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* [PATCH v3] fs/proc/task_mmu: fix PAGEMAP_SCAN written state for PMD holes
@ 2026-07-09 12:16 Kiryl Shutsemau
  2026-07-12  1:14 ` Zenghui Yu
  0 siblings, 1 reply; 3+ messages in thread
From: Kiryl Shutsemau @ 2026-07-09 12:16 UTC (permalink / raw)
  To: akpm
  Cc: usama.anjum, peterx, liam, ljs, vbabka, jannh, pfalcato, david,
	rppt, surenb, mhocko, shuah, linux-mm, linux-kernel,
	linux-fsdevel, linux-kselftest, stable, kernel-team

From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>

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 <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260707151349.92143-1-kirill@shutemov.name
Fixes: 2bad466cc9d9 ("mm/uffd: UFFD_FEATURE_WP_UNPOPULATED")
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Kiryl Shutsemau <kas@kernel.org>
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/20260708103429.150655-1-kirill@shutemov.name/
 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


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2026-07-09 12:16 [PATCH v3] fs/proc/task_mmu: fix PAGEMAP_SCAN written state for PMD holes Kiryl Shutsemau
2026-07-12  1:14 ` Zenghui Yu
2026-07-13  9:16   ` Kiryl Shutsemau

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