1 Pages
Page的概念来源为处理器Processor的部件MMU(Memory Management Unit)，MMU通过设置好的页表(通过设置CR3寄存器，指向页目录所在的物理内存)对内存进行管理，管理操作包括：
a) 建立线性内存地址与物理内存地址的对应关系，即pa()和va()函数；
b) 管理哪些内存页驻存(Resident)于物理内存中，而哪些内存被交换到Swap文件中；
c) 哪些内存页被映射到哪个进程的虚拟地址空间；
d) 管理哪些内存页存储磁盘上（或者文件系统中）文件的缓存；
数据结构， struct page
/*
* Each physical page in the system has a struct page associated with
* it to keep track of whatever it is we are using the page for at the
* moment. Note that we have no way to track which tasks are using
* a page, though if it is a pagecache page, rmap structures can tell us
* who is mapping it.
*/
struct page {
/* Atomic flags, some possibly
* updated asynchronously */
/* Usage count, see below. */
union {
/*
* Count of ptes mapped in
* mms, to show when page is
* mapped & limit reverse map
* searches.
*
* Used also for tail pages
* refcounting instead of
* _count. Tail pages cannot
* be mapped and keeping the
* tail page _count zero at
* all times guarantees
* get_page_unless_zero() will
* never succeed on tail
* pages.
*/
29: atomic_t _mapcount;
30:
/* SLUB */
32: u16 inuse;
33: u16 objects;
34: };
35: };
union {
struct {
/* Mapping-private opaque data:
* usually used for buffer_heads
* if PagePrivate set; used for
* swp_entry_t if PageSwapCache;
* indicates order in the buddy
* system if PG_buddy is set.
*/
/* 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.
*/
52: };
#if USE_SPLIT_PTLOCKS
54: spinlock_t ptl;
#endif
/* SLUB: Pointer to slab */
/* Compound tail pages */
58: };
union {
/* Our offset within mapping. */
/* SLUB: freelist req. slab lock */
62: };
/* Pageout list, eg. active_list
* protected by zone->lru_lock !
*/
/*
* On machines where all RAM is mapped into kernel address space,
* we can simply calculate the virtual address. On machines with
* highmem some memory is mapped into kernel virtual memory
* dynamically, so we need a place to store that address.
* Note that this field could be 16 bits on x86 ... ;)
*
* Architectures with slow multiplication can define
* WANT_PAGE_VIRTUAL in asm/page.h
*/
#if defined(WANT_PAGE_VIRTUAL)
/* Kernel virtual address (NULL if
not kmapped, ie. highmem) */
/* WANT_PAGE_VIRTUAL */
#ifdef CONFIG_WANT_PAGE_DEBUG_FLAGS
/* Use atomic bitops on this */
#endif
83:
#ifdef CONFIG_KMEMCHECK
/*
* kmemcheck wants to track the status of each byte in a page; this
* is a pointer to such a status block. NULL if not tracked.
*/
void *shadow;
#endif
91: };