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.
MADV_DONTNEED has fill-with-zeros semantics: it changes the contents of the range to zeroes (a subsequent read maps the zero page), which write tracking must report as written. 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 -- and the zeroing goes unreported. A write-tracking checkpoint/migration tool (e.g. CRIU) then treats the range as unchanged and keeps its previous contents, so 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. Add a pagemap_ioctl selftest covering the anon-THP PMD-hole case. 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 --- Changes since v2 [1], addressing Andrew's review: - Describe the user-visible effect: MADV_DONTNEED has fill-with-zeros semantics, so the range must be reported written; otherwise a checkpoint/migration tool (CRIU) keeps stale data and the process reads corrupted contents after restore. Add Reported-by/Closes. - Reword the hugetlb carve-out to rest on the category functions: pagemap_hugetlb_category() reads an empty hugetlb entry as not-written, unlike pagemap_page_category(). - Drop the redundant MADV_COLLAPSE fallback #define; it is in <asm-generic/mman-common.h> and used directly by other mm selftests. No functional change to the fix itself since v2. [1] https://lore.kernel.org/all/[email protected]/ fs/proc/task_mmu.c | 27 ++++++++++- tools/testing/selftests/mm/pagemap_ioctl.c | 56 +++++++++++++++++++++- 2 files changed, 80 insertions(+), 3 deletions(-) diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index d45c729ab6bb..03ead4184546 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -3049,12 +3049,35 @@ static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end, { struct pagemap_scan_private *p = walk->private; struct vm_area_struct *vma = walk->vma; + unsigned long categories; int ret, err; - if (!vma || !pagemap_scan_is_interesting_page(p->cur_vma_category, p)) + if (!vma) return 0; - ret = pagemap_scan_output(p->cur_vma_category, p, addr, &end); + /* + * An unpopulated range with no page table -- e.g. a 2MB anon THP + * dropped via MADV_DONTNEED, which pagemap_page_category() never sees + * -- reads as written on a uffd-wp VMA, matching the pte_none case + * there. Reporting it also lets the PM_SCAN_WP_MATCHING arming below + * install markers (uffd_wp_range() allocates the page table under + * WP_UNPOPULATED), so the next scan sees it clean until re-written. + * + * 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. This + * path 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 two agree within the VMA. + */ + categories = p->cur_vma_category; + if (userfaultfd_wp(vma) && !is_vm_hugetlb_page(vma)) + categories |= PAGE_IS_WRITTEN; + + if (!pagemap_scan_is_interesting_page(categories, p)) + return 0; + + ret = pagemap_scan_output(categories, p, addr, &end); if (addr == end) return ret; diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c b/tools/testing/selftests/mm/pagemap_ioctl.c index 550d1f2c059b..560499eb4737 100644 --- a/tools/testing/selftests/mm/pagemap_ioctl.c +++ b/tools/testing/selftests/mm/pagemap_ioctl.c @@ -1102,6 +1102,59 @@ static void unpopulated_scan_test(void) munmap(mem, mem_size); } +/* + * A 2MB anon THP dropped with MADV_DONTNEED leaves a pmd_none hole with no + * page table, which pagemap_page_category() never sees. PAGEMAP_SCAN must + * still report it as written on a uffd-wp VMA, via pagemap_scan_pte_hole(). + */ +static void unpopulated_thp_hole_test(void) +{ + long npages, written = 0, ret, i; + struct page_region regions[16]; + char *area, *mem; + + if (!hpage_size) { + ksft_test_result_skip("%s THP not supported\n", __func__); + return; + } + npages = hpage_size / page_size; + + /* Get a PMD-aligned range so the range can be a single THP. */ + area = mmap(NULL, 2 * hpage_size, PROT_READ | PROT_WRITE, + MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); + if (area == MAP_FAILED) + ksft_exit_fail_msg("%s mmap failed\n", __func__); + mem = (char *)(((unsigned long)area + hpage_size - 1) & ~(hpage_size - 1)); + + memset(mem, 1, hpage_size); + if (madvise(mem, hpage_size, MADV_COLLAPSE) || + !check_huge_anon(mem, 1, hpage_size)) { + ksft_test_result_skip("%s could not form a THP\n", __func__); + munmap(area, 2 * hpage_size); + return; + } + + wp_init(mem, hpage_size); + + /* Drop the whole PMD: it is cleared to a pmd_none hole. */ + if (madvise(mem, hpage_size, MADV_DONTNEED)) + ksft_exit_fail_msg("%s MADV_DONTNEED failed\n", __func__); + + ret = pagemap_ioctl(mem, hpage_size, regions, 16, 0, 0, + PAGE_IS_WRITTEN, 0, 0, PAGE_IS_WRITTEN); + if (ret < 0) + ksft_exit_fail_msg("%s scan failed\n", __func__); + for (i = 0; i < ret; i++) + written += LEN(regions[i]); + + ksft_test_result(written == npages, + "%s pmd-hole reported written (%ld of %ld)\n", + __func__, written, npages); + + wp_free(mem, hpage_size); + munmap(area, 2 * hpage_size); +} + int sanity_tests(void) { unsigned long long mem_size, vec_size; @@ -1610,7 +1663,7 @@ int main(int __attribute__((unused)) argc, char *argv[]) if (!hugetlb_setup_default(4)) ksft_print_msg("HugeTLB test will be skipped\n"); - ksft_set_plan(118); + ksft_set_plan(119); page_size = getpagesize(); hpage_size = read_pmd_pagesize(); @@ -1790,6 +1843,7 @@ int main(int __attribute__((unused)) argc, char *argv[]) /* 18. Unpopulated pte scan-path consistency */ unpopulated_scan_test(); + unpopulated_thp_hole_test(); close(pagemap_fd); ksft_finished(); base-commit: 9795ad96d277c4af049fe30de1cebd4e39d7bcbe -- 2.54.0
