contrib/check-config.sh: fix MEMCG_SWAP_ENABLED

Kernel commit 2d1c498072de69e (which made its way into kernel v5.8-rc1)
removed CONFIG_MEMCG_SWAP_ENABLED Kconfig option, making swap accounting
always enabled (unless swapaccount=0 boot option is provided).

Make the check conditional.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 070f9d9dd35e854f4a30563183987a6ec3276847)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

contrib/check-config.sh

@@ -230,13 +230,21 @@ echo 'Optional Features:'
 	check_flags CGROUP_PIDS
 }
 {
-	CODE=${EXITCODE}
-	check_flags MEMCG_SWAP MEMCG_SWAP_ENABLED
-	if [ -e /sys/fs/cgroup/memory/memory.memsw.limit_in_bytes ]; then
+	check_flags MEMCG_SWAP
+	# Kernel v5.8+ removes MEMCG_SWAP_ENABLED.
+	if [ "$kernelMajor" -lt 5 ] || [ "$kernelMajor" -eq 5 -a "$kernelMinor" -le 8 ]; then
+		CODE=${EXITCODE}
+		check_flags MEMCG_SWAP_ENABLED
+		# FIXME this check is cgroupv1-specific
+		if [ -e /sys/fs/cgroup/memory/memory.memsw.limit_in_bytes ]; then
+			echo "    $(wrap_color '(cgroup swap accounting is currently enabled)' bold black)"
+			EXITCODE=${CODE}
+		elif is_set MEMCG_SWAP && ! is_set MEMCG_SWAP_ENABLED; then
+			echo "    $(wrap_color '(cgroup swap accounting is currently not enabled, you can enable it by setting boot option "swapaccount=1")' bold black)"
+		fi
+	else
+		# Kernel v5.8+ enables swap accounting by default.
 		echo "    $(wrap_color '(cgroup swap accounting is currently enabled)' bold black)"
-		EXITCODE=${CODE}
-	elif is_set MEMCG_SWAP && ! is_set MEMCG_SWAP_ENABLED; then
-		echo "    $(wrap_color '(cgroup swap accounting is currently not enabled, you can enable it by setting boot option "swapaccount=1")' bold black)"
 	fi
 }
 {