+ selftests-mm-added-fork-test-to-verify-global-ksm_zero_pages-counter-behavior.patch added to mm-new branch

+ selftests-mm-added-fork-test-to-verify-global-ksm_zero_pages-counter-behavior.patch added to mm-new branch

[Andrew Morton]

The patch titled
     Subject: selftests/mm: added fork test to verify global ksm_zero_pages counter behavior
has been added to the -mm mm-new branch.  Its filename is
     selftests-mm-added-fork-test-to-verify-global-ksm_zero_pages-counter-behavior.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/selftests-mm-added-fork-test-to-verify-global-ksm_zero_pages-counter-behavior.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
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fixup patches in mm-new.

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------------------------------------------------------
From: Donet Tom <donettom@linux.ibm.com>
Subject: selftests/mm: added fork test to verify global ksm_zero_pages counter behavior
Date: Mon, 15 Sep 2025 20:33:06 +0530

Added a selftest to verify the behavior of the global KSM zero-page
counter during fork.  When a process forks, the per-process zero-page
counter is inherited by the child, and the global counter should be
updated with this inherited value.  This test ensures that the global
counter is correctly updated after fork.

Link: https://lkml.kernel.org/r/5910cb6bfc7c43b169b679a0108667a56d7ebdb8.1757946863.git.donettom@linux.ibm.com
Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Cc: Aboorva Devarajan <aboorvad@linux.ibm.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 tools/testing/selftests/mm/ksm_functional_tests.c |   74 +++++++++++-
 1 file changed, 73 insertions(+), 1 deletion(-)

--- a/tools/testing/selftests/mm/ksm_functional_tests.c~selftests-mm-added-fork-test-to-verify-global-ksm_zero_pages-counter-behavior
+++ a/tools/testing/selftests/mm/ksm_functional_tests.c
@@ -602,6 +602,77 @@ unmap:
 	munmap(map, size);
 }
 
+long ksm_get_global_ksm_zero_pages(void)
+{
+	int global_ksm_zero_pages_fd;
+	char buf[10];
+	ssize_t ret;
+
+	global_ksm_zero_pages_fd = open("/sys/kernel/mm/ksm/ksm_zero_pages",
+								O_RDONLY);
+	if (global_ksm_zero_pages_fd < 0)
+		return -errno;
+
+	ret = pread(global_ksm_zero_pages_fd, buf, sizeof(buf) - 1, 0);
+	close(global_ksm_zero_pages_fd);
+	if (ret <= 0)
+		return -errno;
+	buf[ret] = 0;
+
+	return strtol(buf, NULL, 10);
+}
+
+static void test_fork_global_ksm_zero_pages_count(void)
+{
+	const unsigned int size = 2 * MiB;
+	char *map;
+	pid_t child_pid;
+	int status;
+	long g_zpages_before = 0, g_zpages_after = 0;
+
+	ksft_print_msg("[RUN] %s\n", __func__);
+
+	/* Unmerge all pages before test */
+	if (ksm_stop() < 0) {
+		ksft_test_result_fail("KSM unmerging failed\n");
+		return;
+	}
+	/* Get the global zero page count before test */
+	g_zpages_before = ksm_get_global_ksm_zero_pages();
+	/* Let KSM deduplicate zero pages. */
+	map = mmap_and_merge_range(0x00, size, PROT_READ | PROT_WRITE, KSM_MERGE_MADVISE);
+	if (map == MAP_FAILED)
+		return;
+
+	child_pid = fork();
+	if (!child_pid) {
+		exit(ksm_stop());
+	} else if (child_pid < 0) {
+		ksft_test_result_fail("fork() failed\n");
+		return;
+	}
+	if (waitpid(child_pid, &status, 0) < 0) {
+		ksft_test_result_fail("waitpid() failed\n");
+		return;
+	}
+	status = WEXITSTATUS(status);
+	if (status < 0) {
+		ksft_test_result_fail("KSM unmerging failed in child\n");
+		return;
+	}
+
+	/* Verify global zero-page count remains unchanged */
+	g_zpages_after = ksm_get_global_ksm_zero_pages();
+	if (g_zpages_before != g_zpages_after) {
+		ksft_test_result_fail("Incorrect global ksm zero page count after fork\n");
+		return;
+	}
+
+	ksft_test_result_pass("Global ksm zero page count is correct after fork\n");
+	ksm_stop();
+	munmap(map, size);
+}
+
 static void test_fork_ksm_merging_page_count(void)
 {
 	const unsigned int size = 2 * MiB;
@@ -659,7 +730,7 @@ static void init_global_file_handles(voi
 
 int main(int argc, char **argv)
 {
-	unsigned int tests = 9;
+	unsigned int tests = 10;
 	int err;
 
 	if (argc > 1 && !strcmp(argv[1], FORK_EXEC_CHILD_PRG_NAME)) {
@@ -692,6 +763,7 @@ int main(int argc, char **argv)
 	test_prctl_fork_exec();
 	test_prctl_unmerge();
 	test_fork_ksm_merging_page_count();
+	test_fork_global_ksm_zero_pages_count();
 
 	err = ksft_get_fail_cnt();
 	if (err)
_

Patches currently in -mm which might be from donettom@linux.ibm.com are

mm-ksm-fix-incorrect-ksm-counter-handling-in-mm_struct-during-fork.patch
selftests-mm-added-fork-inheritance-test-for-ksm_merging_pages-counter.patch
selftests-mm-added-fork-test-to-verify-global-ksm_zero_pages-counter-behavior.patch