linux-kernel - [PATCH] Add a new test 'migrate.cow_after_fork' that verifies correct RMAP handling of Copy-On-Write pages after fork(). Before a write, parent and child share the same PFN; after a write, the child’s PFN differs, confirming proper COW duplication.

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Message-Id: <20251014194944.20341-1-dalalitamar@gmail.com>
Date: Tue, 14 Oct 2025 22:49:44 +0300
From: Itamar-Dalal <dalalitamar@...il.com>
To: linux-mm@...ck.org
Cc: linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org,
	david@...hat.com,
	lorenzo.stoakes@...cle.com,
	riel@...riel.com,
	Liam.Howlett@...cle.com,
	vbabka@...e.cz,
	harry.yoo@...cle.com,
	jannh@...gle.com,
	rppt@...nel.org,
	surenb@...gle.com,
	mhocko@...e.com,
	shuah@...nel.org,
	Itamar-Dalal <dalalitamar@...il.com>
Subject: [PATCH] Add a new test 'migrate.cow_after_fork' that verifies correct RMAP handling of Copy-On-Write pages after fork(). Before a write, parent and child share the same PFN; after a write, the child’s PFN differs, confirming proper COW duplication.

Signed-off-by: Itamar-Dalal <dalalitamar@...il.com>
---
 tools/testing/selftests/mm/rmap.c | 45 ++++++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/rmap.c b/tools/testing/selftests/mm/rmap.c
index 13f7bccfd0a9..2ba3361fecf0 100644
--- a/tools/testing/selftests/mm/rmap.c
+++ b/tools/testing/selftests/mm/rmap.c
@@ -430,4 +430,47 @@ TEST_F(migrate, ksm)
 	propagate_children(_metadata, data);
 }
 
-TEST_HARNESS_MAIN
+TEST_F(migrate, cow_after_fork)
+{
+	struct global_data *data = &self->data;
+	int status;
+	pid_t pid;
+	unsigned long parent_pfn, child_pfn;
+	int pagemap_fd;
+	char *region;
+
+	/* Map private anonymous memory and fault it in */
+	region = mmap(NULL, data->mapsize, PROT_READ | PROT_WRITE,
+		MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	ASSERT_NE(region, MAP_FAILED);
+	memset(region, 0xaa, data->mapsize);
+
+	pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+	ASSERT_NE(pagemap_fd, -1);
+	parent_pfn = pagemap_get_pfn(pagemap_fd, region);
+	close(pagemap_fd);
+
+	pid = fork();
+	ASSERT_NE(pid, -1);
+
+	if (pid == 0) {
+		/* Child: write to trigger COW */
+		region[0] = 0xbb;
+
+		pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
+		ASSERT_NE(pagemap_fd, -1);
+		child_pfn = pagemap_get_pfn(pagemap_fd, region);
+		close(pagemap_fd);
+
+		/* Expect PFN to differ after write (COW happened) */
+		if (child_pfn == parent_pfn)
+			_exit(FAIL_ON_CHECK);
+		_exit(0);
+	}
+
+	waitpid(pid, &status, 0);
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+	munmap(region, data->mapsize);
+}
+
+TEST_HARNESS_MAIN
\ No newline at end of file
-- 
2.34.1