Re: [PATCH v3] fs/proc/task_mmu: fix PAGEMAP_SCAN written state for PMD holes

[Zenghui Yu <zenghui.yu@linux.dev>]

Hi Kiryl,

On 7/9/26 8:16 PM, Kiryl Shutsemau wrote:
> 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.

I hit the following compilation error on mm-new:

[root@localhost mm]# make
  CC       pagemap_ioctl
pagemap_ioctl.c: In function 'unpopulated_thp_hole_test':
pagemap_ioctl.c:1130:31: error: 'MADV_COLLAPSE' undeclared (first use in this function); did you mean 'MADV_COLD'?
 1130 |  if (madvise(mem, hpage_size, MADV_COLLAPSE) ||
      |                               ^~~~~~~~~~~~~
      |                               MADV_COLD
pagemap_ioctl.c:1130:31: note: each undeclared identifier is reported only once for each function it appears in
make: *** [../lib.mk:225: /root/code/mm/tools/testing/selftests/mm/pagemap_ioctl] Error 1

Could you consider addressing it like fd5295afae91 ("selftests/mm:
hmm-tests: include linux/mman.h to access MADV_COLLAPSE")?

Thanks,
Zenghui