When pmap_change_attr() changes the PAT setting on a kernel mapping, it has to simultaneously change the PAT setting for the same pages within the direct map region.  This may require the demotion of a 2MB page mapping and the allocation of a page table page.  This revision gives the highest possible priority (VM_ALLOC_INTERRUPT) to this page allocation, so that pmap_change_attr() is less likely to fail.  (In general, kernel page table page allocations have the highest priority, so this is not creating a new precedent.)
When pmap_change_attr() changes the PAT setting on a kernel mapping, it has
to simultaneously change the PAT setting for the same pages within the
direct map region.  This may require the demotion of a 2MB page mapping and
the allocation of a page table page.  This revision gives the highest
possible priority (VM_ALLOC_INTERRUPT) to this page allocation, so that
pmap_change_attr() is less likely to fail.  (In general, kernel page table
page allocations have the highest priority, so this is not creating a new
precedent.)

(Demotion of 1GB page mappings within the direct map already specifies
VM_ALLOC_INTERRUPT to vm_page_alloc(), so only pmap_demote_pde() must be
changed.)

Approved by:	re (kib)

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