+ kselftest-mm-return-fail-when-child-test-result-is-fail-in-khugepaged.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: kselftest: mm: return fail when child test result is fail in khugepaged
has been added to the -mm mm-unstable branch.  Its filename is
     kselftest-mm-return-fail-when-child-test-result-is-fail-in-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-return-fail-when-child-test-result-is-fail-in-khugepaged.patch

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

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via various
branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there most days

------------------------------------------------------
From: Yeoreum Yun <yeoreum.yun@arm.com>
Subject: kselftest: mm: return fail when child test result is fail in khugepaged
Date: Tue, 29 Sep 2026 11:33:18 +0100

Patch series "kselftest: mm: fix intermittent failure khugepaged test", v4.

There are intermittent failures in collapse_max_ptes_swap() and
collapse_max_ptes_shared() when using the khugepaged_context:

  # Run test: collapse_max_ptes_shared (khugepaged:anon)
  # Allocate huge page... OK
  # Share huge page over fork()... OK
  # Trigger CoW on page 1023 of 2048... OK
  # Maybe collapse with max_ptes_shared exceeded.... OK
  # Trigger CoW on page 1024 of 2048... Fail
  Bail out! Unexpected huge page
  # Planned tests != run tests (26 != 23)
  # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0

  # Run test: collapse_max_ptes_swap (khugepaged:anon)
  # Swapout 257 of 2048 pages... OK
  # Maybe collapse with max_ptes_swap exceeded.... OK
  # Swapout 256 of 2048 pages... OK
  Bail out! Unexpected huge page
  # Planned tests != run tests (26 != 17)
  # Totals: pass:17 fail:0 xfail:0 xpass:0 skip:0 error:0

This happens because khugepaged may collapse the pages before
wait_for_scan() is called, causing a sanity check that expects uncollapsed
pages to fail.

For example, in collapse_max_ptes_swap(), after faulting the pages back in
and paging out up to max_ptes_swap pages, khugepaged may collapse them
again before c->collapse() is called.

To prevent this, mark the VMA with MADV_NOHUGEPAGE after it has been
collapsed by wait_for_scan() for anon.  This prevents khugepaged from
collapsing it again before c->collapse() is called.

Also, fix false-positive results when a child process fails in tests such
as collapse_fork*() or collapse_max_ptes_shared():

  # -------------------------
  # running ./khugepaged -s 2
  # -------------------------
  #
  # Run test: collapse_max_ptes_shared (khugepaged:anon)
  # Allocate huge page... OK
  # Share huge page over fork()... OK
  # Trigger CoW on page 1023 of 2048... OK
  # Maybe collapse with max_ptes_shared exceeded.... OK
  # Trigger CoW on page 1024 of 2048... Fail
  Bail out! Unexpected huge page
  # Planned tests != run tests (26 != 23)
  # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0  // child failed.
  # Check if parent still has huge page... OK              // parent hpage success
  ok 24 collapse_max_ptes_shared                           // considered as success
  ...
  # Totals: pass:26 fail:0 xfail:0 xpass:0 skip:0 error:0

This failure was observed on NVIDIA Spark with 16KB page.


This patch (of 2):

Although the child process in collapse_fork*() or
collapse_max_ptes_shared() reports `KSFT_FAIL`, the result is ignored
because the test only checks whether the parent's page was collapsed into
a huge page.

As a result, the test is considered successful whenever the parent's page
is a huge page, even if the child test fails, as shown below:

  #
  # Run test: collapse_max_ptes_shared (khugepaged:anon)
  # Allocate huge page... OK
  # Share huge page over fork()... OK
  # Trigger CoW on page 1023 of 2048... OK
  # Maybe collapse with max_ptes_shared exceeded.... OK
  # Trigger CoW on page 1024 of 2048... Fail
  Bail out! Unexpected huge page
  # Planned tests != run tests (26 != 23)
  # Totals: pass:23 fail:0 xfail:0 xpass:0 skip:0 error:0  // child failed.
  # Check if parent still has huge page... OK              // parent hpage success
  ok 24 collapse_max_ptes_shared                           // considered as success
  ...
  # Totals: pass:26 fail:0 xfail:0 xpass:0 skip:0 error:0

To address this, propagate the child's failure and skip the subsequent
check in the parent.

Link: https://lore.kernel.org/20260929-fix_khugepagd_fail-v4-0-2169c18f2576@arm.com
Link: https://lore.kernel.org/20260929-fix_khugepagd_fail-v4-1-2169c18f2576@arm.com
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Zi Yan <ziy@nvidia.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: Kiryl Shutsemau <kas@kernel.org>
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/khugepaged.c |    9 +++++++++
 1 file changed, 9 insertions(+)

--- a/tools/testing/selftests/mm/khugepaged.c~kselftest-mm-return-fail-when-child-test-result-is-fail-in-khugepaged
+++ a/tools/testing/selftests/mm/khugepaged.c
@@ -1053,6 +1053,8 @@ static void collapse_fork(struct collaps
 
 	wait(&wstatus);
 	exit_status = WEXITSTATUS(wstatus);
+	if (exit_status == KSFT_FAIL)
+		goto out;
 
 	ksft_print_msg("Check if parent still has small page...");
 	if (ops->check_huge(p, hpage_pmd_size, 0, hpage_pmd_size))
@@ -1060,6 +1062,7 @@ static void collapse_fork(struct collaps
 	else
 		fail("Fail");
 	validate_memory(p, 0, page_size);
+out:
 	ops->cleanup_area(p, hpage_pmd_size);
 	ksft_test_result_report(exit_status, "%s\n", __func__);
 }
@@ -1100,6 +1103,8 @@ static void collapse_fork_compound(struc
 
 	wait(&wstatus);
 	exit_status = WEXITSTATUS(wstatus);
+	if (exit_status == KSFT_FAIL)
+		goto out;
 
 	ksft_print_msg("Check if parent still has huge page...");
 	if (ops->check_huge(p, hpage_pmd_size, 1, hpage_pmd_size))
@@ -1107,6 +1112,7 @@ static void collapse_fork_compound(struc
 	else
 		fail("Fail");
 	validate_memory(p, 0, hpage_pmd_size);
+out:
 	ops->cleanup_area(p, hpage_pmd_size);
 	ksft_test_result_report(exit_status, "%s\n", __func__);
 }
@@ -1158,6 +1164,8 @@ static void collapse_max_ptes_shared(str
 
 	wait(&wstatus);
 	exit_status = WEXITSTATUS(wstatus);
+	if (exit_status == KSFT_FAIL)
+		goto out;
 
 	ksft_print_msg("Check if parent still has huge page...");
 	if (ops->check_huge(p, hpage_pmd_size, 1, hpage_pmd_size))
@@ -1165,6 +1173,7 @@ static void collapse_max_ptes_shared(str
 	else
 		fail("Fail");
 	validate_memory(p, 0, hpage_pmd_size);
+out:
 	ops->cleanup_area(p, hpage_pmd_size);
 	ksft_test_result_report(exit_status, "%s\n", __func__);
 }
_

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