tmpdir-setup: Avoid flush of TMPDIR

Use a dm-device (linear target) between the filesystem and the loop
device and then use this sequence for teardown:

- fcntl EXT4_IOC_SHUTDOWN with EXT4_GOING_FLAGS_NOLOGFLUSH
- dmestup reload $dmname --table "0 $sectors zero"
- dmsetup resume $dmname --noflush
- umount $mountpoint
- dmsetup remove --deferred $dmname
- rmdir $mountpoint

helper/tmpdir-setup

@@ -20,6 +20,8 @@ cmd_create() {
 
     filename=$tmpdir/$MXQ_JOBID.tmp
     mountpoint=$mntdir/$MXQ_JOBID
+    dmname=mxq.$MXQ_JOBID
+    dmdevice=/dev/mapper/$dmname
 
     umask 006
     mkdir -p $tmpdir
@@ -29,15 +31,19 @@ cmd_create() {
 
     if fallocate -l ${MXQ_SIZE}G $filename; then
         if loopdevice=$(losetup --find --show $filename); then
-            if mkfs.ext4 \
-                    -q \
-                    -m 0 \
-                    -E nodiscard,mmp_update_interval=300,lazy_journal_init=1,root_owner=$MXQ_UID:0 \
-                    -O '64bit,ext_attr,filetype,^has_journal,huge_file,inline_data,^mmp,^quota,sparse_super2' \
-                    $loopdevice \
-                    && mkdir -p $mountpoint && mount -Odata=writeback,barrier=0  $loopdevice $mountpoint; then
-                rmdir $mountpoint/lost+found
-                status=0
+            if dmsetup create $dmname --table "0 $((MXQ_SIZE*1024*1024*2)) linear $loopdevice 0"; then
+                if mkfs.ext4 \
+                        -q \
+                        -m 0 \
+                        -E nodiscard,mmp_update_interval=300,lazy_journal_init=1,root_owner=$MXQ_UID:0 \
+                        -O '64bit,ext_attr,filetype,^has_journal,huge_file,inline_data,^mmp,^quota,sparse_super2' \
+                        $dmdevice \
+                        && mkdir -p $mountpoint && mount -Odata=writeback,barrier=0  $dmdevice $mountpoint; then
+                    rmdir $mountpoint/lost+found
+                    status=0
+                else
+                    dmsetup remove --force --deferred $dmname
+                fi
             fi
             losetup -d $loopdevice
         fi
@@ -52,12 +58,24 @@ cmd_cleanup() {
     (( $# == 1 )) || usage
     MXQ_JOBID=$1
 
-    (
-        shopt -s dotglob;
-        rm -rf /dev/shm/mxqd/mnt/job/$MXQ_JOBID/*
-        umount /dev/shm/mxqd/mnt/job/$MXQ_JOBID
-        rmdir /dev/shm/mxqd/mnt/job/$MXQ_JOBID
-    ) &
+    mountpoint=$mntdir/$MXQ_JOBID
+    dmname=mxq.$MXQ_JOBID
+    dmdevice=/dev/mapper/$dmname
+    minor=$(( 0x$(stat -c%T $dmdevice) ))
+    sectors=$(cat /sys/block/dm-$minor/size)
+
+    # fs/ext4/ext4.h: #define EXT4_IOC_SHUTDOWN _IOR ('X', 125, __u32)
+    # (_IOC_READ << 30) + (sizeof(_u32) << 16) + ( 'X' << 8 ) + 125
+    # ( 2 << 30 ) + ( 4 << 16 ) + ( 88 << 8 ) + 125 = 2147768445
+    # fs/ext4/ext4.h: #define EXT4_GOING_FLAGS_NOLOGFLUSH       0x2 /* don't flush log nor data */
+
+    /usr/local/system/perl/bin/perl -e "open F, '<', '$mountpoint/.' ; \$d=pack('L', 2) ; ioctl (F, 2147768445, \$d)"
+
+    dmsetup reload $dmname --table "0 $sectors zero"
+    dmsetup resume $dmname --noflush
+    umount --lazy $mountpoint
+    dmsetup remove $dmname
+    rmdir $mountpoint
 }
 
 (( $# > 0 )) || usage