selftests/vm: only run 128TBswitch with 5-level paging

The test va_128TBswitch.c expects to be able to pass mmap an address hint
and length that cross the address 1<<47.  On x86_64, this is not possible
without 5-level page tables, so the test fails.

The test is already only run on 64-bit powerpc and x86_64 archs, but this
patch adds an additional check on x86_64 that skips the test if
PG_TABLE_LEVELS < 5.  There is precedent for checking /proc/config.gz in
selftests, e.g.  in selftests/firmware.

Running the tests produces the desired output:

sudo make -C tools/testing/selftests TARGETS=vm run_tests
---------------------------
running ./va_128TBswitch.sh
---------------------------
./va_128TBswitch.sh: PG_TABLE_LEVELS=4, must be >= 5 to run this test
[SKIP]
-------------------------------

[adam@wowsignal.io: restrict the check to x86_64]
  Link: https://lkml.kernel.org/r/20220628163654.337600-1-adam@wowsignal.io
[adam@wowsignal.io: fix formatting issues, rename "die" to "fail"]
  Link: https://lkml.kernel.org/r/20220701163030.415735-1-adam@wowsignal.io
Link: https://lkml.kernel.org/r/20220627163912.5581-1-adam@wowsignal.io
Signed-off-by: Adam Sindelar <adam@wowsignal.io>
Cc: Adam Sindelar <ats@fb.com>
Cc: David Vernet <void@manifault.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/vm/Makefile

@@ -93,6 +93,7 @@ TEST_PROGS := run_vmtests.sh
 
 TEST_FILES := test_vmalloc.sh
 TEST_FILES += test_hmm.sh
+TEST_FILES += va_128TBswitch.sh
 
 KSFT_KHDR_INSTALL := 1
 include ../lib.mk

tools/testing/selftests/vm/run_vmtests.sh

@@ -151,7 +151,7 @@ if [ $VADDR64 -ne 0 ]; then
 	run_test ./virtual_address_range
 
 	# virtual address 128TB switch test
-	run_test ./va_128TBswitch
+	run_test ./va_128TBswitch.sh
 fi # VADDR64
 
 # vmalloc stability smoke test

tools/testing/selftests/vm/va_128TBswitch.sh

@@ -0,0 +1,54 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Copyright (C) 2022 Adam Sindelar (Meta) <adam@wowsignal.io>
+#
+# This is a test for mmap behavior with 5-level paging. This script wraps the
+# real test to check that the kernel is configured to support at least 5
+# pagetable levels.
+
+# 1 means the test failed
+exitcode=1
+
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
+fail()
+{
+	echo "$1"
+	exit $exitcode
+}
+
+check_supported_x86_64()
+{
+	local config="/proc/config.gz"
+	[[ -f "${config}" ]] || config="/boot/config-$(uname -r)"
+	[[ -f "${config}" ]] || fail "Cannot find kernel config in /proc or /boot"
+
+	# gzip -dcfq automatically handles both compressed and plaintext input.
+	# See man 1 gzip under '-f'.
+	local pg_table_levels=$(gzip -dcfq "${config}" | grep PGTABLE_LEVELS | cut -d'=' -f 2)
+
+	if [[ "${pg_table_levels}" -lt 5 ]]; then
+		echo "$0: PGTABLE_LEVELS=${pg_table_levels}, must be >= 5 to run this test"
+		exit $ksft_skip
+	fi
+}
+
+check_test_requirements()
+{
+	# The test supports x86_64 and powerpc64. We currently have no useful
+	# eligibility check for powerpc64, and the test itself will reject other
+	# architectures.
+	case `uname -m` in
+		"x86_64")
+			check_supported_x86_64
+		;;
+		*)
+			return 0
+		;;
+	esac
+}
+
+check_test_requirements
+./va_128TBswitch