Re: [PATCH v3 4/5] guest_memfd: add support for userfaultfd minor mode

[Nikita Kalyazin <kalyazin@amazon.com>]

On 01/12/2025 18:35, Peter Xu wrote:
> 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/Z9GsIDVYWoV8d8-C@x1.local
> 
> 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()?

No, UFFDIO_COPY would work perfectly fine.  We will still use write() 
whenever we resolve stage-2 faults as they aren't visible to UFFD.  When 
a userfault occurs at an offset that already has a page in the cache, we 
will have to keep using UFFDIO_CONTINUE so it looks like both will be 
required:

  - user mapping major fault -> UFFDIO_COPY (fills the cache and sets up 
userspace PT)
  - user mapping minor fault -> UFFDIO_CONTINUE (only sets up userspace PT)
  - stage-2 fault -> write() (only fills the cache)

> 
> 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.

This would equally work for us.  I suppose this MINOR+MAJOR semantics 
would be more intrusive from the API point of view though.

> 
> 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
>