Architectures lacking MMU don'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:

- Detect NOMMU at runtime from the MmapCopy field in /proc/meminfo
  and avoid timeout, which requires fork(2).
- Document how to build and run the NOMMU tests, including the procfs
  prerequisite for runtime detection.

Build and run the tests with:

 $ make ARCH=um O=build kselftest-all TARGETS=nommu
 $ make ARCH=um O=build kselftest-install TARGETS=nommu
 $ ./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 <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>

--

v4 => v5
- remove build-time selection and detect NOMMU at runtime

v3 => v4
- fix typos in commit message

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/

rfc: https://lore.kernel.org/linux-mm/20260813063401.1786548-1-thehajime@gmail.com/
---
 Documentation/dev-tools/kselftest.rst       | 18 ++++++++++++++++++
 tools/testing/selftests/kselftest/runner.sh | 11 ++++++++++-
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/Documentation/dev-tools/kselftest.rst b/Documentation/dev-tools/kselftest.rst
index 64c0ec742..fc4dc1f9d 100644
--- a/Documentation/dev-tools/kselftest.rst
+++ b/Documentation/dev-tools/kselftest.rst
@@ -230,6 +230,24 @@ section::
 
 .. _tar's auto-compress: https://www.gnu.org/software/tar/manual/html_node/gzip.html#auto_002dcompress
 
+Build and test on nommu target
+==============================
+
+The nommu selftests detect NOMMU at runtime by checking for the ``MmapCopy``
+entry in ``/proc/meminfo``. Ensure procfs is mounted at ``/proc`` before
+building and running the tests.
+
+::
+
+  $ make ARCH=um O=build kselftest-all TARGETS=nommu  # <= build-only
+  $ make ARCH=um O=build kselftest-install TARGETS=nommu
+  $ ./build/kselftest/kselftest_install/run_kselftest.sh -p -c nommu
+
+The C tests report a skip if ``/proc/meminfo`` is unavailable. On a NOMMU
+system, the runner needs that file to avoid wrapping the tests in ``timeout``
+before they can report a skip. Mounting procfs is therefore a prerequisite
+for running this target through the kselftest runner.
+
 Contributing new tests
 ======================
 
diff --git a/tools/testing/selftests/kselftest/runner.sh b/tools/testing/selftests/kselftest/runner.sh
index 311811dc5..45fb8ec38 100644
--- a/tools/testing/selftests/kselftest/runner.sh
+++ b/tools/testing/selftests/kselftest/runner.sh
@@ -36,10 +36,19 @@ tap_prefix()
 	fi
 }
 
+is_nommu()
+{
+	grep -q '^MmapCopy:' /proc/meminfo 2>/dev/null
+}
+
 tap_timeout()
 {
+	# nommu doesn't support timeout command (missing fork(2))
+	if is_nommu; 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
-- 
2.52.0