helper/tmpdir-setup: Use device mapper to shortcut unmount writeback

This is very EXPERIMENTAL.

This change fixes the problem of unnecessary writeback when we unmount the
TMODIR image which sometimes take a long time (minutes!) and wastes i/O
bandwidth.

We sometimes trigger

[2966308.931208] EXT4-fs (dm-1): ext4_writepages: jbd2_start: 4096 pages, ino 11; err -5

with this. It is yet unclear, id this is a problem or not.

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
@@ -53,8 +59,21 @@ cmd_cleanup() {
     MXQ_JOBID=$1
 
     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)
 
-    umount $mountpoint
+    # fs/ext4/ext4.h: #define EXT4_IOC_SHUTDOWN _IOR ('X', 125, __u32)
+    # (_IOC_READ << 30) + (sizeof(_u32) << 16) + ( 'X' << 8 ) + 125
+    # ( 2 << 30 ) + ( 4 << 16 ) + ( 88 << 8 ) + 125 = 2147768445
+    # fs/ext4/ext4.h: #define EXT4_GOING_FLAGS_NOLOGFLUSH       0x2 /* don't flush log nor data */
+
+    /usr/local/system/perl/bin/perl -e "open F, '<', '$mountpoint/.' ; \$d=pack('L', 2) ; ioctl (F, 2147768445, \$d)"
+
+    dmsetup reload $dmname --table "0 $sectors zero"
+    dmsetup resume $dmname --noflush
+    umount --lazy $mountpoint
     rmdir $mountpoint
 }