Linux 3/1 virtual address split

Question

I am missing something when it comes to understanding the need for highmem to address more than 1GB of RAM. Could someone point out where I go wrong? Thanks!

What I know:

• 1 GB of a processes' virtual memory (high memory region) is reserved for kernel operations. The user space can use the remaining 3 GB. This is the 3/1 split.

• The virtual memory features of the VM map the (continuous) virtual memory pages to physical pages (RAM).

What I don't know:

• What operations use the kernel virtual memory? I suppose things like kmalloc(...) in kernel-space would use kernel virtual memory.

• I would think that 4GB of RAM could be used under this scheme. I don't get why the kernel 1 GB virtual space is the limiting factor when addressing physical space. This is where my understanding breaks down. Please advise.

I've been reading this (http://kerneltrap.org/node/2450), which is great. But it doesn't quite address my question to my liking.

Answer 1

The reason that kernel virtual space is a limiting factor on useable physical memory is because the kernel needs access to all physical memory, and the way it accesses physical memory is through kernel virtual addresses. The kernel doesn't use special instructions that allow direct access to physical memory locations - it has to set up page table entries for any physical ranges that it wants to talk to.

In the "old style" scheme, the kernel set things up so that every process's page tables mapped virtual addresses from 0xC0000000 to 0xFFFFFFFF directly to physical addresses from 0x00000000 to 0x3FFFFFFF (these pages were marked so that they were only accessible in ring 0 - kernel mode). These are the "kernel virtual addresses". Under this scheme, the kernel could directly read and write any physical memory location without having to fiddle with the MMU to change the mappings.

Under the HIGHMEM scheme, the mappings from kernel virtual addresses to physical addresses aren't fixed - parts of physical memory are mapped in and out of the kernel virtual address space as the kernel needs access to that memory. This allows more physical memory to be used, but at the cost of having to constantly change the virtual-to-physical mappings, which is quite an expensive operation.