Use device-mapper for per-mount test devices

We don't directly mount the underlying devices for each mount because
the kernel notices multiple mounts and doesn't setup a new super block
for each.

Previously the script used loopback devices to create the local shared
block construct 'cause it was easy.  This introduced corruption of
blocks that saw concurrent read and write IOs.  The buffered kernel file
IO paths that loopback eventually degrades into by default (via splice)
could have buffered readers copying out of pages without the page lock
while writers modified the page.  This manifest as occasional crc
failure of blocks that we knowingly issue concurrent reads and writes to
from multiple mounts (the quorum and super blocks).

This changes the script to use device-mapper linear passthrough devices.
Their IOs don't hit a caching layer and don't provide an opportunity to
corrupt blocks.

Signed-off-by: Zach Brown <zab@versity.com>

tests/README.md

@@ -25,8 +25,9 @@ All options can be seen by running with -h.
 This script is built to test multi-node systems on one host by using
 different mounts of the same devices.  The script creates a fake block
 device in front of each fs block device for each mount that will be
-tested.  Currently it will create free loop devices and will mount on
-/mnt/test.[0-9].
+tested.  It will create predictable device mapper devices and mounts
+them on /mnt/test.N.  These static device names and mount paths limit
+the script to a single execution per host.
 
 All tests will be run by default.  Particular tests can be included or
 excluded by providing test name regular expressions with the -I and -E
@@ -104,8 +105,8 @@ used during the test.
 
 | Variable         | Description          | Origin          | Example           |
 | ---------------- | -------------------  | --------------- | ----------------- |
-| T\_MB[0-9]       | per-mount meta bdev  | created per run | /dev/loop0        |
-| T\_DB[0-9]       | per-mount data bdev  | created per run | /dev/loop1        |
+| T\_MB[0-9]       | per-mount meta bdev  | created per run | /dev/mapper/\_scoutfs\_test\_meta\_[0-9]        |
+| T\_DB[0-9]       | per-mount data bdev  | created per run | /dev/mapper/\_scoutfs\_test\_data\_[0-9]        |
 | T\_D[0-9]        | per-mount test dir   | made for test   | /mnt/test.[0-9]/t |
 | T\_META\_DEVICE  | main FS meta bdev    | -M              | /dev/vda          |
 | T\_DATA\_DEVICE  | main FS data bdev    | -D              | /dev/vdb          |

tests/run-tests.sh

@@ -326,16 +326,10 @@ unmount_all() {
 		cmd wait $p
 	done
 
-	# delete all temp meta devices
-	for dev in $(losetup --associated "$T_META_DEVICE" | cut -d : -f 1); do
-		if [ -e "$dev" ]; then
-			cmd losetup -d "$dev"
-		fi
-	done
-	# delete all temp data devices
-	for dev in $(losetup --associated "$T_DATA_DEVICE" | cut -d : -f 1); do
-		if [ -e "$dev" ]; then
-			cmd losetup -d "$dev"
+	# delete all temp devices
+	for dev in /dev/mapper/_scoutfs_test_*; do
+		if [ -b "$dev" ]; then
+			cmd dmsetup remove $dev
 		fi
 	done
 }
@@ -434,6 +428,12 @@ $T_UTILS/fenced/scoutfs-fenced > "$T_FENCED_LOG" 2>&1 &
 fenced_pid=$!
 fenced_log "started fenced pid $fenced_pid in the background"
 
+# setup dm tables
+echo "0 $(blockdev --getsz $T_META_DEVICE) linear $T_META_DEVICE 0" > \
+	$T_RESULTS/dmtable.meta
+echo "0 $(blockdev --getsz $T_DATA_DEVICE) linear $T_DATA_DEVICE 0" > \
+	$T_RESULTS/dmtable.data
+
 #
 # mount concurrently so that a quorum is present to elect the leader and
 # start a server.
@@ -442,10 +442,13 @@ msg "mounting $T_NR_MOUNTS mounts on meta $T_META_DEVICE data $T_DATA_DEVICE"
 pids=""
 for i in $(seq 0 $((T_NR_MOUNTS - 1))); do
 
-	meta_dev=$(losetup --find --show $T_META_DEVICE)
-	test -b "$meta_dev" || die "failed to create temp device $meta_dev"
-	data_dev=$(losetup --find --show $T_DATA_DEVICE)
-	test -b "$data_dev" || die "failed to create temp device $data_dev"
+	name="_scoutfs_test_meta_$i"
+	cmd dmsetup create "$name" --table "$(cat $T_RESULTS/dmtable.meta)"
+	meta_dev="/dev/mapper/$name"
+
+	name="_scoutfs_test_data_$i"
+	cmd dmsetup create "$name" --table "$(cat $T_RESULTS/dmtable.data)"
+	data_dev="/dev/mapper/$name"
 
 	dir="/mnt/test.$i"
 	test -d "$dir" || cmd mkdir -p "$dir"