+ kselftest-mm-prevent-random-failure-of-huge-page-split-for-khugepaged.patch added to mm-new branch

+ kselftest-mm-prevent-random-failure-of-huge-page-split-for-khugepaged.patch added to mm-new branch

[Andrew Morton]

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The patch titled
     Subject: kselftest: mm: prevent random failure of huge page split for khugepaged
has been added to the -mm mm-new branch.  Its filename is
     kselftest-mm-prevent-random-failure-of-huge-page-split-for-khugepaged.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/kselftest-mm-prevent-random-failure-of-huge-page-split-for-khugepaged.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
notice and to finish up reviews.  Please do not hesitate to respond to
review feedback and post updated versions to replace or incrementally
fixup patches in mm-new.

The mm-new branch of mm.git is not included in linux-next

If a few days of testing in mm-new is successful, the patch will me moved
into mm.git's mm-unstable branch, which is included in linux-next

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------------------------------------------------------
From: Yeoreum Yun <yeoreum.yun@arm.com>
Subject: kselftest: mm: prevent random failure of huge page split for khugepaged
Date: Thu, 24 Sep 2026 20:11:16 +0100

Patch series "kselftest: mm: fix some failure of split_huge_page_test", v8.

split_huge_page_test can fail for the following reasons:

  1. During the test, khugepaged may collapse previously split pages again,
     causing intermittent failures.

  2. 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(). Moreover, a subsequent
     madvise(MADV_HUGEPAGE) call does not split the VMA because it already
     has the same advice.

     This causes the 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.

Address these issues by applying MADV_NOHUGEPAGE after faulting in the
huge page, preventing khugepaged from collapsing it again, and 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 check_huge_shmem() was required to distinguish shmem huge pages
because /proc/self/smaps reports them using a dedicated
“ShmemPmdMapped” entry, as opposed to “FilePmdMapped” for
file-backed huge pages.

Now that /proc/self/smaps is no longer used to detect huge pages and
/proc/kpageflags is used instead, it is sufficient to distinguish between
file-backed and anonymous pages since the ShmemPmdMapped is also kind of
file-backed.

Therefore, remove check_huge_shmem() and use check_huge_file() instead.


This patch (of 4):

There're some random failure for split_huge_page_test when khugepaged
collapses pages into pmd again which had split by the test.

Prevent the khugepaged's collapses for split page by setting the mapped
pmd-huge-page with MADV_NOHUGEPAGE before split.

Link: https://lore.kernel.org/20260924-fix_split-v8-0-cba7359d882a@arm.com
Link: https://lore.kernel.org/20260924-fix_split-v8-1-cba7359d882a@arm.com
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Kevin Brodsky <kevin.brodsky@arm.com>
Suggested-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
---

 tools/testing/selftests/mm/split_huge_page_test.c |   19 ++++++++++++
 1 file changed, 19 insertions(+)

--- a/tools/testing/selftests/mm/split_huge_page_test.c~kselftest-mm-prevent-random-failure-of-huge-page-split-for-khugepaged
+++ a/tools/testing/selftests/mm/split_huge_page_test.c
@@ -108,6 +108,18 @@ static char *allocate_zero_filled_hugepa
 	return result;
 }
 
+static void disable_khugepaged(void *addr, size_t len)
+{
+	/*
+	 * Disables khugepaged from collapsing THPs in range, existing THP
+	 * pages remain.
+	 */
+	if (!madvise(addr, len, MADV_NOHUGEPAGE))
+		return;
+
+	ksft_exit_fail_msg("MADV_NOHUGEPAGE failed, err=%d\n", errno);
+}
+
 static void verify_rss_anon_split_huge_page_all_zeroes(char *one_page, int nr_hpages, size_t len)
 {
 	unsigned long rss_anon_before, rss_anon_after;
@@ -120,6 +132,8 @@ static void verify_rss_anon_split_huge_p
 	if (!rss_anon_before)
 		ksft_exit_fail_msg("No RssAnon is allocated before split\n");
 
+	disable_khugepaged(one_page, len);
+
 	/* split all THPs */
 	write_debugfs(PID_FMT, getpid(), (uint64_t)one_page,
 		      (uint64_t)one_page + len, 0);
@@ -167,6 +181,8 @@ static void split_pmd_thp_to_order(int o
 	if (!check_huge_anon(one_page, 4 * pmd_pagesize, 4, pmd_pagesize))
 		ksft_exit_fail_msg("No THP is allocated\n");
 
+	disable_khugepaged(one_page, len);
+
 	/* split all THPs */
 	write_debugfs(PID_FMT, getpid(), (uint64_t)one_page,
 		(uint64_t)one_page + len, order);
@@ -215,6 +231,8 @@ static void split_pte_mapped_thp(void)
 		goto out;
 	}
 
+	disable_khugepaged(thp_area, thp_area_size);
+
 	/*
 	 * To challenge spitting code, we will mremap a single page of each
 	 * THP (page[i] of thp[i]) in the thp_area into page_area. This will
@@ -482,6 +500,7 @@ static int create_pagecache_thp_and_fd(c
 		ksft_test_result_skip("Pagecache folio split skipped\n");
 		return -2;
 	}
+	disable_khugepaged(*addr, fd_size);
 	return 0;
 err_out_close:
 	close(*fd);
_

Patches currently in -mm which might be from yeoreum.yun@arm.com are

kselftest-mm-fix-potential-failure-for-merged-vma-in-guard-regions.patch
kselftest-mm-remove-exclusion-of-building-soft-dirty-test-in-arm64.patch
kselftest-mm-return-fail-when-child-test-result-is-fail-in-khugepaged.patch
kselftest-mm-fix-intermittent-failure-khugepaged-test.patch
kselftest-mm-prevent-random-failure-of-huge-page-split-for-khugepaged.patch
kselftest-mm-replace-usage-of-proc-self-smaps-for-__check_pmd_huge.patch
kselftest-mm-integrate-huge-page-checks.patch
kselftest-mm-remove-check_huge_shmem.patch