[PATCH] selftests/mm: Fix soft-dirty kselftest supported check

[Audra Mitchell <audra@redhat.com>]

On architectures with separate user address space, such as s390 or
those without an MMU, the soft-dirty kselftest may fail when checking
to see if the feature is supported.

  # --------------------
  # running ./soft-dirty
  # --------------------
  # TAP version 13
  # 1..15
  # Bail out! PAGEMAP_SCAN succeeded unexpectedly
  # # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
  # [FAIL]
  not ok 1 soft-dirty # exit=1
  # SUMMARY: PASS=0 SKIP=0 FAIL=1

The soft-dirty test will initate an ioctl with the PAGEMAP_SCAN flag with
an invalid address for the page_region. This is done intentionally to
have the ioctl return with an expected EFAULT and with the correct
categories returned.

However, on architectures with separate user address space, such as s390 or
those without an MMU, the call to __access_ok (used to validate the
variables provided with the ioctl) will always return true and we will not
fail as expected.

        if (IS_ENABLED(CONFIG_ALTERNATE_USER_ADDRESS_SPACE) ||
            !IS_ENABLED(CONFIG_MMU))
                return true;

Let's simplify the check for PAGEMAP_SCAN and provide a valid page_region
address so that we get a non-errno return if it is supported.

Fixes: 600bca580579 ("selftests/mm: check that PAGEMAP_SCAN returns correct categories")

Signed-off-by: Audra Mitchell <audra@redhat.com>
---
 tools/testing/selftests/mm/vm_util.c | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/mm/vm_util.c b/tools/testing/selftests/mm/vm_util.c
index a6d4ff7dfdc0..82998406b335 100644
--- a/tools/testing/selftests/mm/vm_util.c
+++ b/tools/testing/selftests/mm/vm_util.c
@@ -67,20 +67,26 @@ static uint64_t pagemap_scan_get_categories(int fd, char *start)
 }
 
 /* `start` is any valid address. */
-static bool pagemap_scan_supported(int fd, char *start)
+static bool pagemap_scan_supported(int fd)
 {
+	const size_t pagesize = getpagesize();
 	static int supported = -1;
-	int ret;
+	struct page_region r;
+	void *test_area;
 
 	if (supported != -1)
 		return supported;
 
-	/* Provide an invalid address in order to trigger EFAULT. */
-	ret = __pagemap_scan_get_categories(fd, start, (struct page_region *) ~0UL);
-	if (ret == 0)
-		ksft_exit_fail_msg("PAGEMAP_SCAN succeeded unexpectedly\n");
-
-	supported = errno == EFAULT;
+	test_area = mmap(0, pagesize, PROT_READ | PROT_WRITE,
+			MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
+	if (test_area == MAP_FAILED) {
+		ksft_print_msg("WARN: mmap() failed: %s\n", strerror(errno));
+		supported = 0;
+	} else {
+		supported = __pagemap_scan_get_categories(fd, test_area, &r) >= 0;
+		ksft_print_msg("errno: %d\n", errno);
+		munmap(test_area, pagesize);
+	}
 
 	return supported;
 }
@@ -90,7 +96,7 @@ static bool page_entry_is(int fd, char *start, char *desc,
 {
 	bool m = pagemap_get_entry(fd, start) & pagemap_flags;
 
-	if (pagemap_scan_supported(fd, start)) {
+	if (pagemap_scan_supported(fd)) {
 		bool s = pagemap_scan_get_categories(fd, start) & pagescan_flags;
 
 		if (m == s)
-- 
2.52.0