Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

[Peter Xu]

On Mon, Oct 02, 2023 at 10:00:03AM +0200, David Hildenbrand wrote:
> In case we cannot simply remap the page, the fallback sequence (from the
> cover letter) would be triggered.
> 
> 1) UFFDIO_COPY
> 2) MADV_DONTNEED
> 
> So we would just handle the operation internally without a fallback.

Note that I think there will be a slight difference on whole remap
atomicity, on what happens if the page is modified after UFFDIO_COPY but
before DONTNEED.

UFFDIO_REMAP guarantees full atomicity when moving the page, IOW, threads
can be updating the pages when ioctl(UFFDIO_REMAP), data won't get lost
during movement, and it will generate a missing event after moved, with
latest data showing up on dest.

I'm not sure that means such a fallback is a problem, Suren may know
better with the use case.

Thanks,

-- 
Peter Xu