linux – Understanding High Memory and Low Memory on Linux

kernellinuxmemory

I'm interested in the difference between Highmem and Lowmem:

  1. Why is there such a differentiation?
  2. What do we gain by doing so?
  3. What features does each have?

Best Answer

On a 32-bit architecture, the address space range for addressing RAM is: 

0x00000000 - 0xffffffff

or 4'294'967'295 (4 GB).

The linux kernel splits that up 3/1 (could also be 2/2, or 1/3 1) into user space (high memory) and kernel space (low memory) respectively.

The user space range: 

0x00000000 - 0xbfffffff

Every newly spawned user process gets an address (range) inside this area. User processes are generally untrusted and therefore are forbidden to access the kernel space. Further, they are considered non-urgent, as a general rule, the kernel tries to defer the allocation of memory to those processes.

The kernel space range: 

0xc0000000 - 0xffffffff

A kernel processes gets its address (range) here. The kernel can directly access this 1 GB of addresses (well, not the full 1 GB, there are 128 MB reserved for high memory access).

Processes spawned in kernel space are trusted, urgent and assumed error-free, the memory request gets processed instantaneously.

Every kernel process can also access the user space range if it wishes to. And to achieve this, the kernel maps an address from the user space (the high memory) to its kernel space (the low memory), the 128 MB mentioned above are especially reserved for this.

1 Whether the split is 3/1, 2/2, or 1/3 is controlled by the CONFIG_VMSPLIT_... option; you can probably check under /boot/config* to see which option was selected for your kernel.

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