Allow for fluctuation of MemTotal for automatic swapfile size (#4016)

As the reported MemTotal can fluctuate a bit on some systems, e.g. because the
reserved memory changes between kernel version or other factors affect it like
VRAM, the swap file can be recreated unnecessarily between boots. Allow for
some fluctuation (up to +-32MB) before the swapfile is recreated.

This was a problem already before the recent haos-swapfile changes, however,
before it checked if the existing swapfile isn't smaller than the desired
value. If the MemTotal fluctuated there, the swapfile size eventually settled
on the highest value seen and it wasn't recreated anymore. With this change,
things should be stable even more.

Author: sairon

buildroot-external/rootfs-overlay/usr/libexec/haos-swapfile

@@ -21,11 +21,13 @@ SWAPFILE="/mnt/data/swapfile"
 
 # Swap size in kilobytes (as it's also what meminfo shows)
 SWAPSIZE="$(size2kilobytes "${SWAPSIZE}")"
+SWAPSIZE_TOLERANCE=0
 
 if [ -z "${SWAPSIZE}" ] || [ "${SWAPSIZE}" = "-1" ]; then
 	# Default to 33% of total memory
 	SWAPSIZE="$(awk '/MemTotal/{ print int($2 * 0.33) }' /proc/meminfo)"
 	echo "[INFO] Using default swapsize of 33% RAM (${SWAPSIZE} kB)"
+	SWAPSIZE_TOLERANCE=$((32*1024))  # allow for 32MB fluctuations
 fi
 
 # Swap space in 4k blocks
@@ -42,7 +44,12 @@ if [ "${SWAPSIZE_BLOCKS}" -lt 10 ]; then
     exit 0
 fi
 
-if [ ! -s "${SWAPFILE}" ] || [ "$(stat "${SWAPFILE}" -c '%s')" -ne $((SWAPSIZE_BLOCKS * 4096)) ]; then
+CURRENT_SIZE="$([ -f "${SWAPFILE}" ] && stat "${SWAPFILE}" -c '%s' || echo 0)"
+
+if [ -s "${SWAPFILE}" ] && [ "${CURRENT_SIZE}" -ge $(((SWAPSIZE - SWAPSIZE_TOLERANCE) * 1024)) ] \
+    && [ "${CURRENT_SIZE}" -le $(((SWAPSIZE + SWAPSIZE_TOLERANCE) * 1024)) ]; then
+    echo "[INFO] Swapfile already exists with size ${CURRENT_SIZE} bytes"
+elif [ ! -s "${SWAPFILE}" ] || [ "${CURRENT_SIZE}" -ne $((SWAPSIZE_BLOCKS * 4096)) ]; then
     # Check free space (in 4k blocks)
     if [ "$(stat -f /mnt/data -c '%f')" -lt "${SWAPSIZE_BLOCKS}" ]; then
         echo "[ERROR] Not enough space to allocate swapfile"