+ selftests-mm-handle-child-status-safely-in-khugepaged.patch added to mm-new branch

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

The patch titled
     Subject: selftests/mm: handle child status safely in khugepaged
has been added to the -mm mm-new branch.  Its filename is
     selftests-mm-handle-child-status-safely-in-khugepaged.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-handle-child-status-safely-in-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: Muhammad Mahdi Amirpour <ampr.mhd@gmail.com>
Subject: selftests/mm: handle child status safely in khugepaged
Date: Mon, 02 Nov 2026 22:21:30 +0330

collapse_fork(), collapse_fork_compound() and collapse_max_ptes_shared()
take the child result from WEXITSTATUS() without checking WIFEXITED(). 
For a child killed by a signal WEXITSTATUS() is 0, which is KSFT_PASS: the
parent runs its own checks and the test reports ok even though the child
crashed.

Fail the test when the child did not exit normally, and print the signal
that killed it.  Also fail when fork() fails, instead of reading an
uninitialised status.

Use get_child_status() in all four fork tests, and use wait_for_child() to
handle fork() and waitpid() in the three blocking-wait tests.

Link: https://lore.kernel.org/20261102185130.30504-1-ampr.mhd@gmail.com
Fixes: bd212a7dbdb1 ("kselftest: mm: return fail when child test result is fail in khugepaged")
Signed-off-by: Muhammad Mahdi Amirpour <ampr.mhd@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Yeoreum Yun <yeoreum.yun@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
---

 tools/testing/selftests/mm/khugepaged.c |   61 +++++++++++++++++-----
 1 file changed, 48 insertions(+), 13 deletions(-)

--- a/tools/testing/selftests/mm/khugepaged.c~selftests-mm-handle-child-status-safely-in-khugepaged
+++ a/tools/testing/selftests/mm/khugepaged.c
@@ -1006,9 +1006,44 @@ static void collapse_compound_extreme(st
 	ksft_test_result_report(exit_status, "%s\n", __func__);
 }
 
-static void collapse_fork(struct collapse_context *c, struct mem_ops *ops)
+static int get_child_status(int wstatus)
+{
+	char buf[64];
+
+	if (WIFEXITED(wstatus))
+		return WEXITSTATUS(wstatus);
+
+	if (WIFSIGNALED(wstatus)) {
+		snprintf(buf, sizeof(buf), "Child killed by signal %d",
+			 WTERMSIG(wstatus));
+		fail(buf);
+	} else {
+		fail("Child terminated abnormally");
+	}
+
+	return KSFT_FAIL;
+}
+
+static int wait_for_child(pid_t pid)
 {
 	int wstatus;
+
+	if (pid < 0) {
+		fail("fork() failed");
+		return KSFT_FAIL;
+	}
+
+	if (waitpid(pid, &wstatus, 0) == -1) {
+		fail("waitpid() failed");
+		return KSFT_FAIL;
+	}
+
+	return get_child_status(wstatus);
+}
+
+static void collapse_fork(struct collapse_context *c, struct mem_ops *ops)
+{
+	pid_t pid;
 	void *p;
 
 	p = ops->setup_area(1);
@@ -1021,7 +1056,8 @@ static void collapse_fork(struct collaps
 		fail("Fail");
 
 	ksft_print_msg("Share small page over fork()...");
-	if (!fork()) {
+	pid = fork();
+	if (pid == 0) {
 		/* Do not touch settings on child exit */
 		if (ops->check_huge(p, hpage_pmd_size, 0, hpage_pmd_size))
 			success("OK");
@@ -1037,8 +1073,7 @@ static void collapse_fork(struct collaps
 		_exit(exit_status);
 	}
 
-	wait(&wstatus);
-	exit_status = WEXITSTATUS(wstatus);
+	exit_status = wait_for_child(pid);
 	if (exit_status == KSFT_FAIL)
 		goto out;
 
@@ -1055,12 +1090,13 @@ out:
 
 static void collapse_fork_compound(struct collapse_context *c, struct mem_ops *ops)
 {
-	int wstatus;
+	pid_t pid;
 	void *p;
 
 	p = alloc_hpage(ops);
 	ksft_print_msg("Share huge page over fork()...");
-	if (!fork()) {
+	pid = fork();
+	if (pid == 0) {
 		/* Do not touch settings on child exit */
 		if (ops->check_huge(p, hpage_pmd_size, 1, hpage_pmd_size))
 			success("OK");
@@ -1087,8 +1123,7 @@ static void collapse_fork_compound(struc
 		_exit(exit_status);
 	}
 
-	wait(&wstatus);
-	exit_status = WEXITSTATUS(wstatus);
+	exit_status = wait_for_child(pid);
 	if (exit_status == KSFT_FAIL)
 		goto out;
 
@@ -1106,12 +1141,13 @@ out:
 static void collapse_max_ptes_shared(struct collapse_context *c, struct mem_ops *ops)
 {
 	int max_ptes_shared = thp_read_num("khugepaged/max_ptes_shared");
-	int wstatus;
+	pid_t pid;
 	void *p;
 
 	p = alloc_hpage(ops);
 	ksft_print_msg("Share huge page over fork()...");
-	if (!fork()) {
+	pid = fork();
+	if (pid == 0) {
 		/* Do not touch settings on child exit */
 		if (ops->check_huge(p, hpage_pmd_size, 1, hpage_pmd_size))
 			success("OK");
@@ -1148,8 +1184,7 @@ static void collapse_max_ptes_shared(str
 		_exit(exit_status);
 	}
 
-	wait(&wstatus);
-	exit_status = WEXITSTATUS(wstatus);
+	exit_status = wait_for_child(pid);
 	if (exit_status == KSFT_FAIL)
 		goto out;
 
@@ -1250,7 +1285,7 @@ static void collapse_fork_cow_race(struc
 	for (; i < n; i++)
 		ip[i * stride] = i + 0xbeef0000;
 	/* A child that died reading the racing pages is a failure, not a zero */
-	child_status = WIFEXITED(wstatus) ? WEXITSTATUS(wstatus) : KSFT_FAIL;
+	child_status = get_child_status(wstatus);
 
 	ksft_print_msg("Check the parent sees only its own writes...");
 	for (i = 0; i < n; i++)
_

Patches currently in -mm which might be from ampr.mhd@gmail.com are

selftests-mm-handle-child-status-safely-in-khugepaged.patch