linux物理内存描述

linux使用于广泛的体系结构，因此需要用一种与体系结构无关的方式来描述内存。linux用VM描述和管理内存。在VM中兽药的普遍概念就是非一致内存访问。对于大型机器而言，内存会分成许多簇，依据簇与处理器“距离”的不同，访问不同的簇会有不同的代价。

每个簇都被认为是一个节点（pg_data_t)，每个节点被分成很多的成为管理区（zone)的块，用于表示内存中的某个范围。除了ZONE_DMA,ZONE_NORMAL,ZONE_HIGHMEM以外，linux2.6.32中引入了ZONE_MOVABLE，用于适应大块连续内存的分配。

每个物理页面由一个page结构体描述，所有的结构都存储在一个全局的mem_map数组中（非平板模式），该数组通常存放在ZONE_NORMAL的首部，或者就在校内存系统中为装入内核映像而预留的区域之后。

节点

内存的每个节点都有pg_data_t描述，在分配一个页面时，linux采用节点局部分配的策略，从最靠近运行中的CPU的节点分配内存。由于进程往往是在同一个CPU上运行，因此从当前节点得到的内存很可能被用到。

/*
 * The pg_data_t structure is used in machines with CONFIG_DISCONTIGMEM
 * (mostly NUMA machines?) to denote a higher-level memory zone than the
 * zone denotes.
 *
 * On NUMA machines, each NUMA node would have a pg_data_t to describe
 * it's memory layout.
 *
 * Memory statistics and page replacement data structures are maintained on a
 * per-zone basis.
 */
struct bootmem_data;
typedef struct pglist_data {
	 /*该节点内的内存区。可能的区域类型用zone_type表示。 */
	struct zone node_zones[MAX_NR_ZONES];
	 /* 该节点的备用内存区。当节点没有可用内存时，就从备用区中分配内存。*/
	struct zonelist node_zonelists[MAX_ZONELISTS];
	  /*可用内存区数目，即node_zones数据中保存的最后一个有效区域的索引*/
	int nr_zones;
#ifdef CONFIG_FLAT_NODE_MEM_MAP	/* means !SPARSEMEM */
	 /* 在平坦型的内存模型中，它指向本节点第一个页面的描述符。 */
	struct page *node_mem_map;
#ifdef CONFIG_CGROUP_MEM_RES_CTLR
	/*cgroup相关*/
	struct page_cgroup *node_page_cgroup;
#endif
#endif
  /**
          * 在内存子系统初始化以前，即boot阶段也需要进行内存管理。
          * 此结构用于这个阶段的内存管理。
          */
	struct bootmem_data *bdata;
#ifdef CONFIG_MEMORY_HOTPLUG
	/*
	 * Must be held any time you expect node_start_pfn, node_present_pages
	 * or node_spanned_pages stay constant.  Holding this will also
	 * guarantee that any pfn_valid() stays that way.
	 *
	 * Nests above zone->lock and zone->size_seqlock.
	 */
	    /*当系统支持内存热插拨时，用于保护本结构中的与节点大小相关的字段。
          	哪调用node_start_pfn，node_present_pages，node_spanned_pages相关的代码时，需要使用该锁。
          */
	spinlock_t node_size_lock;
#endif
	/*起始页面帧号，指出该节点在全局mem_map中
	的偏移*/
	unsigned long node_start_pfn;
	unsigned long node_present_pages; /* total number of physical pages */
	unsigned long node_spanned_pages; /* total size of physical page range, including holes */
	/*节点编号*/					    
	int node_id;
	/*等待该节点内的交换守护进程的等待队列。将节点中的页帧换出时会用到。*/
	wait_queue_head_t kswapd_wait;
	/*负责该节点的交换守护进程。*/
	struct task_struct *kswapd;
	/*由页交换子系统使用，定义要释放的区域大小。*/
	int kswapd_max_order;
} pg_data_t;

