Remove the code to dyanmically change the pv_entry limits. Go back
to a single fixed kva reservation for pv entries, like was done
before when using the uma zone. Go back to never freeing pages
back to the free pool after they are no longer used, just like
before.
This stops the lock order reversal due to aquiring the kernel map
lock while pmap was locked.
This fixes the recursive panic if invariants are enabled.
The problem was that allocating/freeing kva causes vm_map_entry
nodes to be allocated/freed. That can recurse back into pmap as
new pages are hooked up to kvm and hence all the problem.
Allocating/freeing kva indirectly allocate/frees memory.
So, by going back to a single fixed size kva block and an index,
we avoid the recursion panics and the LOR.
The problem is that now with a linear block of kva, we have no
mechanism to track holes once pages are freed. UMA has the same
problem when using custom object for a zone and a fixed reservation
of kva. Simple solutions like having a bitmap would work, but would
be very inefficient when there are hundreds of thousands of bits
in the map. A first-free pointer is similarly flawed because pages
can be freed at random and the first-free pointer would be rewinding
huge amounts. If we could allocate memory for tree strucures or
an external freelist, that would work. Except we cannot allocate/free
memory here because we cannot allocate/free address space to use
it in. Anyway, my change here reverts back to the UMA behavior of
not freeing pages for now, thereby avoiding holes in the map.
ups@ had a truely evil idea that I'll investigate. It should allow
freeing unused pages again by giving us a no-cost way to track the
holes in the kva block. But in the meantime, this should get people
booting with witness and/or invariants again.
Footnote: amd64 doesn't have this problem because of the direct map
access method. I'd done all my witness/invariants testing there. I'd
never considered that the harmless-looking kmem_alloc/kmem_free calls
would cause such a problem and it didn't show up on the boot test.
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