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

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,