From: Peter Xu Subject: mm/khugepaged: don't recycle vma pgtable if uffd-wp registered When we're trying to collapse a 2M huge shmem page, don't retract pgtable pmd page if it's registered with uffd-wp, because that pgtable could have pte markers installed. Recycling of that pgtable means we'll lose the pte markers. That could cause data loss for an uffd-wp enabled application on shmem. Instead of disabling khugepaged on these files, simply skip retracting these special VMAs, then the page cache can still be merged into a huge thp, and other mm/vma can still map the range of file with a huge thp when proper. Note that checking VM_UFFD_WP needs to be done with mmap_sem held for write, that avoids race like: khugepaged user thread ========== =========== check VM_UFFD_WP, not set UFFDIO_REGISTER with uffd-wp on shmem wr-protect some pages (install markers) take mmap_sem write lock erase pmd and free pmd page --> pte markers are dropped unnoticed! Link: https://lkml.kernel.org/r/20220405014921.14994-1-peterx@redhat.com Signed-off-by: Peter Xu Cc: Alistair Popple Cc: Andrea Arcangeli Cc: Axel Rasmussen Cc: David Hildenbrand Cc: Hugh Dickins Cc: Jerome Glisse Cc: "Kirill A . Shutemov" Cc: Matthew Wilcox Cc: Mike Kravetz Cc: Mike Rapoport Cc: Nadav Amit Signed-off-by: Andrew Morton --- mm/khugepaged.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) --- a/mm/khugepaged.c~mm-khugepaged-dont-recycle-vma-pgtable-if-uffd-wp-registered +++ a/mm/khugepaged.c @@ -1474,6 +1474,10 @@ void collapse_pte_mapped_thp(struct mm_s if (!hugepage_vma_check(vma, vma->vm_flags | VM_HUGEPAGE)) return; + /* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */ + if (userfaultfd_wp(vma)) + return; + hpage = find_lock_page(vma->vm_file->f_mapping, linear_page_index(vma, haddr)); if (!hpage) @@ -1609,7 +1613,15 @@ static void retract_page_tables(struct a * reverse order. Trylock is a way to avoid deadlock. */ if (mmap_write_trylock(mm)) { - if (!khugepaged_test_exit(mm)) + /* + * When a vma is registered with uffd-wp, we can't + * recycle the pmd pgtable because there can be pte + * markers installed. Skip it only, so the rest mm/vma + * can still have the same file mapped hugely, however + * it'll always mapped in small page size for uffd-wp + * registered ranges. + */ + if (!khugepaged_test_exit(mm) && !userfaultfd_wp(vma)) collapse_and_free_pmd(mm, vma, addr, pmd); mmap_write_unlock(mm); } else { _