Linux 3/1 virtual address split

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.

Mapping 1 GB to kernel in each process allows processes to switch to kernel mode without also performing a context switch. Responses to system calls such as read(), mmap() and others can then be appropriately processed in the calling process' address space.

If space for the kernel were not reserved in each process, switching to "kernel mode" in between executing user space code would be more expensive, and be unable to use virtual address mapping through the hardware MMU (memory management unit) for the system calls being serviced.

Systems running a 32bit kernel with more than 1GB of physical memory, are able to assign physical memory locations in ZONE_HIGHMEM (roughly above the 1GB mark), which can require the kernel to jump through hoops for certain operations to interact with them. The addition of PAE (physical address extension), extends this problem by allowing upto 64GB of physical memory, decreasing the ratio of memory within the 1GB physical address memory, to regions allocated in ZONE_HIGHMEM.

1. For example the system calls use the kernel space.
2. You can have 64GB of physical ram, but on 32-bit platforms processors can access only 4gb because of the 32-bit virtual addressing. Actually, you can have 1GB of RAM and 3GB of swap and virtual addressing will make it look like you have 4GB. On 64-bit platforms virtual addressing is practically unlimited.