On Mon, Dec 01, 2025 at 04:48:22PM +0000, Nikita Kalyazin wrote: > I believe I found the precise point where we convinced ourselves that minor > support was sufficient: [1]. If at this moment we don't find that reasoning > valid anymore, then indeed implementing missing is the only option. > > [1] https://lore.kernel.org/kvm/[email protected]
Now after I re-read the discussion, I may have made a wrong statement there, sorry. I could have got slightly confused on when the write() syscall can be involved. I agree if you want to get an event when cache missed with the current uffd definitions and when pre-population is forbidden, then MISSING trap is required. That is, with/without the need of UFFDIO_COPY being available. Do I understand it right that UFFDIO_COPY is not allowed in your case, but only write()? One way that might work this around, is introducing a new UFFD_FEATURE bit allowing the MINOR registration to trap all pgtable faults, which will change the MINOR fault semantics. That'll need some further thoughts, meanwhile we may also want to make sure the old shmem/hugetlbfs semantics are kept (e.g. they should fail MINOR registers if the new feature bit is enabled in the ctx somehow; or support them properly in the codebase). Thanks, -- Peter Xu
