7eeda22793
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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