GENERIC 12.0-RELEASE-p7 (amd64) panic when rebooting

[pomoke]

I have a reproducible panic on my VPS just before powering off or
rebooting.Suggesting umounting fs before swapoff?

Message of the crash:
   Waiting (max 60 seconds) for system process `vnlru' to stop... done
   Waiting (max 60 seconds) for system process `syncer' to stop... 
   Syncing disks, vnodes remaining... 12 12 12 3 1 1 0 0 0 0 done
   Waiting (max 60 seconds) for system thread `bufdaemon' to stop...
   done
   Waiting (max 60 seconds) for system thread `bufspacedaemon-0' to
   stop... done
   All buffers synced.
   swap_pager: I/O error - pagein failed; blkno 3291,size 4096, error 5
   panic: swap_pager_force_pagein: read from swap failed
   cpuid = 0
   time = 1559891849
   KDB: stack backtrace:
   #0 0xffffffff80be7977 at kdb_backtrace+0x67
   #1 0xffffffff80b9b563 at vpanic+0x1a3
   #2 0xffffffff80b9b3b3 at panic+0x43
   #3 0xffffffff80ed14bd at swapoff_one+0x80d
   #4 0xffffffff80ed15e7 at swapoff_all+0x117
   #5 0xffffffff80c4905a at bufshutdown+0x2fa
   #6 0xffffffff80b9aee8 at kern_reboot+0x228
   #7 0xffffffff80b9acb1 at sys_reboot+0x411
   #8 0xffffffff81075449 at amd64_syscall+0x369
   #9 0xffffffff8104fd1d at fast_syscall_common+0x101
   Uptime: 20h4m14s
   Automatic reboot in 15 seconds - press a key on the console to abort
   Rebooting...

/etc/fstab :
   /dev/ufs/rootfs	/		ufs	rw,userquota,groupquota	0 0
   md11 none swap sw,file=/var/swap,late 0 0

swapinfo (upon reboot) :
   /dev/md11          1572864        0  1572864     0%
   -- 
   Darui Zhuo <pomoke at typeof.pw>
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