On 7/15/26 16:42, Kiryl Shutsemau wrote: > From: "Kiryl Shutsemau (Meta)" <[email protected]> > > PAGEMAP_SCAN reports an unpopulated PTE in a uffd-wp VMA as written, but > a range with no page table at all -- a PMD hole -- is skipped: > pagemap_scan_pte_hole() tests p->cur_vma_category, which never carries > PAGE_IS_WRITTEN, so the hole is neither reported nor (under > PM_SCAN_WP_MATCHING) armed. > > In a uffd-wp VMA, WP_UNPOPULATED installs uffd-wp markers when protecting > a range, allocating page tables as needed, so an unpopulated slot is > treated as written -- see the pte_none() handling in > pagemap_page_category(). A missing marker therefore means the range was > zapped, e.g. via MADV_DONTNEED. This applies to anon and shmem VMAs. > > An anonymous THP is write-protected in place as a huge PMD, so a > full-PMD MADV_DONTNEED clears it to pmd_none -- a hole with no page table > -- and pagemap_scan_pte_hole() misses it. For a MAP_PRIVATE|MAP_ANON > mapping MADV_DONTNEED has fill-with-zeros semantics, so a write-tracking > checkpoint/migration tool (e.g. CRIU) treats the range as unchanged and > keeps its previous contents; after restore or live migration the process > reads stale data instead of zeroes -- data corruption. > > Report a hole in a non-hugetlb uffd-wp VMA as written, matching the > pte_none handling in pagemap_page_category(); the existing > PM_SCAN_WP_MATCHING path then arms it via uffd_wp_range(). > > hugetlb is excluded: pagemap_hugetlb_category() reports an empty hugetlb > entry (huge_pte_none) as not-written, unlike pagemap_page_category(), > which reports pte_none as written. pagemap_scan_pte_hole() fires for a > hugetlb slot only when it has no page table; keeping that not-written > matches how an allocated-but-empty hugetlb entry reads, so the hole and > the empty-entry cases agree within the VMA. > > Reported-by: Sashiko AI review <[email protected]> > Closes: > https://sashiko.dev/#/patchset/[email protected] > Fixes: 2bad466cc9d9 ("mm/uffd: UFFD_FEATURE_WP_UNPOPULATED") > Cc: Muhammad Usama Anjum <[email protected]> > Cc: Peter Xu <[email protected]> > Cc: [email protected] > Signed-off-by: Kiryl Shutsemau <[email protected]> > Assisted-by: Claude:claude-fable-5 > ---
Thanks! Acked-by: David Hildenbrand (Arm) <[email protected]> -- Cheers, David
