问题描述
从this question中，我可以得出结论，对于32位系统，页面条目大小通常为4个字节。
但是查看Linux的源代码以了解页面如何表示，here，我可以看到该结构的大小远远大于4个字节。
page结构的代码：
struct page {
unsigned long flags; /* Atomic flags,some possibly
* updated asynchronously */
/*
* Five words (20/40 bytes) are available in this union.
* WARNING: bit 0 of the first word is used for PageTail(). That
* means the other users of this union MUST NOT use the bit to
* avoid collision and false-positive PageTail().
*/
union {
struct { /* Page cache and anonymous pages */
/**
* @lru: Pageout list,eg. active_list protected by
* pgdat->lru_lock. Sometimes used as a generic list
* by the page owner.
*/
struct list_head lru;
/* See page-flags.h for PAGE_MAPPING_FLAGS */
struct address_space *mapping;
pgoff_t index; /* Our offset within mapping. */
/**
* @private: Mapping-private opaque data.
* Usually used for buffer_heads if PagePrivate.
* Used for swp_entry_t if PageSwapCache.
* Indicates order in the buddy system if PageBuddy.
*/
unsigned long private;
};
struct { /* page_pool used by netstack */
/**
* @dma_addr: might require a 64-bit value even on
* 32-bit architectures.
*/
dma_addr_t dma_addr;
};
struct { /* slab,slob and slub */
union {
struct list_head slab_list;
struct { /* Partial pages */
struct page *next;
#ifdef CONfig_64BIT
int pages; /* Nr of pages left */
int pobjects; /* Approximate count */
#else
short int pages;
short int pobjects;
#endif
};
};
struct kmem_cache *slab_cache; /* not slob */
/* Double-word boundary */
void *freelist; /* first free object */
union {
void *s_mem; /* slab: first object */
unsigned long counters; /* SLUB */
struct { /* SLUB */
unsigned inuse:16;
unsigned objects:15;
unsigned frozen:1;
};
};
};
struct { /* Tail pages of compound page */
unsigned long compound_head; /* Bit zero is set */
/* First tail page only */
unsigned char compound_dtor;
unsigned char compound_order;
atomic_t compound_mapcount;
unsigned int compound_nr; /* 1 << compound_order */
};
struct { /* Second tail page of compound page */
unsigned long _compound_pad_1; /* compound_head */
atomic_t hpage_pinned_refcount;
/* For both global and memcg */
struct list_head deferred_list;
};
struct { /* Page table pages */
unsigned long _pt_pad_1; /* compound_head */
pgtable_t pmd_huge_pte; /* protected by page->ptl */
unsigned long _pt_pad_2; /* mapping */
union {
struct mm_struct *pt_mm; /* x86 pgds only */
atomic_t pt_frag_refcount; /* powerpc */
};
#if ALLOC_SPLIT_PTLOCKS
spinlock_t *ptl;
#else
spinlock_t ptl;
#endif
};
struct { /* ZONE_DEVICE pages */
/** @pgmap: Points to the hosting device page map. */
struct dev_pagemap *pgmap;
void *zone_device_data;
/*
* ZONE_DEVICE private pages are counted as being
* mapped so the next 3 words hold the mapping,index,* and private fields from the source anonymous or
* page cache page while the page is migrated to device
* private memory.
* ZONE_DEVICE MEMORY_DEVICE_FS_DAX pages also
* use the mapping,and private fields when
* pmem backed DAX files are mapped.
*/
};
/** @rcu_head: You can use this to free a page by RCU. */
struct rcu_head rcu_head;
};
union { /* This union is 4 bytes in size. */
/*
* If the page can be mapped to userspace,encodes the number
* of times this page is referenced by a page table.
*/
atomic_t _mapcount;
/*
* If the page is neither PageSlab nor mappable to userspace,* the value stored here may help determine what this page
* is used for. See page-flags.h for a list of page types
* which are currently stored here.
*/
unsigned int page_type;
unsigned int active; /* SLAB */
int units; /* SLOB */
};
/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
atomic_t _refcount;
#ifdef CONfig_MEMCG
union {
struct mem_cgroup *mem_cgroup;
struct obj_cgroup **obj_cgroups;
};
#endif
/*
* 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 LAST_cpuPID_NOT_IN_PAGE_FLAGS
int _last_cpupid;
#endif
} _struct_page_alignment;
内核是否为页面表以外的页面维护另一种表示形式？这个结构和页表项之间是什么关系？
内核是否使用此结构仅表示使用过的页面？这个结构是做什么用的？
图片对我来说不太清楚。
解决方法
页表条目存储层次结构中下一页表的地址以及其他一些标志，具体取决于体系结构。看Paging - OSDev
comments非常清楚struct page是什么。
/*
* 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.
*/
最后，它只是将信息存储在框架上。