前言
页帧page是物理内存管理的基本单位，struct page记录了任意时刻page的所有状态，因此每一个物理页帧都需一个对应的struct page结构体记录状态，对于内存多计算机系统来说需要的struct page本身就需要大量内存进行存储，因此该结构体中每增加一个变量带来的代价会很大，需要仔细控制该结构体的size。内核中广泛使用了union，将同一时刻不会同时使用的变量作为一个union来减少内存开销。
struct page
struct page的声明如下：
flags: 标志位_count: 使用计数，关系到内存释放。_mapcount&inuse:_mapcount是页表项的映射个数，当page用于slab分配器管理时使用inuse表示包含的对象个数。priave: page用作pagecahe时private存放struct buffer_head;page内容被换出时，private存放swap_entry_t;page位于伙伴系统时，存放ordermapping: 如果low bit没有set，指向inode的address_space，如果有low bit，指向aono_vma对象，此时clear低位bit后就可以访问到有效指针。指针都是字节对齐的，低位bit正常情况为0，可以在低位bit中添加信息。slab: 指向所属的slab分配器kmem_cache结构体。first_page: 用于复合页中tail page指向head page。virtual: 高端内存页的虚拟地址。
struct page {
unsigned long flags; /* Atomic flags, some possibly
* updated asynchronously */
atomic_t _count; /* Usage count, see below. */
union {
atomic_t _mapcount; /* Count of ptes mapped in mms,
* to show when page is mapped
* & limit reverse map searches.
*/
unsigned int inuse; /* SLUB: Nr of objects */
};
union {
struct {
unsigned long private; /* 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.
*/
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.
*/
};
#if NR_CPUS >= CONFIG_SPLIT_PTLOCK_CPUS
spinlock_t ptl;
#endif
struct kmem_cache *slab; /* SLUB: Pointer to slab */
struct page *first_page; /* Compound tail pages */
};
union {
pgoff_t index; /* Our offset within mapping. */
void *freelist; /* SLUB: freelist req. slab lock */
};
struct list_head lru; /* 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)
void *virtual; /* Kernel virtual address (NULL if
not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */
#ifdef CONFIG_CGROUP_MEM_RES_CTLR
unsigned long page_cgroup;
#endif
};
swap、地址映射、复合页、slab分配器等机制都会操作page结构体。
PG_FLAG
PG_FLAG是page的标志位，可以表示page当前所处的状态。一些标志位的说明如下：
- PG_locked: 锁定，避免竞态。
- PG_error: IO error
- PG_reference & PG_active: 和page swap相关。
- PG_uptodate: 数据已从块设备读取并且未出错。
- PG_dirty: 数据未同步到磁盘。
- PG_lru: 页面回收和切换相关，表示在active_list or inactive_list中，如果PG_active同时设置表明在active_list中。
- PG_slab: 用于slab分配器。
- PG_private: private变量不为空时使用。
- PG_writeback: 处于IO写回的过程中。
- PG_compound: 复合页。
- PG_swapcache: 处于swap cache。
- PG_reclaim: 内核决定回收该page时需要标记可回收。
- PG_buddy: 位于伙伴系统中，空闲状态。
#define PG_locked 0 /* Page is locked. Don't touch. */
#define PG_error 1
#define PG_referenced 2
#define PG_uptodate 3
#define PG_dirty 4
#define PG_lru 5
#define PG_active 6
#define PG_slab 7 /* slab debug (Suparna wants this) */
#define PG_owner_priv_1 8 /* Owner use. If pagecache, fs may use*/
#define PG_arch_1 9
#define PG_reserved 10
#define PG_private 11 /* If pagecache, has fs-private data */
#define PG_writeback 12 /* Page is under writeback */
#define PG_compound 14 /* Part of a compound page */
#define PG_swapcache 15 /* Swap page: swp_entry_t in private */
#define PG_mappedtodisk 16 /* Has blocks allocated on-disk */
#define PG_reclaim 17 /* To be reclaimed asap */
#define PG_buddy 19 /* Page is free, on buddy lists */
/* PG_readahead is only used for file reads; PG_reclaim is only for writes */
#define PG_readahead PG_reclaim /* Reminder to do async read-ahead */
/* PG_owner_priv_1 users should have descriptive aliases */
#define PG_checked PG_owner_priv_1 /* Used by some filesystems */
#define PG_pinned PG_owner_priv_1 /* Xen pinned pagetable */