If we crashed in the middle of mapping in Regions, some of the regions
may not have a page directory yet, and will result in a crash when
Region::remap() is called.
When a page fault led to the mapping of a new physical page, we were
updating the page tables for *every* region that shared the same
underlying VMObject.
Let's just not do that, avoiding a bunch of unnecessary page table
updates and TLB invalidations.
This allows us to enable Write-Combine on e.g. framebuffers,
significantly improving performance on bare metal.
To keep things simple we right now only use one of up to three bits
(bit 7 in the PTE), which maps to the PA4 entry in the PAT MSR, which
we set to the Write-Combine mode on each CPU at boot time.
We were already using a non-intrusive RedBlackTree, and since the kernel
regions tree is non-owning, this is a trivial conversion that makes a
bunch of the tree operations infallible (by being allocation-free.) :^)
When deleting an entire AddressSpace, we don't need to do TLB flushes
at all (since the entire page directory is going away anyway).
We also don't need to deallocate VM ranges one by one, since the entire
VM range allocator will be deleted anyway.
We now use AK::Error and AK::ErrorOr<T> in both kernel and userspace!
This was a slightly tedious refactoring that took a long time, so it's
not unlikely that some bugs crept in.
Nevertheless, it does pass basic functionality testing, and it's just
real nice to finally see the same pattern in all contexts. :^)
...and also RangeAllocator => VirtualRangeAllocator.
This clarifies that the ranges we're dealing with are *virtual* memory
ranges and not anything else.
