正片从现在开始了。

1.结构体关联

        当DDR初始化后，整个内存就可以访问了。但是需要合理的管理，防止内存碎片以及安全相关的问题。因此需要对物理内存进行严格的管理。

        物理内存分为：页， 分区，内存节点。DMA需要连续的内存，因此需要单独的内存。（大概16MB）。

物理页帧结构体，每个物理块都通过一个struct page描述。

• 定义头文件：include/linux/mm_type.h 描述不同类型的页。
• 每个物理页帧page frame 都使用结构体page表示。
• 结构体struct page核心成员分析。
• 思考

• • 物理页帧和struct page之间的关系：通过page结构体找到对应的页帧号。因为page结构体是线性存放。
  • 不同类型的page分别有什么作用。
  • 物理页帧号pfn 和物理地址的关系：通过页帧号找到对应的page结构体。
  • struct page存储在哪里。page本身是需要占用内存的，每个页空间，都会用page结构体描述它。比如说系统总共有1024个page，那么就需要1024个page结构体来描述整个系统内存。此时需要类似struct page mm_map[1024]的信息来保存。
  • 全局变量：mem_map。它保存了系统所有page的地址，因为系统所有的page都有对应的地址，需要这么一块内存指向它。

         假设物理页帧号是0x10240000,0x10250000,0x10260000 ... 0x20470000, 如果用arr[1024]数组保存它,arr[0] = 0x10240000,arr[1] = 0x10250000, ... arr[1023] = 0x20470000。因此物理页帧号对应存放的地址：__page_to_pfn(page) ==> (page - mem_map)

示例1：如果已知page，要去求pfn

struct page my_page=mem_map[22]
my_page_pfn = (my_page - mem_map) + ARCH_PFN_OFFSET

示例2：如果已知某个pfn，去求page

my_pfn = 0x10250000
struct page my_page = mem_map + (pfn - ARCH_FPN_OFFSET)

2.内存页：page结构体

struct page 是 Linux 内核中的一个重要数据结构，用于表示和管理物理内存页。struct page 结构体的设计非常复杂，因为它必须能够适应多种不同的用途，包括但不限于页缓存、匿名页面、页表管理、内存池管理、内存控制组等。为了支持这些多样的用途，struct page 中包含了多个 union 联合体。

unsigned long flags 这个成员包含了一些标志位，用于表示页面的状态，如是否可写、是否脏、是否被锁定等。

struct page {
	unsigned long flags; /* 用于表示页面的状态，如是否可写、是否脏、是否被锁定等。
	/*
	 * 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 on
			 * 32-bit architectures.
			 */
			unsigned long dma_addr[2];
		};
		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, index, 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;

3.内存区域：struct zone

• 定义：mmzone.h
• 重要结构体成员：_watermark watermark_boost zone_start_pfn present_pages free_area

• • zone_start_pfn 起始页帧号 present_pages 该zone当前有多少个页
  • lowmem_reseve备用内存，怎么理解？？

• 初始化：zone_sizes_init

zone_type

enum zone_type {
#ifdef CONFIG_ZONE_DMA
	ZONE_DMA,
#endif
#ifdef CONFIG_ZONE_DMA32
	ZONE_DMA32,
#endif
	ZONE_NORMAL,
#ifdef CONFIG_HIGHMEM
	ZONE_HIGHMEM,
#endif
	ZONE_MOVABLE,
#ifdef CONFIG_ZONE_DEVICE
	ZONE_DEVICE,
#endif
	__MAX_NR_ZONES
};

struct zone

struct zone {
	/* Read-mostly fields */

	/* zone watermarks, access with *_wmark_pages(zone) macros */
	unsigned long _watermark[NR_WMARK]; 表示该内存区域的水印阈值，用于内存管理策略。
	unsigned long watermark_boost; 用于提高水印阈值的临时值。
	unsigned long nr_reserved_highatomic; 表示为高原子操作预留的页面数量。
	long lowmem_reserve[MAX_NR_ZONES]; 用于保留低内存区域的页面数量，以防上层内存充足而下层内存不足的情况。
#ifdef CONFIG_NEED_MULTIPLE_NODES
	int node; 表示该内存区域所在的 NUMA 节点编号。
#endif
	struct pglist_data	*zone_pgdat; 用于描述整个内存区域的元数据。
	struct per_cpu_pageset __percpu *pageset;

