眼花的页结构体
页结构体(struct page)可以说是我见过的最为繁杂的结构体定义了，至今我只能说是略懂。然而这个结构体定义又是那么的重要，因为物理意义上的每个页面都对应一个结构体。可以说这个结构体是整个系统上所有内存的守护者。
每个细分成员的作用就不在本节展开了，这里主要是作为一个全局参考，从整体上对页结构体有个了解。具体的作用将在对应的章节中描述。
庞然大物
struct page (include/linux/mm_types.h)
page
+--------------------------------------------------------------+
|flags | page-flags-layout.h
| (unsigned long) | page-flags.h
--+-- +==============================================================+
| |..............................................................|
| |page cache and anonymous pages |
| | +---------------------------------------------------------+
| | |lru |
| | | (struct list_head) |
| |mapping |
| | (struct address_space*) |
| |index |
| | (pgoff_t) |
5 words | |private |
union | | (unsigned long) |
| +---------------------------------------------------------+
|..............................................................|
has |slab, slob, slub |
| +---------------------------------------------------------+
7 usage | |.........................................................|
| | .+---------------------------|
| |slab_list .|next |
| | (struct list_head) .| (struct page*) |
| | .|pages |
| | .|pobjects |
| | .| (int) |
| | .+---------------------------|
| |.........................................................|
| +---------------------------------------------------------+
| |slab_cache |
| | (struct kmem_cache*) |
| |freelist |
| | (void*) |
| +---------------------------------------------------------+
| |.........................................................|
| |s_mem .counters .+---------------------------|
| | (void*) . (unsigned long) .|inuse |
| | . .|objects |
| | . .|frozen |
| | . .| (unsigned) |
| | . .+---------------------------|
| |.........................................................|
| +---------------------------------------------------------+
|..............................................................|
|Tail pages of compound page |
| +---------------------------------------------------------+
| |compound_head |
| | (unsigned long) |
| |compound_dtor |
| |compound_order |
| | (unsigned char) |
| |compound_mapcount |
| | (atomic_t) |
| +---------------------------------------------------------+
|..............................................................|
|Second tail page of compound page |
| +---------------------------------------------------------+
| |_compound_pad_1 |
| |_compound_pad_2 |
| | (unsigned long) |
| |deferred_list |
| | (struct list_head) |
| +---------------------------------------------------------+
|..............................................................|
|Page table pages |
| +---------------------------------------------------------+
| |_pt_pad_1 |
| | (unsigned long) |
| |pmd_huge_pte |
| | (pgtable_t) |
| |_pt_pad_2 |
| | (unsigned long) |
| |.........................................................|
| |pt_mm .pt_frag_refcount |
| | (struct mm_struct*) . (atomic_t) |
| |.........................................................|
| |ptl |
| | (spinlock_t/spinlock_t *) |
| +---------------------------------------------------------+
|..............................................................|
|ZONE_DEVICE pages |
| +---------------------------------------------------------+
| |pgmap |
| | (struct dev_pagemap*) |
| |hmm_data |
| |_zd_pad_1 |
| | | (unsigned long) |
| | +---------------------------------------------------------+
| |..............................................................|
| |rcu_head |
| | (struct rcu_head) |
| |..............................................................|
--+-- +==============================================================+
| |..............................................................|
| . . . |
4 bytes |_mapcount .page_type .active .units |
union | (atomic_t). (unsigned int). (unsigned int). (int) |
| . . . |
| |..............................................................|
--+-- +==============================================================+
|_refcount |
| (atomic_t) |
|mem_cgroup |
| (struct mem_cgroup) |
|virtual |
| (void *) |
|_last_cpupid |
| (int) |
+--------------------------------------------------------------+page->flags
在页结构体众多成员中，flasgs字段在任何用途下都标示了对应页的属性。除此之外内核中还对这个字段做了非常巧（变）妙（态）的布局。
内核中有两个头文件和flags的定义有着密切的联系
* include/linux/page-flags-layout.h
* include/linux/page-flags.h前者定义了该字段的布局，后者则定义了该字段具体的意义和相关操作的宏定义。
布局
以下对字段布局的描述摘自page-flags-layout.h
/*
* page->flags layout:
*
* There are five possibilities for how page->flags get laid out. The first
* pair is for the normal case without sparsemem. The second pair is for
* sparsemem when there is plenty of space for node and section information.
* The last is when there is insufficient space in page->flags and a separate
* lookup is necessary.
*
* No sparsemem or sparsemem vmemmap: | NODE | ZONE | ... | FLAGS |
* " plus space for last_cpupid: | NODE | ZONE | LAST_CPUPID ... | FLAGS |
* classic sparse with space for node:| SECTION | NODE | ZONE | ... | FLAGS |
* " plus space for last_cpupid: | SECTION | NODE | ZONE | LAST_CPUPID ... | FLAGS |
* classic sparse no space for node: | SECTION | ZONE | ... | FLAGS |
*/其中FLAGS部分的定义就在include/linux/page-flags.h文件中，这部分也是在内核代码中用来判断页属性的关键部分。
FLAGS定义
FLAGS是一个按照比特位来定义的属性集合，比如我们可以看一下定义从而大致了解一下这个FLAGS中都有哪些内容。
enum pageflags {
PG_locked, /* Page is locked. Don't touch. */
PG_referenced,
PG_uptodate,
PG_dirty,
PG_lru,
PG_active,
PG_workingset,
...
}整个FLAGS的长度为__NR_PAGEFLAGS，并且内核定义又了NR_PAGEFLAGS。这两者的值一致。
并且在page-flags-layout.h中，用下面的代码保证了这些神秘的东西加起来不会放不下。
#if ZONES_WIDTH + SECTIONS_WIDTH + NODES_WIDTH + KASAN_TAG_WIDTH + LAST_CPUPID_WIDTH \
> BITS_PER_LONG - NR_PAGEFLAGS
#error "Not enough bits in page flags"
#endifFLAGS操作
有了定义，接下来就是如何操作。这里就是体现C语言博大精深的时刻了，其中对字段操作的宏定义总结如下：
* PageXXX()
* SetPageXXX()
* ClearPage()这三个宏分别用来判断、设置、清楚页结构体中相应的属性。在内核代码中使用非常广泛，以后看到长成这样的，就可以到这个头文件中搜索。
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