Getting 'vm_thread_new: kstack allocation failed' errors after latest drm updates.

[Alan Cox]

Roughly speaking, vm.kvm_* pertain to the kernel's address space and
vm.kmem_* pertain to the kernel's heap within its address space.  Your
settings of vm.kmem_* have made the heap so large that there is little or no
free space left in the address space for other things.  These other things,
e.g., the buffer cache, are non-trivial in size.

So, the short answer is that you need to reduce vm.kmem_size.

Alan