[RFC PATCH 5/6] selftests: run tests on nommu architecture

[Mark Brown]

On Fri, Aug 14, 2026 at 01:50:50PM +0100, Lorenzo Stoakes (ARM) wrote:

> So at this point you're not really running much of the selftests at all,
> and are only happening to run those that don't do the very basic stuff
> nommu can't deal with.

> I think an audit of existing tests to figure out what works with nommu vs
> what doesn't is really unreasonable again, given nobody tests or seemingly
> uses these arches.

> So this patch might beget more nommu carve outs and exceptions and
> therefore nommu workload and maintenance, which isn't really sustainable.

> I know I complain about lack of testing on nommu, but at the same time I
> don't think running arbitrary tests that happen to work on it and who knows
> if they are valid asserts anyway really fixes things.

> And given nobody really is doing testing, it's not really a great RoI
> here...

Perhaps a good first step here is to start off with getting some public
CI set up, then build up to getting things running cleanly.  That way
there'll be visible ongoing coverage which will help motivate getting
whatever changes integrated, and if the changes are done bit by bit
there should be clearer explanations for why they make sense on nommu.

You could perhaps use a skiplist to ignore tests that don't work yet so
the CI starts off clean?

> 
> ...OTOH I suppose it's a minimal change we can largely ignore as long as
> this doesn't lead to additional work/auditing from anybody but nommu
> enthusiasts.
> 
> But I'm just not sure that will be the case :(
> 
> (I'm amazed we support a mode in linux that can't fork() in 2026 :)
> 
> 
> > ---
> >  Documentation/dev-tools/kselftest.rst       | 12 ++++++++++++
> >  tools/testing/selftests/kselftest/runner.sh |  9 +++++++--
> >  tools/testing/selftests/kselftest_harness.h |  4 ++++
> >  tools/testing/selftests/lib.mk              |  8 ++++++++
> >  4 files changed, 31 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/dev-tools/kselftest.rst b/Documentation/dev-tools/kselftest.rst
> > index 64c0ec7428a2..800b2b688aff 100644
> > --- a/Documentation/dev-tools/kselftest.rst
> > +++ b/Documentation/dev-tools/kselftest.rst
> > @@ -230,6 +230,18 @@ 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
> > +  $ make ARCH=um NOMMU=1 -C tools/testing/selftests/mm run_tests
> > +  $ NOMMU=1 /tmp/kselftest_install/run_kselftest.sh -s -c mm
> > +
> >  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..8eee7b14f824 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_msg("harness test doesn't support on NOMMU architecture (no fork(2)).\n");
> > +	return KSFT_SKIP;
> > +#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 2cc819006424..4734b5ce613f 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
> >
> 
> --
> Cheers, Lorenzo
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