selftests/mm: skip soft-dirty tests on arm64

arm64 does not support the soft-dirty PTE bit.  However, the `soft-dirty`
test suite is currently run unconditionally and therefore generates
spurious test failures on arm64.  There are also some tests in
`madv_populate` which assume it is supported.

For `soft-dirty` lets disable the whole suite for arm64; it is no longer
built and run_vmtests.sh will skip it if its not present.

For `madv_populate`, we need a runtime mechanism so that the remaining
tests continue to be run.  Unfortunately, the only way to determine if the
soft-dirty dirty bit is supported is to write to a page, then see if the
bit is set in /proc/self/pagemap.  But the tests that we want to
conditionally execute are testing precicesly this.  So if we introduced
this feature check, we could accedentally turn a real failure (on a system
that claims to support soft-dirty) into a skip.  So instead, do the check
based on architecture; for arm64, we report that soft-dirty is not
supported.

Link: https://lkml.kernel.org/r/20230724082522.1202616-3-ryan.roberts@arm.comSigned-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Florent Revest <revest@chromium.org>
Cc: Jérôme Glisse <jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/Makefile

@@ -64,12 +64,15 @@ TEST_GEN_PROGS += thuge-gen
 TEST_GEN_PROGS += transhuge-stress
 TEST_GEN_PROGS += uffd-stress
 TEST_GEN_PROGS += uffd-unit-tests
-TEST_GEN_PROGS += soft-dirty
 TEST_GEN_PROGS += split_huge_page_test
 TEST_GEN_PROGS += ksm_tests
 TEST_GEN_PROGS += ksm_functional_tests
 TEST_GEN_PROGS += mdwe_test
 
+ifneq ($(ARCH),arm64)
+TEST_GEN_PROGS += soft-dirty
+endif
+
 ifeq ($(ARCH),x86_64)
 CAN_BUILD_I386 := $(shell ./../x86/check_cc.sh "$(CC)" ../x86/trivial_32bit_program.c -m32)
 CAN_BUILD_X86_64 := $(shell ./../x86/check_cc.sh "$(CC)" ../x86/trivial_64bit_program.c)

tools/testing/selftests/mm/madv_populate.c

@@ -264,14 +264,35 @@ static void test_softdirty(void)
 	munmap(addr, SIZE);
 }
 
+static int system_has_softdirty(void)
+{
+	/*
+	 * There is no way to check if the kernel supports soft-dirty, other
+	 * than by writing to a page and seeing if the bit was set. But the
+	 * tests are intended to check that the bit gets set when it should, so
+	 * doing that check would turn a potentially legitimate fail into a
+	 * skip. Fortunately, we know for sure that arm64 does not support
+	 * soft-dirty. So for now, let's just use the arch as a corse guide.
+	 */
+#if defined(__aarch64__)
+	return 0;
+#else
+	return 1;
+#endif
+}
+
 int main(int argc, char **argv)
 {
+	int nr_tests = 16;
 	int err;
 
 	pagesize = getpagesize();
 
+	if (system_has_softdirty())
+		nr_tests += 5;
+
 	ksft_print_header();
-	ksft_set_plan(21);
+	ksft_set_plan(nr_tests);
 
 	sense_support();
 	test_prot_read();
@@ -279,7 +300,8 @@ int main(int argc, char **argv)
 	test_holes();
 	test_populate_read();
 	test_populate_write();
-	test_softdirty();
+	if (system_has_softdirty())
+		test_softdirty();
 
 	err = ksft_get_fail_cnt();
 	if (err)

tools/testing/selftests/mm/run_vmtests.sh

@@ -322,7 +322,10 @@ then
 	CATEGORY="pkey" run_test ./protection_keys_64
 fi
 
-CATEGORY="soft_dirty" run_test ./soft-dirty
+if [ -x ./soft-dirty ]
+then
+	CATEGORY="soft_dirty" run_test ./soft-dirty
+fi
 
 # COW tests
 CATEGORY="cow" run_test ./cow