[Bug 224479] kernel panic in reboot+swapoff sys call

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=224479

--- Comment #32 from ota at j.email.ne.jp ---
(In reply to Conrad Meyer from comment #31)

if tmpfs uses large memory so that files on tmpfs were swapped out, the 1st
swapoff_all() can fail due to insufficient memory.

vfs_unmountall() will unmount and release such tmpfs memory so that 2nd
swapoff_all() may be able to swapoff this time; however, if swap spaces left
are still NFS files or mdconfig files, we will still get panic.

So, I changed swapoff_all() to release in the reverse order swap spaces are
added.  System adds swap devices at start (given the penalty of mdconfig/NFS,
raw devices are best and most suitable for default entries).

When memory and swap space go short, I add md-file or NFS files to avoid
immediate issues.  So, reverse order in swap_all will take outs manually added
entries first to reduce chances of panic.

Actually... if all file systems are already unmounted, any processes get
swapped-in from swap spaces cannot do much useful anyway; indeed, we may not
need 2nd(existing) swapoff_all()...

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