[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.

[Itamar-Dalal]

Signed-off-by: Itamar-Dalal <dalalita...@gmail.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