mm/huge_memory: preserve pmd_swp_uffd_wp on device-private PMD downgrade

change_non_present_huge_pmd() rewrites a writable device-private PMD swap
entry into a readable one without carrying pmd_swp_uffd_wp() across.  The
PTE-level change_softleaf_pte() does this correctly; mirror that here,
matching what copy_huge_pmd() does for the fork path.  Without the carry,
a plain mprotect() over a UFFD_WP-marked device-private THP strips the bit
and the trap is bypassed on swap-in.

Link: https://lore.kernel.org/20260529172331.356655-5-kas@kernel.org
Fixes: 368076f52ebe ("mm/huge_memory: add device-private THP support to PMD operations")
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Reviewed-by: Balbir Singh <balbirs@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index da851a5696d5aa2f1713fe6324321f63f47492b8..a5176653ba1f214c350ce16689f374f8ea5cb0d8 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2565,6 +2565,8 @@ static void change_non_present_huge_pmd(struct mm_struct *mm,
        } else if (softleaf_is_device_private_write(entry)) {
                entry = make_readable_device_private_entry(swp_offset(entry));
                newpmd = swp_entry_to_pmd(entry);
+               if (pmd_swp_uffd_wp(*pmd))
+                       newpmd = pmd_swp_mkuffd_wp(newpmd);
        } else {
                newpmd = *pmd;
        }