分类：Linux kernel 笔记

MMU（Memory Management Unit）用来管理内存的最小单位是一个物理页面（physical page，在32-bit系统上通常每个页面为4k，64-bit系统上为8k）。Linux kernel使用struct page结构体（定义在<linux/mm_types.h>）来代表每个物理页面：

struct page {
    /* First double word block */
    unsigned long flags;        /* Atomic flags, some possibly
                     * updated asynchronously */
    struct address_space *mapping;  /* If low bit clear, points to
                     * inode address_space, or NULL.
                     * If page mapped as anonymous
                     * memory, low bit is set, and
                     * it points to anon_vma object:
                     * see PAGE_MAPPING_ANON below.
                     */
    ......
}  

受制于一些硬件限制，有些物理页面不能用来进行某种操作。所以kernel把页面分成不同的zone：
ZONE_DMA：用来进行DMA操作的页面；
ZONE_DMA32：也是用来进行DMA操作的页面，不过仅针对32-bit设备；
ZONE_NORMAL：包含常规的，用来映射的页面；
ZONE_HIGHMEM：包含不能永久地映射到kernel的地址空间（address space）的页面。
还有其它的zone定义（例如：ZONE_MOVABLE）。zone_type定义在 <linux/mmzone.h>文件里：

enum zone_type {
#ifdef CONFIG_ZONE_DMA
    /*
     * ZONE_DMA is used when there are devices that are not able
     * to do DMA to all of addressable memory (ZONE_NORMAL). Then we
     * carve out the portion of memory that is needed for these devices.
     * The range is arch specific.
     *
     * Some examples
     *
     * Architecture     Limit
     * ---------------------------
     * parisc, ia64, sparc  <4G
     * s390         <2G
     * arm          Various
     * alpha        Unlimited or 0-16MB.
     *
     * i386, x86_64 and multiple other arches
     *          <16M.
     */
    ZONE_DMA,
#endif
#ifdef CONFIG_ZONE_DMA32
    /*
     * x86_64 needs two ZONE_DMAs because it supports devices that are
     * only able to do DMA to the lower 16M but also 32 bit devices that
     * can only do DMA areas below 4G.
     */
    ZONE_DMA32,
#endif
    /*
     * Normal addressable memory is in ZONE_NORMAL. DMA operations can be
     * performed on pages in ZONE_NORMAL if the DMA devices support
     * transfers to all addressable memory.
     */
    ZONE_NORMAL,
#ifdef CONFIG_HIGHMEM
    /*
     * A memory area that is only addressable by the kernel through
     * mapping portions into its own address space. This is for example
     * used by i386 to allow the kernel to address the memory beyond
     * 900MB. The kernel will set up special mappings (page
     * table entries on i386) for each page that the kernel needs to
     * access.
     */
    ZONE_HIGHMEM,
#endif
    ZONE_MOVABLE,
    __MAX_NR_ZONES
};  

如果CPU体系结构允许DMA操作访问任何地址空间，那就没有ZONE_DMA。同理，Intel X86_64处理器可以访问所有的内存空间，也就不存在ZONE_HIGHMEM。 关于“High memory”，可以参考这篇文章。