#ifndef CONFIG_SPARSEMEM
	unsigned long		*pageblock_flags; 用于标记页面块的标志，例如页面是否可移动等。
#endif /* CONFIG_SPARSEMEM */

	/* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
	unsigned long		zone_start_pfn; 示该内存区域起始的物理页号。

	atomic_long_t		managed_pages; 表示该内存区域中由 buddy 系统管理的页面数量。
	unsigned long		spanned_pages; 表示该内存区域覆盖的总页数，包括空洞区域。
	unsigned long		present_pages; 表示该内存区域存在的物理页数，不包括空洞区域。

	const char		*name;

#ifdef CONFIG_MEMORY_ISOLATION
	unsigned long		nr_isolate_pageblock;
#endif

#ifdef CONFIG_MEMORY_HOTPLUG
	/* see spanned/present_pages for more description */
	seqlock_t		span_seqlock;
#endif

	int initialized; 表示该内存区域是否已经初始化。
	/* Write-intensive fields used from the page allocator */
	ZONE_PADDING(_pad1_)
	/* free areas of different sizes */
	struct free_area	free_area[MAX_ORDER]; 用于表示不同大小的空闲页面区域。

	/* zone flags, see below */
	unsigned long		flags; 包含该内存区域的标志位，如是否支持 NUMA 等

	/* Primarily protects free_area */
	spinlock_t		lock;

	/* Write-intensive fields used by compaction and vmstats. */
	ZONE_PADDING(_pad2_)

	/*
	 * When free pages are below this point, additional steps are taken
	 * when reading the number of free pages to avoid per-cpu counter
	 * drift allowing watermarks to be breached
	 */
	unsigned long percpu_drift_mark;

#if defined CONFIG_COMPACTION || defined CONFIG_CMA
	/* pfn where compaction free scanner should start */
	unsigned long		compact_cached_free_pfn;
	/* pfn where compaction migration scanner should start */
	unsigned long		compact_cached_migrate_pfn[ASYNC_AND_SYNC];
	unsigned long		compact_init_migrate_pfn;
	unsigned long		compact_init_free_pfn;
#endif

#ifdef CONFIG_COMPACTION
	unsigned int		compact_considered;
	unsigned int		compact_defer_shift;
	int			compact_order_failed;
#endif

#if defined CONFIG_COMPACTION || defined CONFIG_CMA
	/* Set to true when the PG_migrate_skip bits should be cleared */
	bool			compact_blockskip_flush; 表示是否应清除 PG_migrate_skip 标志。
#endif

	bool			contiguous; 表示该内存区域是否支持连续内存分配。
	ZONE_PADDING(_pad3_)
	/* Zone statistics */
	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS]; 维护关于该内存区域的各种统计信息。
	atomic_long_t		vm_numa_stat[NR_VM_NUMA_STAT_ITEMS]; 维护关于该内存区域的 NUMA 相关统计信息。
} ____cacheline_internodealigned_in_smp;

4.内存节点：node(struct pglist_data)

• 内存模型：uma numa
• struct pglist_data 表示node中的内存资源
• 定义：include/linux/mmzone.h
• 结构体：struct pglist_data
• node_data数组：保存所有node的的pglist_data结构体

物理内存管理架构：

• 在numa系统中，每个node节点将有一个pglist_data结构体去描述它。如果在uma上，这里将只有一个pglist_data.
• 接下来就是zone：一般分为dma区，nomal区和highmem区，当然这里是不一定的。
• zone区通过struct zone结构体描述，unsigned long zone_start_pfn;描述具体page页帧号。