selftests/powerpc: Test for spurious kernel memory faults on radix

Previously when mapping kernel memory on radix, no ptesync was
included which would periodically lead to unhandled spurious faults.
Mapping kernel memory is used when code patching with Strict RWX
enabled. As suggested by Chris Riedl, turning ftrace on and off does a
large amount of code patching so is a convenient way to see this kind
of fault.

Add a selftest to try and trigger this kind of a spurious fault. It
tests for 30 seconds which is usually long enough for the issue to
show up.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
[mpe: Rename it to better reflect what it does, rather than the symptom]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210208032957.1232102-2-jniethe5@gmail.com

tools/testing/selftests/powerpc/mm/Makefile

@@ -5,6 +5,7 @@ noarg:
 TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors wild_bctr \
 		  large_vm_fork_separation bad_accesses pkey_exec_prot \
 		  pkey_siginfo stack_expansion_signal stack_expansion_ldst
+TEST_PROGS := stress_code_patching.sh
 
 TEST_GEN_PROGS_EXTENDED := tlbie_test
 TEST_GEN_FILES := tempfile

tools/testing/selftests/powerpc/mm/stress_code_patching.sh

@@ -0,0 +1,49 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+TIMEOUT=30
+
+DEBUFS_DIR=`cat /proc/mounts | grep debugfs | awk '{print $2}'`
+if [ ! -e "$DEBUFS_DIR" ]
+then
+	echo "debugfs not found, skipping" 1>&2
+	exit 4
+fi
+
+if [ ! -e "$DEBUFS_DIR/tracing/current_tracer" ]
+then
+	echo "Tracing files not found, skipping" 1>&2
+	exit 4
+fi
+
+
+echo "Testing for spurious faults when mapping kernel memory..."
+
+if grep -q "FUNCTION TRACING IS CORRUPTED" "$DEBUFS_DIR/tracing/trace"
+then
+	echo "FAILED: Ftrace already dead. Probably due to a spurious fault" 1>&2
+	exit 1
+fi
+
+dmesg -C
+START_TIME=`date +%s`
+END_TIME=`expr $START_TIME + $TIMEOUT`
+while [ `date +%s` -lt $END_TIME ]
+do
+	echo function > $DEBUFS_DIR/tracing/current_tracer
+	echo nop > $DEBUFS_DIR/tracing/current_tracer
+	if dmesg | grep -q 'ftrace bug'
+	then
+		break
+	fi
+done
+
+echo nop > $DEBUFS_DIR/tracing/current_tracer
+if dmesg | grep -q 'ftrace bug'
+then
+	echo "FAILED: Mapping kernel memory causes spurious faults" 1>&2
+	exit 1
+else
+	echo "OK: Mapping kernel memory does not cause spurious faults"
+	exit 0
+fi