base-files: flush kernel memory cache during sysupgrade

Flush kernel memory caches during sysupgrade in order
to mitigate the impact from memory consumption spikes
in low-RAM devices.

This may help to prevent sysupgrade causing a reboot
before the actual flashing starts.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: maurerr <mariusd84@gmail.com>

package/base-files/files/lib/upgrade/common.sh

@@ -297,6 +297,7 @@ indicate_upgrade() {
 # $(2): (optional) pipe command to extract firmware, e.g. dd bs=n skip=m
 default_do_upgrade() {
 	sync
+	echo 3 > /proc/sys/vm/drop_caches
 	if [ -n "$UPGRADE_BACKUP" ]; then
 		get_image "$1" "$2" | mtd $MTD_ARGS $MTD_CONFIG_ARGS -j "$UPGRADE_BACKUP" write - "${PART_NAME:-image}"
 	else

package/base-files/files/lib/upgrade/stage2

@@ -123,6 +123,7 @@ kill_remaining KILL 1
 
 sleep 1
 
+echo 3 > /proc/sys/vm/drop_caches
 
 if [ -n "$IMAGE" ] && type 'platform_pre_upgrade' >/dev/null 2>/dev/null; then
 	platform_pre_upgrade "$IMAGE"