page 的种类，swap的选择

对于swap页面时，页面可以分为一下几类：

1.unredaimable：

free page,reserved pages,pages dynamically allocated by the kernel,pages in the kernel mode stacks of the processes,temporarilly locked pages,memory locked pages

2.swappable:

anonymous pages in the user mode address spaces

mapped pages of tmpfs filesystem

3.syncable:

mapped pages in User mode address spaces

pages included in the page cache and containing data of disk files

block device buffer pages

pages of some disk caches

4.discardable:

unused pages included in memory caches,usused pages of the dentry cache

page reclaim算法：

它是一个混合的启发式算法：

1.根据page的类别进行选择

2.根据page的最近使用时间,因为linux使用的是LRU算法

3.page的状态，比方说，dirty page和普通page，则会优先换出普通page