[PATCH v3 2/3] selftests: run tests on nommu architecture

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 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.
- 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=nommu
 $ make ARCH=um NOMMU=1 O=build kselftest-install TARGETS=nommu
 $ NOMMU=1 ./build/kselftest/kselftest_install/run_kselftest.sh -p \
   -c 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 <redacted>
Cc: "Ricardo B. Marliere" <redacted>
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 <redacted>

--

v2 => v3
- remove unnecessary modification
- new target is moved from TARGETS=mm/nommu to TARGETS=nommu
- rename CONFIG_NOMMU to NOMMU to avoid confusion to kernel config

rfc => v2
- reformat ktap header, reported by Sashiko review
- https://lore.kernel.org/linux-mm/20260825015945.141739-2-thehajime@gmail.com/ (local)

rfc: https://lore.kernel.org/linux-mm/20260813063401.1786548-1-thehajime@gmail.com/ (local)
---
 Documentation/dev-tools/kselftest.rst       | 14 ++++++++++++++
 tools/testing/selftests/kselftest/runner.sh |  6 +++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kselftest.rst b/Documentation/dev-tools/kselftest.rst
index 64c0ec7428a2..c35b819947b7 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=nommu  # <= build-only
+  $ make ARCH=um NOMMU=1 O=build kselftest-install TARGETS=nommu
+  $ NOMMU=1 ./build/kselftest/kselftest_install/run_kselftest.sh -p -c nommu
+
 Contributing new tests
 ======================
 

diff --git a/tools/testing/selftests/kselftest/runner.sh b/tools/testing/selftests/kselftest/runner.sh
index 8fc1018d6fec..95b5ebb8bbb9 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

-- 
2.43.0