[PATCH mm-unstable v3 09/10] selftests/vm: add file/shmem MADV_COLLAPSE selftest for cleared pmd

["Zach O'Keefe" <zokeefe@google.com>]

This test tests that MADV_COLLAPSE acting on file/shmem memory for which
(1) the file extent mapping by the memory is already a huge page in the
page cache, and (2) the pmd mapping this memory in the target process
is none.

In practice, (1)+(2) is the state left over after khugepaged has
successfully collapsed file/shmem memory for a target VMA, but the
memory has not yet been refaulted. So, this test in-effect tests
MADV_COLLAPSE racing with khugepaged to collapse the memory first.

Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 tools/testing/selftests/vm/khugepaged.c | 30 +++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/tools/testing/selftests/vm/khugepaged.c b/tools/testing/selftests/vm/khugepaged.c
index 05d9945daa48..730507e38c58 100644
--- a/tools/testing/selftests/vm/khugepaged.c
+++ b/tools/testing/selftests/vm/khugepaged.c
@@ -1323,6 +1323,33 @@ static void madvise_collapse_existing_thps(struct collapse_context *c,
 	ops->cleanup_area(p, hpage_pmd_size);
 }
 
+/*
+ * Test race with khugepaged where page tables have been retracted and
+ * pmd cleared.
+ */
+static void madvise_retracted_page_tables(struct collapse_context *c,
+					  struct mem_ops *ops)
+{
+	void *p;
+	int nr_hpages = 1;
+	unsigned long size = nr_hpages * hpage_pmd_size;
+
+	p = ops->setup_area(nr_hpages);
+	ops->fault(p, 0, size);
+
+	/* Let khugepaged collapse and leave pmd cleared */
+	if (wait_for_scan("Collapse and leave PMD cleared", p, nr_hpages,
+			  ops)) {
+		fail("Timeout");
+		return;
+	}
+	success("OK");
+	c->collapse("Install huge PMD from page cache", p, nr_hpages, ops,
+		    true);
+	validate_memory(p, 0, size);
+	ops->cleanup_area(p, size);
+}
+
 static void usage(void)
 {
 	fprintf(stderr, "\nUsage: ./khugepaged <test type> [dir]\n\n");
@@ -1494,5 +1521,8 @@ int main(int argc, const char **argv)
 	TEST(madvise_collapse_existing_thps, madvise_context, file_ops);
 	TEST(madvise_collapse_existing_thps, madvise_context, shmem_ops);
 
+	TEST(madvise_retracted_page_tables, madvise_context, file_ops);
+	TEST(madvise_retracted_page_tables, madvise_context, shmem_ops);
+
 	restore_settings(0);
 }
-- 
2.37.2.789.g6183377224-goog