When pmap_pinit0() allocates a page for proc0's page directory, kernal page table may need to be extended.  But while growing the kernel page table (pmap_growkernel()), newly allocated kernel page table pages are entered into every process' page directory. For proc0, the page directory is not allocated yet, and results in a page fault.  Eventually, the machine panics with "lockmgr: not holding exclusive lock".
When pmap_pinit0() allocates a page for proc0's page directory,
kernal page table may need to be extended.  But while growing the
kernel page table (pmap_growkernel()), newly allocated kernel page
table pages are entered into every process' page directory. For
proc0, the page directory is not allocated yet, and results in a
page fault.  Eventually, the machine panics with "lockmgr: not
holding exclusive lock".

PR:		5458
Reviewed by:	phk
Submitted by:	Luoqi Chen <luoqi@luoqi.watermarkgroup.com>

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