[PATCH v4 2/2] mm: selftests: Adjust the MADV_COLLAPSE selftests for uffd-minor

[James Houghton <jthoughton@google.com>]

The behavior of MADV_COLLAPSE with uffd-minor VMAs has changed.
Previously, MADV_COLLAPSE would install PMDs in parts of a VMA where
PTEs may be missing, essentially suppressing userfaultfd minor faults.
Now, MADV_COLLAPSE returns -EINVAL for uffd-minor VMAs. Update the
selftest to demonstrate this new behavior.

The selftest, if left unmodified, will SKIP when it gets EINVAL back
from MADV_COLLAPSE.
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 69 ++++++++++++++++++--
 1 file changed, 62 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index 580178630ede..ab195c2c44d3 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -518,19 +518,32 @@ static void uffd_wp_fork_pin_with_event_test(uffd_global_test_opts_t *gopts, uff
 	uffd_wp_fork_pin_test_common(gopts, args, true);
 }
 
-static void check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+static int __check_memory_contents(unsigned long offset, unsigned long nr_pages,
+		uffd_global_test_opts_t *gopts, char *p)
 {
 	unsigned long i, j;
 	uint8_t expected_byte;
 
-	for (i = 0; i < gopts->nr_pages; ++i) {
+	if (nr_pages + offset < nr_pages)
+		err("overflow in memory check");
+	if (nr_pages + offset > gopts->nr_pages)
+		err("out of bounds memory check");
+
+	for (i = offset; i < offset + nr_pages; ++i) {
 		expected_byte = ~((uint8_t)(i % ((uint8_t)-1)));
 		for (j = 0; j < gopts->page_size; j++) {
 			uint8_t v = *(uint8_t *)(p + (i * gopts->page_size) + j);
 			if (v != expected_byte)
-				err("unexpected page contents");
+				return 1;
 		}
 	}
+
+	return 0;
+}
+
+static int check_memory_contents(uffd_global_test_opts_t *gopts, char *p)
+{
+	return __check_memory_contents(0, gopts->nr_pages, gopts, p);
 }
 
 static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_collapse, bool test_wp)
@@ -538,8 +551,8 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 	unsigned long p;
 	pthread_t uffd_mon;
 	char c = '\0';
-	struct uffd_args args = { 0 };
-	args.gopts = gopts;
+	struct uffd_args args = { .gopts = gopts };
+	bool bad_contents;
 
 	/*
 	 * NOTE: MADV_COLLAPSE is not yet compatible with WP, so testing
@@ -564,20 +577,59 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 	if (pthread_create(&uffd_mon, NULL, uffd_poll_thread, &args))
 		err("uffd_poll_thread create");
 
+	if (test_collapse) {
+		/*
+		 * Read just a single page and try collapsing. The collapse
+		 * should either be rejected or be a no-op.
+		 */
+		if (__check_memory_contents(0, 1, gopts, gopts->area_dst_alias))
+			err("unexpected memory contents before collapse");
+
+		/* MADV_COLLAPSE might return EINVAL for uffd-minor VMAs. */
+		if (madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
+			      MADV_COLLAPSE) && errno != EINVAL)
+			err("MADV_COLLAPSE failed unexpectedly");
+		/*
+		 * If the above collapse mapped pages that were not explicitly
+		 * CONTINUE'd, the below __check_memory_contents() will not
+		 * fault on some pages, resulting in incorrect contents.
+		 * The page we already touched should not be unmapped, so a
+		 * second userfault on it should not be generated.
+		 */
+	}
+
 	/*
 	 * Read each of the pages back using the UFFD-registered mapping. We
 	 * expect that the first time we touch a page, it will result in a minor
 	 * fault. uffd_poll_thread will resolve the fault by bit-flipping the
 	 * page's contents, and then issuing a CONTINUE ioctl.
 	 */
-	check_memory_contents(gopts, gopts->area_dst_alias);
+	bad_contents = !!check_memory_contents(gopts, gopts->area_dst_alias);
 
 	if (write(gopts->pipefd[1], &c, sizeof(c)) != sizeof(c))
 		err("pipe write");
 	if (pthread_join(uffd_mon, NULL))
 		err("join() failed");
 
+	if (bad_contents && test_collapse) {
+		uffd_test_fail("unexpected memory contents after collapse");
+		return;
+	}
+
+	if (bad_contents) {
+		uffd_test_fail("unexpected memory contents");
+		return;
+	}
+
 	if (test_collapse) {
+		/*
+		 * MADV_COLLAPSE will fail unless userfaultfd-minor is
+		 * unregistered.
+		 */
+		if (uffd_unregister(gopts->uffd, gopts->area_dst_alias,
+				    gopts->nr_pages * gopts->page_size))
+			err("uffd_unregister before MADV_COLLAPSE failed");
+
 		if (madvise(gopts->area_dst_alias, gopts->nr_pages * gopts->page_size,
 			    MADV_COLLAPSE)) {
 			/* It's fine to fail for this one... */
@@ -593,7 +645,10 @@ static void uffd_minor_test_common(uffd_global_test_opts_t *gopts, bool test_col
 		 * This won't cause uffd-fault - it purely just makes sure there
 		 * was no corruption.
 		 */
-		check_memory_contents(gopts, gopts->area_dst_alias);
+		if (check_memory_contents(gopts, gopts->area_dst_alias)) {
+			uffd_test_fail("unexpected memory contents");
+			return;
+		}
 	}
 
 	if (args.missing_faults != 0 || args.minor_faults != gopts->nr_pages)
-- 
2.56.0.rc1.315.gc6ed9934b7-goog