On 9/25/26 06:48, Andrew Morton wrote: > On Fri, 25 Sep 2026 13:29:06 +0900 Donggeun Yoo > <[email protected]> wrote: > >> UFFDIO_MOVE on a swapped-out page installs the source PTE at the >> destination unchanged, so a uffd bit set on the source lands in a >> destination VMA that was never registered for write protection. It >> can't be unset there, and THP collapse can't happen either, because a >> swap entry with the uffd bit set makes the scan bail. >> >> I don't think it's intentional because for an unswapped page the bit >> doesn't come along, and I don't see why being swapped out should change >> that. >> >> Clear the uffd bit on the moved swap entry unless the destination is >> RWP-registered. > > Thanks, but... > > When fixing a bug, please always fully describe the userspace-visible > effects of that bug.
It would also be helpful to describe if this was actually hit in practice so far. IOW, how relevant is this in practice. Will this trigger a kernel splat later? Someone would have to use UFFD_WP/UFFD_RWP with UFFDIO_MOVE, and have UFFD_WP/UFFD_RWP disabled on the destination vma ... and then work on a swap entry. Code itself LGTM -- Cheers, David
