We have seen cases where the map fails, but we return the region
to the caller, causing them to page fault later on when they touch
the region.
The fix is to always observe the return code of map/remap.
The quickmap_page() and unquickmap_page() functions are used to map a
single physical page at a kernel virtual address for temporary access.
These use the per-CPU quickmap buffer in the page tables, and access to
this is guarded by the MM lock. To prevent bugs, quickmap_page() should
not *take* the MM lock, but rather verify that it is already held!
This exposed two situations where we were using quickmap without holding
the MM lock during page fault handling. This patch is forced to fix
these issues (which is great!) :^)
This has several benefits:
1) We no longer just blindly derefence a null pointer in various places
2) We will get nicer runtime error messages if the current process does
turn out to be null in the call location
3) GCC no longer complains about possible nullptr dereferences when
compiling without KUBSAN
This makes for nicer handling of errors compared to checking whether a
RefPtr is null. Additionally, this will give way to return different
types of errors in the future.
First off: unregister the region from MemoryManager before unmapping it.
The order of operations here was a bit strange, presumably to avoid a
situation where a fault would happen while unmapping, and the fault
handler would find the MemoryManager region list in an invalid state.
Unregistering it before unmapping sidesteps the whole problem, and
allows us to easily fix another problem: a deadlock could occur due
to inconsistent acquisition order (PageDirectory must come before MM.)
It may happen that CPU A manages to page in from the same inode
while we're just entering the same page fault handler on CPU B.
Handle it gracefully by checking if the data has already been paged in
(instead of VERIFY'ing that it hasn't) and then remap the page if that's
the case.
When booting AP's, we identity map a region at 0x8000 while doing the
initial bringup sequence. This is the only thing in the kernel that
requires an identity mapping, yet we had a bunch of generic API's and a
dedicated VirtualRangeAllocator in every PageDirectory for this purpose.
This patch simplifies the situation by moving the identity mapping logic
to the AP boot code and removing the generic API's.
...and also RangeAllocator => VirtualRangeAllocator.
This clarifies that the ranges we're dealing with are *virtual* memory
ranges and not anything else.
