[PATCH] selftests/mm: add fork CoW guard page test

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

When we fork anonymous pages, apply a guard page then remove it, the
previous CoW mapping is cleared.

This might not be obvious to an outside observer without taking some time
to think about how the overall process functions, so document that this is
the case through a test, which also usefully asserts that the behaviour is
as we expect.

This is grouped with other, more important, fork tests that ensure that
guard pages are correctly propagated on fork.

Fix a typo in a nearby comment at the same time.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 tools/testing/selftests/mm/guard-pages.c | 73 +++++++++++++++++++++++-
 1 file changed, 72 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/guard-pages.c b/tools/testing/selftests/mm/guard-pages.c
index 7cdf815d0d63..d8f8dee9ebbd 100644
--- a/tools/testing/selftests/mm/guard-pages.c
+++ b/tools/testing/selftests/mm/guard-pages.c
@@ -990,7 +990,7 @@ TEST_F(guard_pages, fork)
 		   MAP_ANON | MAP_PRIVATE, -1, 0);
 	ASSERT_NE(ptr, MAP_FAILED);

-	/* Establish guard apges in the first 5 pages. */
+	/* Establish guard pages in the first 5 pages. */
 	ASSERT_EQ(madvise(ptr, 5 * page_size, MADV_GUARD_INSTALL), 0);

 	pid = fork();
@@ -1029,6 +1029,77 @@ TEST_F(guard_pages, fork)
 	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
 }

+/*
+ * Assert expected behaviour after we fork populated ranges of anonymous memory
+ * and then guard and unguard the range.
+ */
+TEST_F(guard_pages, fork_cow)
+{
+	const unsigned long page_size = self->page_size;
+	char *ptr;
+	pid_t pid;
+	int i;
+
+	/* Map 10 pages. */
+	ptr = mmap(NULL, 10 * page_size, PROT_READ | PROT_WRITE,
+		   MAP_ANON | MAP_PRIVATE, -1, 0);
+	ASSERT_NE(ptr, MAP_FAILED);
+
+	/* Populate range. */
+	for (i = 0; i < 10 * page_size; i++) {
+		char chr = 'a' + (i % 26);
+
+		ptr[i] = chr;
+	}
+
+	pid = fork();
+	ASSERT_NE(pid, -1);
+	if (!pid) {
+		/* This is the child process now. */
+
+		/* Ensure the range is as expected. */
+		for (i = 0; i < 10 * page_size; i++) {
+			char expected = 'a' + (i % 26);
+			char actual = ptr[i];
+
+			ASSERT_EQ(actual, expected);
+		}
+
+		/* Establish guard pages across the whole range. */
+		ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_INSTALL), 0);
+		/* Remove it. */
+		ASSERT_EQ(madvise(ptr, 10 * page_size, MADV_GUARD_REMOVE), 0);
+
+		/*
+		 * By removing the guard pages, the page tables will be
+		 * cleared. Assert that we are looking at the zero page now.
+		 */
+		for (i = 0; i < 10 * page_size; i++) {
+			char actual = ptr[i];
+
+			ASSERT_EQ(actual, '\0');
+		}
+
+		exit(0);
+	}
+
+	/* Parent process. */
+
+	/* Parent simply waits on child. */
+	waitpid(pid, NULL, 0);
+
+	/* Ensure the range is unchanged in parent anon range. */
+	for (i = 0; i < 10 * page_size; i++) {
+		char expected = 'a' + (i % 26);
+		char actual = ptr[i];
+
+		ASSERT_EQ(actual, expected);
+	}
+
+	/* Cleanup. */
+	ASSERT_EQ(munmap(ptr, 10 * page_size), 0);
+}
+
 /*
  * Assert that forking a process with VMAs that do have VM_WIPEONFORK set
  * behave as expected.
--
2.47.1