管理区

每个管理区由一个zone结构体描述，对于管理区的类型描述如下

enum zone_type {
#ifdef CONFIG_ZONE_DMA
	/*
	 * ZONE_DMA is used when there are devices that are not able
	 * to do DMA to all of addressable memory (ZONE_NORMAL). Then we
	 * carve out the portion of memory that is needed for these devices.
	 * The range is arch specific.
	 *
	 * Some examples
	 *
	 * Architecture		Limit
	 * ---------------------------
	 * parisc, ia64, sparc	<4G
	 * s390			<2G
	 * arm			Various
	 * alpha		Unlimited or 0-16MB.
	 *
	 * i386, x86_64 and multiple other arches
	 * 			<16M.
	 */
	ZONE_DMA,
#endif
#ifdef CONFIG_ZONE_DMA32
	/*
	 * x86_64 needs two ZONE_DMAs because it supports devices that are
	 * only able to do DMA to the lower 16M but also 32 bit devices that
	 * can only do DMA areas below 4G.
	 */
	ZONE_DMA32,
#endif
	/*
	 * Normal addressable memory is in ZONE_NORMAL. DMA operations can be
	 * performed on pages in ZONE_NORMAL if the DMA devices support
	 * transfers to all addressable memory.
	 */
	ZONE_NORMAL,
#ifdef CONFIG_HIGHMEM
	/*
	 * A memory area that is only addressable by the kernel through
	 * mapping portions into its own address space. This is for example
	 * used by i386 to allow the kernel to address the memory beyond
	 * 900MB. The kernel will set up special mappings (page
	 * table entries on i386) for each page that the kernel needs to
	 * access.
	 */
	ZONE_HIGHMEM,
#endif
	/*
          这是一个伪内存段。为了防止形成物理内存碎片，
          可以将虚拟地址对应的物理地址进行迁移。
          */
	ZONE_MOVABLE,
	__MAX_NR_ZONES
};

 

里面的英文注释已经写的很详细了。

管理区用于跟踪诸如页面使用情况统计数，空闲区域信息和锁信息等。

struct zone {
	/* Fields commonly accessed by the page allocator */

	/* zone watermarks, access with *_wmark_pages(zone) macros */
	/*本管理区的三个水线值:高水线(比较充足)、低水线、MIN水线。*/
	unsigned long watermark[NR_WMARK];

	/*
	 * We don't know if the memory that we're going to allocate will be freeable
	 * or/and it will be released eventually, so to avoid totally wasting several
	 * GB of ram we must reserve some of the lower zone memory (otherwise we risk
	 * to run OOM on the lower zones despite there's tons of freeable ram
	 * on the higher zones). This array is recalculated at runtime if the
	 * sysctl_lowmem_reserve_ratio sysctl changes.
	 */
	  /**
          * 当高端内存、normal内存区域中无法分配到内存时，需要从normal、DMA区域中分配内存。
          * 为了避免DMA区域被消耗光，需要额外保留一些内存供驱动使用。
          * 该字段就是指从上级内存区退到回内存区时，需要额外保留的内存数量。
          */
	unsigned long		lowmem_reserve[MAX_NR_ZONES];

#ifdef CONFIG_NUMA
	/*所属的NUMA节点。*/
	int node;
	/*
	 * zone reclaim becomes active if more unmapped pages exist.
	 */
	 /*当可回收的页超过此值时，将进行页面回收。*/
	unsigned long		min_unmapped_pages;
	/*当管理区中，用于slab的可回收页大于此值时，将回收slab中的缓存页。*/
	unsigned long		min_slab_pages;
	/*
          * 每CPU的页面缓存。
          * 当分配单个页面时，首先从该缓存中分配页面。这样可以:
          *避免使用全局的锁
          * 避免同一个页面反复被不同的CPU分配，引起缓存行的失效。
          * 避免将管理区中的大块分割成碎片。
          */
	struct per_cpu_pageset	*pageset[NR_CPUS];
#else
	struct per_cpu_pageset	pageset[NR_CPUS];
#endif
	/*
	 * free areas of different sizes
	 */
	 /*该锁用于保护伙伴系统数据结构。即保护free_area相关数据。*/
	spinlock_t		lock;
#ifdef CONFIG_MEMORY_HOTPLUG
	/* see spanned/present_pages for more description */
	/*用于保护spanned/present_pages等变量。这些变量几乎不会发生变化，除非发生了内存热插拨操作。
           这几个变量并不被lock字段保护。并且主要用于读，因此使用读写锁。*/
	seqlock_t		span_seqlock;
#endif
	/*伙伴系统的主要变量。这个数组定义了11个队列，每个队列中的元素都是大小为2^n的页面*/
	struct free_area	free_area[MAX_ORDER];

#ifndef CONFIG_SPARSEMEM
	/*
	 * Flags for a pageblock_nr_pages block. See pageblock-flags.h.
	 * In SPARSEMEM, this map is stored in struct mem_section
	 */
	 /*本管理区里的页面标志数组*/
	unsigned long		*pageblock_flags;
#endif /* CONFIG_SPARSEMEM */

	/*填充的未用字段，确保后面的字段是缓存行对齐的*/
	ZONE_PADDING(_pad1_)

