From b67d86c6d37bb0b5417cfa95506982dae87b95db Mon Sep 17 00:00:00 2001
From: Jonathan Bauer
Date: Mon, 18 Nov 2019 11:53:24 +0100
Subject: [slx-dmsetup] dedicated tmpfs for cow fallback

---
 builder/modules.d/slx-dmsetup/module-setup.sh      |  2 +-
 .../slx-dmsetup/scripts/dmsetup-slx-device         | 34 +++++++++++++++++++---
 2 files changed, 31 insertions(+), 5 deletions(-)

diff --git a/builder/modules.d/slx-dmsetup/module-setup.sh b/builder/modules.d/slx-dmsetup/module-setup.sh
index 68e9b7b4..001104aa 100755
--- a/builder/modules.d/slx-dmsetup/module-setup.sh
+++ b/builder/modules.d/slx-dmsetup/module-setup.sh
@@ -11,7 +11,7 @@ install() {
 	inst_hook pre-pivot 10 "$moddir/scripts/generate-fstab-swap.sh"
 	inst_hook pre-pivot 00 "$moddir/scripts/grow-rootfs.sh"
 	# deliberatly left ext helpers out for now, since we don't really use it.
-	inst_multiple blockdev xxd \
+	inst_multiple blockdev fallocate xxd \
 		mkfs.ext4 resize2fs \
 		xfs_repair xfs_growfs
 }
diff --git a/builder/modules.d/slx-dmsetup/scripts/dmsetup-slx-device b/builder/modules.d/slx-dmsetup/scripts/dmsetup-slx-device
index 7a371123..89dab344 100755
--- a/builder/modules.d/slx-dmsetup/scripts/dmsetup-slx-device
+++ b/builder/modules.d/slx-dmsetup/scripts/dmsetup-slx-device
@@ -133,10 +133,36 @@ create_snapshot() {
 # This will call terminate the whole script by calling finish_setup, if successful
 ramdisk_fallback() {
 	echo "$0: Falling back to regular dm-snapshot on a RAMdisk."
-	local file="$(mktemp -u -p /run/openslx dnbd_cow.XXX)"
-	local size="$SLX_RAMDISK_SIZE_IN_MB"
-	[ -z "$size" ] && size="$(awk '/MemTotal/ {printf("%d\n", $2 / 2 / 1024 )}' /proc/meminfo)"
-	dd of="$file" seek="$size" bs=1M count=0 &> /dev/null
+	# first we create a dedicated tmpfs to control the maximum size
+	local cow_tmpfs="/run/openslx/dnbd-cow"
+	if ! mkdir -p "$cow_tmpfs"; then
+		cow_tmpfs="${cow_tmpfs}.$$"
+		if ! mkdir -p "$cow_tmpfs"; then
+			# ultimate fallback on regular /run/openslx
+			cow_tmpfs="/run/openslx"
+		fi
+	fi
+
+	local qcow_size_in_kb="$(( writable_device_size / 2 ))"
+	local ram_size_in_kb="$(awk '/MemTotal/ {printf("%d\n", $2 )}' /proc/meminfo)"
+
+	# Use the qcow size + half the RAM as tmpfs size.
+	local cow_size="$(( qcow_size_in_kb + ( ram_size_in_kb / 2 ) ))"
+	if ! mount -t tmpfs tmpfs -o size="${cow_size}k" "$cow_tmpfs"; then
+		emergency_shell "Failed to prepare dedicated CoW tmpfs in $cow_tmpfs of size $cow_size."
+	fi
+
+	local file="$(mktemp -u -p "$cow_tmpfs" dnbd_cow.XXX)"
+	local fail_count=0
+	while ! fallocate -l "${cow_size}K" "$file" ; do
+		(( cow_size -= 4 ))
+		if [ "$(( fail_count ++ ))" -eq 1000 ]; then
+			break
+		fi
+	done
+	if [ "$fail_count" -eq 1000 ]; then
+		emergency_shell "Failed to allocate CoW file $file after 1000 tries. :("
+	fi
 	writable_device="$(losetup --show --find "$file")"
 	cow_device_candidate="root"
 	while [ -b "/dev/mapper/$cow_device_candidate" ]; do
-- 
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