Re: [PATCH v5 2/3] kselftest: mm: replace usage of /proc/self/smaps for check_huge_xxx() helper

On 9/10/26 12:54, Yeoreum Yun wrote:

quoted

On 9/7/26 10:19, Yeoreum Yun wrote:

quoted

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.

Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Suggested-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Yeoreum Yun <redacted>
---
 tools/testing/selftests/mm/vm_util.c | 145 ++++++++++++++++++++++-------------
 tools/testing/selftests/mm/vm_util.h |   1 +
 2 files changed, 92 insertions(+), 54 deletions(-)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index 4821a3563036..dc62ce84e143 100644

[...]

quoted

 
-	if (hpage_size == pmd_pagesize)
-		return __check_pmd_huge(addr, "AnonHugePages: ", nr_hpages, hpage_size);
+static bool check_huge_type(uint64_t categories, uint64_t kpageflags,
+			    enum check_huge_type type)
+{
+	const bool file = categories & PAGE_IS_FILE;
+	const bool swapbacked = kpageflags & KPF_SWAPBACKED;
+
+	switch (type) {
+	case CHECK_HUGE_ANON:
+		return !file;
+	case CHECK_HUGE_FILE:
+		return file && !swapbacked;
+	case CHECK_HUGE_SHMEM:
+		return file & swapbacked;
+	}

Why do we even need CHECK_HUGE_SHMEM?

That's really just a legacy thing for using smaps to identify huge pages.

It's sufficient to identify CHECK_HUGE_FILE if you know that you have shmem mapping.

You will not arbitrarily have non-shmem folios in a shmem mapping :)

Agree. but there seems the case where discern whethr the mapping is
with regular file or shmem like tmpfs -- khugepaged test where using
__shmem_ops. So I think it would be better to keep this as-is.

Let's not add or maintain unnecessary complexity.

hmem_check_huge can really just become check_huge_file.

Even file_check_huge can just be simplified.

There must be a pretty good reason why we'd want to keep this.

-- 
Cheers,

David