linux-kernel - selftests/mm/rmap: verify correct RMAP handling of COW pages after fork()

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Message-ID:
	<SN6PR04MB4048CF9A242A640B29A588EDFEE8A@SN6PR04MB4048.namprd04.prod.outlook.com>
Date: Wed, 15 Oct 2025 08:43:33 +0000
From: Itamar Dalal <dalalitamar@...il.com>
To: "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "david@...hat.com"
	<david@...hat.com>, "lorenzo.stoakes@...cle.com"
	<lorenzo.stoakes@...cle.com>, "riel@...riel.com" <riel@...riel.com>,
	"Liam.Howlett@...cle.com" <Liam.Howlett@...cle.com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "harry.yoo@...cle.com" <harry.yoo@...cle.com>,
	"jannh@...gle.com" <jannh@...gle.com>, "rppt@...nel.org" <rppt@...nel.org>,
	"surenb@...gle.com" <surenb@...gle.com>, "mhocko@...e.com" <mhocko@...e.com>,
	"shuah@...nel.org" <shuah@...nel.org>
Subject: selftests/mm/rmap: verify correct RMAP handling of COW pages after
 fork()

Add a new test `migrate.cow_after_fork` that verifies correct RMAP handling
of Copy-On-Write (COW) pages after fork().

Before a write, the parent and child share the same PFN. After a write, the
child’s PFN differs. This confirms that proper COW duplication occurred and
that RMAP correctly tracks page ownership transitions during COW events.

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