highmem tactics

this article copy from: http://linux-mm.org/HighMemory

Basically the system uses the following tactics (for highmem):

• Memory above the physical address of 896MB are temporarily mapped into kernel virtual memory whenever the kernel needs to access that memory.

• Data which the kernel frequently needs to access is allocated in the lower 896MB of memory (ZONE_NORMAL) and can be immediately accessed by the kernel (see Temporary mapping).

• Data which the kernel only needs to access occasionally, including page cache, process memory and page tables, are preferentially allocated from ZONE_HIGHMEM.
• The system can have additional physical memory zones to deal with devices that can only perform DMA to a limited amount of physical memory, ZONE_DMA and ZONE_DMA32.

• Allocations and pageout pressure on the various memory zones need to be balanced (see Memory Balancing).