reserved page frame

linux保留了一定的reserved page frame
数量由以下方式决定：
The amount of the reserved memory (in kilobytes) is stored in the min_free_kbytes variable. Its initial value is set during kernel initialization and
depends on the amount of physical memory that is directly mapped in the kernel's fourth gigabyte of linear addressesthat is, it depends on the number
of page frames included in the ZONE_DMA and ZONE_NORMAL memory zones.(有个公式，详见《understanding the linux kernel》302页）
之所以要保留这些页面，是因为对于有些kernel control path是不能因缺页而被阻塞的，那些操作又被称为atomic page allcation request。
这些保留的页面都分布在ZONE_DMA和ZONE_NORMAL区，比例按ZONE_DMA 和 ZONE_NORMAL的比例进行保留。
The ZONE_DMA and ZONE_NORMAL memory zones contribute to the reserved memory with a number of page frames proportional to their relative sizes. For
instance, if the ZONE_NORMAL zone is eight times bigger than ZONE_DMA, seven-eighths of the page frames will be taken from ZONE_NORMAL and one-eighth
from ZONE_DMA.

2009/02/02 一