selftests/vm: add file/shmem MADV_COLLAPSE selftest for cleared pmd

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.

Link: https://lkml.kernel.org/r/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Zach O'Keefe <[e-mail]>
Cc: Axel Rasmussen <[e-mail]>
Cc: Chris Kennelly <[e-mail]>
Cc: David Hildenbrand <[e-mail]>
Cc: David Rientjes <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: James Houghton <[e-mail]>
Cc: "Kirill A. Shutemov" <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Miaohe Lin <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Pasha Tatashin <[e-mail]>
Cc: Peter Xu <[e-mail]>
Cc: Rongwei Wang <[e-mail]>
Cc: SeongJae Park <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Yang Shi <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/vm/khugepaged.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+ * Test race with khugepaged where page tables have been retracted and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");
···
 	TEST(madvise_collapse_existing_thps, madvise_context, anon_ops);
 	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);
 }