	/* Fields commonly accessed by the page reclaim scanner */
 	/*
          * lru相关的字段用于内存回收。这个字段用于保护这几个回收相关的字段。
          * lru用于确定哪些字段是活跃的，哪些不是活跃的，并据此确定应当被写回到磁盘以释放内存。
          */
	spinlock_t		lru_lock;	
	/* 匿名活动页、匿名不活动页、文件活动页、文件不活动页链表头*/
	struct zone_lru {
		struct list_head list;
	} lru[NR_LRU_LISTS];
	/*页面回收状态*/
	struct zone_reclaim_stat reclaim_stat;
	/*自从最后一次回收页面以来，扫过的页面数*/
	unsigned long		pages_scanned;	   /* since last reclaim */
	unsigned long		flags;		   /* zone flags, see below */

	/* Zone statistics */
	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS];

	/*
	 * prev_priority holds the scanning priority for this zone.  It is
	 * defined as the scanning priority at which we achieved our reclaim
	 * target at the previous try_to_free_pages() or balance_pgdat()
	 * invokation.
	 *
	 * We use prev_priority as a measure of how much stress page reclaim is
	 * under - it drives the swappiness decision: whether to unmap mapped
	 * pages.
	 *
	 * Access to both this field is quite racy even on uniprocessor.  But
	 * it is expected to average out OK.
	 */
	int prev_priority;

	/*
	 * The target ratio of ACTIVE_ANON to INACTIVE_ANON pages on
	 * this zone's LRU.  Maintained by the pageout code.
	 */
	unsigned int inactive_ratio;

	/*为cache对齐*/
	ZONE_PADDING(_pad2_)
	/* Rarely used or read-mostly fields */

	/*
	 * wait_table		-- the array holding the hash table
	 * wait_table_hash_nr_entries	-- the size of the hash table array
	 * wait_table_bits	-- wait_table_size == (1 << wait_table_bits)
	 *
	 * The purpose of all these is to keep track of the people
	 * waiting for a page to become available and make them
	 * runnable again when possible. The trouble is that this
	 * consumes a lot of space, especially when so few things
	 * wait on pages at a given time. So instead of using
	 * per-page waitqueues, we use a waitqueue hash table.
	 *
	 * The bucket discipline is to sleep on the same queue when
	 * colliding and wake all in that wait queue when removing.
	 * When something wakes, it must check to be sure its page is
	 * truly available, a la thundering herd. The cost of a
	 * collision is great, but given the expected load of the
	 * table, they should be so rare as to be outweighed by the
	 * benefits from the saved space.
	 *
	 * __wait_on_page_locked() and unlock_page() in mm/filemap.c, are the
	 * primary users of these fields, and in mm/page_alloc.c
	 * free_area_init_core() performs the initialization of them.
	 */
	wait_queue_head_t	* wait_table;
	unsigned long		wait_table_hash_nr_entries;
	unsigned long		wait_table_bits;

	/*
	 * Discontig memory support fields.
	 */
	 /*管理区属于的节点*/
	struct pglist_data	*zone_pgdat;
	/* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
	/*管理区的页面在mem_map中的偏移*/
	unsigned long		zone_start_pfn;

	/*
	 * zone_start_pfn, spanned_pages and present_pages are all
	 * protected by span_seqlock.  It is a seqlock because it has
	 * to be read outside of zone->lock, and it is done in the main
	 * allocator path.  But, it is written quite infrequently.
	 *
	 * The lock is declared along with zone->lock because it is
	 * frequently read in proximity to zone->lock.  It's good to
	 * give them a chance of being in the same cacheline.
	 */
	unsigned long		spanned_pages;	/* total size, including holes */
	unsigned long		present_pages;	/* amount of memory (excluding holes) */

	/*
	 * rarely used fields:
	 */
	const char		*name;
} ____cacheline_internodealigned_in_smp;

没有说明的地方，内核中的英文注释已经写得很清楚了。

页面

系统中每个物理页面都有一个相关联的page用于记录该页面的状态。

/*
 * 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 {
	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.
					 */
		struct {		/* SLUB */
			u16 inuse;
			u16 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 USE_SPLIT_PTLOCKS
	    spinlock_t ptl;
#endif
	    struct kmem_cache *slab;	/* SLUB: Pointer to slab */
	/* 如果属于伙伴系统，并且不是伙伴系统中的第一个页
	则指向第一个页*/
	    struct page *first_page;	/* Compound tail pages */
	};
	union {/*如果是文件映射，那么表示本页面在文件中的位置(偏移)*/
		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_WANT_PAGE_DEBUG_FLAGS
	unsigned long debug_flags;	/* Use atomic bitops on this */
#endif

#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
};

linux中主要的结构描述体现了linux物理内存管理的设计。后面会介绍linux内存管理的各个细节。