if we do that's a more severe one. > > The behavior between an ordinary THP and a huge zeropage differs. For THP, > we handle the split correctly and don't lose the marker. Assuming the huge > zeropage woud be disabled, the behavior would be (IMHO) correct. The test > case would pass. > > For example, QEMU with uffd-wp based snapshotting will make sure that all > virtual addresses are populated (e.g., mapping the shared, eventually the > huge zeropage -- populate_read_range()), before protecting using uffd-wp. > Losing a uffd-wp marker would be problematic. > > The good news is that we barely will end up PTE-mapping the huge zeropage > unless there is real user-space interaction (mprotect(), mremap(), mmap()), > so this shouldn't trigger in the QEMU use-case. Ah yes, I forgot that part. If it's not affected then it's better. > > > Anyhow, I'll send a patch in a couple of days and we can discuss further. > It's independent of the other discussion, just wanted to report my findings > after staring at that code for way too long today. Thanks, that works for me. -- Peter Xu
