Re: [PATCH v7 2/3] kselftest: mm: replace usage of /proc/self/smaps for check_huge_xxx() helper

[Yeoreum Yun <yeoreum.yun@arm.com>]

On Mon, Sep 21, 2026 at 11:05:15PM +0200, David Hildenbrand (Arm) wrote:
> On 9/21/26 20:48, Yeoreum Yun wrote:
> > Hi,
> > 
> >> On 9/21/26 12:40, Yeoreum Yun wrote:
> >>> Since glibc commit 321e1fc73f (“malloc: Enable 2MB THP by default on AArch64”),
> >>> glibc may call madvise(MADV_HUGEPAGE) for sufficiently large allocations
> >>> made by memalign().
> >>>
> >>> The underlying VMA may start at a different address from the aligned
> >>> address returned by memalign(). Furthermore, a subsequent
> >>> madvise(MADV_HUGEPAGE) call does not split the VMA because the flag is
> >>> already set.
> >>>
> >>> This causes split_huge_page_test to fail because the check_huge_xxx()
> >>> helpers incorrectly require the address returned by memalign() to
> >>> match the VMA start address reported in /proc/self/smaps.
> >>>
> >>> Instead of relying on /proc/self/smaps, use /proc/self/pagemap and
> >>> /proc/kpageflags to detect huge-page mappings and large folios:
> >>>
> >>>   1. If hpage_size == pmd_pagesize, check PAGE_IS_HUGE instead of
> >>>      using check_large_folios(), since only the mapping type matters.
> >>>      This identifies PMD-mapped huge pages.
> >>>   2. Otherwise, use check_large_folios() to detect large folios. This
> >>>      covers mTHP cases.
> >>>   3. Check the folio flags according to the type of huge page.
> >>>
> >>> Since shmem pages are also file-backed, simply check whether the page
> >>> is file-backed.
> >>>
> >>> Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
> >>> Suggested-by: Zi Yan <ziy@nvidia.com>
> >>
> >> Do we need a Fixes: tag?
> > 
> > Since this change is required for the chaning of behavior of glibc,
> > I don't think we don't need to add Fixes tag in here?
> > 
> Our test made wrong assumptions on the underlying behavior of the user space
> allocator (no VMA merging would happen). So I'd argue that we should add a
> Fixes: for the test.

Okay. while I do
  - add intermediate patch before this as you suggeeted.
  - on the intermediate patch, add fix tag.
  - based the patch, rebase this patch to add anon/file check for
    other check_large_folios() type.

Thanks.

> 
> -- 
> Cheers,
> 
> David

-- 
Sincerely,
Yeoreum Yun