[PATCH v2 1/2] selftests: run tests on nommu architecture

[Hajime Tazaki <thehajime@gmail.com>]

Architectures lacks MMU doesn't support fork(2) syscall and only
vfork(2) is available with limitations.  Thus, we cannot run kselftest
on nommu architecture as is.

This commit addresses this issue with the following changes:

- on build stage, add -DCONFIG_NOMMU to CFLAGS when NOMMU=1 variable
  added to the build/make argument.
- on test run stage, avoid calling timeout command when NOMMU=1 variable
  added to environmental variable, since timeout command uses fork
  syscall which nommu platform doesn't support.
- kselftest_harness.h: skip the tests if the file is include when
  executing on NOMMU platform, as there is no fork(2) syscall.
- replace "cd -" use as it is not available a shell supported on nommu
  (e.g., busybox hush), use cd "$OLDDIR" instead.
- describe the difference of nommu tests in the document.

So command line to build/execute tests for nommu should be like below:

 $ make ARCH=um NOMMU=1 O=build kselftest-all TARGETS=mm/nommu
 $ make ARCH=um NOMMU=1 O=build kselftest-install TARGETS=mm/nommu
 $ NOMMU=1 ./build/kselftest/kselftest_install/run_kselftest.sh -p \
   -c mm/nommu

Cc: Shuah Khan <shuah@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <kees@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Hangbin Liu <liuhangbin@gmail.com>
Cc: "Ricardo B. Marliere" <rbm@suse.com>
Cc: linux-kselftest@vger.kernel.org
Cc: linux-um@lists.infradead.org
Cc: linux-mm@kvack.org
Cc: linux-doc@vger.kernel.org
Closes: https://sashiko.dev/#/patchset/20260813063401.1786548-1-thehajime%40gmail.com
Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

--

rfc => v2
- reformat ktap header, reported by Sashiko review

rfc: https://lore.kernel.org/linux-mm/20260813063401.1786548-1-thehajime@gmail.com/
---
 Documentation/dev-tools/kselftest.rst       | 14 ++++++++++++++
 tools/testing/selftests/kselftest/runner.sh |  9 +++++++--
 tools/testing/selftests/kselftest_harness.h |  4 ++++
 tools/testing/selftests/lib.mk              |  8 ++++++++
 4 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/Documentation/dev-tools/kselftest.rst b/Documentation/dev-tools/kselftest.rst
index 64c0ec7428a2..a65f3ebc5906 100644
--- a/Documentation/dev-tools/kselftest.rst
+++ b/Documentation/dev-tools/kselftest.rst
@@ -230,6 +230,20 @@ section::
 
 .. _tar's auto-compress: https://www.gnu.org/software/tar/manual/html_node/gzip.html#auto_002dcompress
 
+Build and test on nommu target
+==============================
+
+If you (cross-)build kselftests for nommu targets, or run tests on nommu targets, use
+``NOMMU=1`` as a make variable/environment setting to tell build system to do the additional
+checks.  These nommu targets may differ in several ways, such as not supporting fork(2) or
+using musl or another libc.  Set this variable to apply the necessary build and test adjustments.
+
+::
+
+  $ make ARCH=um NOMMU=1 O=build kselftest-all TARGETS=mm/nommu  # <= build-only
+  $ make ARCH=um NOMMU=1 O=build kselftest-install TARGETS=mm/nommu
+  $ NOMMU=1 ./build/kselftest/kselftest_install/run_kselftest.sh -p -c mm/nommu
+
 Contributing new tests
 ======================
 
diff --git a/tools/testing/selftests/kselftest/runner.sh b/tools/testing/selftests/kselftest/runner.sh
index 311811dc55a0..7287d8290b6c 100644
--- a/tools/testing/selftests/kselftest/runner.sh
+++ b/tools/testing/selftests/kselftest/runner.sh
@@ -38,8 +38,12 @@ tap_prefix()
 
 tap_timeout()
 {
+	# nommu doesn't support timeout command (missing fork(2))
+	if [ "$NOMMU" = "1" ] ; then
+		echo "timeout isn't supported for nommu"
+		$1
 	# Make sure tests will time out if utility is available.
-	if [ -x /usr/bin/timeout ] ; then
+	elif [ -x /usr/bin/timeout ] ; then
 		/usr/bin/timeout --foreground "$kselftest_timeout" \
 			/usr/bin/timeout "$kselftest_timeout" $1
 	else
@@ -130,6 +134,7 @@ run_one()
 				return $KSFT_FAIL
 			fi
 		fi
+		OLDDIR=$(pwd)
 		cd `dirname $TEST` > /dev/null
 		(((( tap_timeout "$cmd" 2>&1; echo $? >&3) |
 			tap_prefix >&4) 3>&1) |
@@ -147,7 +152,7 @@ run_one()
 		*)
 			ktap_test_fail "$TEST_HDR_MSG # exit=$rc";;
 		esac
-		cd - >/dev/null
+		cd "$OLDDIR" >/dev/null
 	fi
 
 	return $rc
diff --git a/tools/testing/selftests/kselftest_harness.h b/tools/testing/selftests/kselftest_harness.h
index 261e4df94d9d..0d05946a5e7a 100644
--- a/tools/testing/selftests/kselftest_harness.h
+++ b/tools/testing/selftests/kselftest_harness.h
@@ -1274,6 +1274,10 @@ static int test_harness_run(int argc, char **argv)
 	unsigned int count = 0;
 	unsigned int pass_count = 0;
 
+#ifdef CONFIG_NOMMU
+	ksft_print_header();
+	ksft_exit_skip("kselftest harness requires fork(2), unavailable on NOMMU\n");
+#endif /* CONFIG_NOMMU */
 	ret = test_harness_argv_check(argc, argv);
 	if (ret != KSFT_PASS)
 		return ret;
diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index f02cc8a2e4ae..fdb895967768 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk
@@ -97,6 +97,14 @@ TEST_GEN_PROGS := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_PROGS))
 TEST_GEN_PROGS_EXTENDED := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_PROGS_EXTENDED))
 TEST_GEN_FILES := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_FILES))
 
+# detect if users request NOMMU build or not
+# User can set NOMMU to 1 to build/test for NOMMU platforms
+NOMMU ?= 0
+ifeq ($(NOMMU),1)
+CFLAGS += -DCONFIG_NOMMU
+export NOMMU
+endif
+
 all: $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED) $(TEST_GEN_FILES) \
 	$(if $(TEST_GEN_MODS_DIR),gen_mods_dir)
 
-- 
2.43.0