Linux物理内存概述

        Linux可以支持大量的架构，所以需要用一种与架构无关的方式去描述内存。在linux的内存管理中，我们首先要明确的一个概念就是NUMA(Non-Uniform Memory Access,关于NUMA的介绍可以参考我前面的文章)。很多大型机器都采用NUMA架构，将内存和CPU分为很多组，每一组称为一个节点(node)。节点与节点之间的互相访问，会因为“距离”的不同导致不同的开销。Linux通过struct pglist_data这个结构体来描述节点，对于UMA架构，Linux同样会保留节点的概念，只是整个系统就是一个节点，只需要一个struct pglist_data来描述，它叫作contig_page_data.

struct pglist_data结构描述如下,现在只需了解其中的关键项即可

 

????typedef struct pglist_data {
    struct zone node_zones[MAX_NR_ZONES];          /*节点中的管理区*/
    struct zonelist node_zonelists[MAX_ZONELISTS]; /*list中zone的顺序代表了分配内存的顺序，前者分配内存失败将会到后者的区域中分配内存*/
    int nr_zones;                    /*节点中管理区的数目，不一定为3个，有的节点中可能不存在ZONE_DMA*/
#ifdef CONFIG_FLAT_NODE_MEM_MAP          /* means !SPARSEMEM */
    struct page *node_mem_map;
#ifdef CONFIG_CGROUP_MEM_RES_CTLR
    struct page_cgroup *node_page_cgroup;
#endif
#endif
    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.
     */
    spinlock_t node_size_lock;
#endif
    unsigned long node_start_pfn;     /*该节点的起始页框编号*/
    unsigned long node_present_pages; /* total number of physical pages */
    unsigned long node_spanned_pages; /* total size of physical page
                         range, including holes */
    int node_id;                      /*节点标识符，代表当前节点是系统中的第几个节点*/
    wait_queue_head_t kswapd_wait;    /*页换出进程使用的等待队列*/
    struct task_struct *kswapd;       /*指向页换出进程的进程描述符*/
    int kswapd_max_order;             /*kswapd将要创建的空闲块的大小取对数的值*/

       每个节点的内存会被分为几个块，我们称之为管理区(zone),对于一个管理区，我们使用struct zone结构体来描述。管理区的类型可以分为ZONE_NORMAL,ZONE_DMA,ZONE_HIGHMEM三种。这三个管理区在物理内存上的布局为

ZONE_DMA:  0~16MB

ZONE_NORMAL:  16MB~896MB

ZONE_HIGHMEM:896MB~end   

 

struct zone结构描述如下：

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

    /* zone watermarks, access with *_wmark_pages(zone) macros */
    unsigned long watermark[NR_WMARK];/*该管理区的三个水平线值，min,low,high*/

    /*
     * 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;

    /*
     * 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.
     */
    unsigned long       lowmem_reserve[MAX_NR_ZONES];  /*每个管理区必须保留的页框数*/

#ifdef CONFIG_NUMA           /*如果定义了NUMA*/
    int node;           /*该管理区所属节点的节点号*/
    /*
     * zone reclaim becomes active if more unmapped pages exist.
     */
    unsigned long       min_unmapped_pages;  /*当可回收的页面数大于该变量时，管理区将回收页面*/
    unsigned long       min_slab_pages;      /*同上，只不过该标准用于slab回收页面中*/
    struct per_cpu_pageset  *pageset[NR_CPUS];   /*每个CPU使用的页面缓存*/
#else
    struct per_cpu_pageset  pageset[NR_CPUS];
#endif
    /*
     * free areas of different sizes
     */
    spinlock_t      lock;       /*保护该管理区的自旋锁*/
#ifdef CONFIG_MEMORY_HOTPLUG
    /* see spanned/present_pages for more description */
    seqlock_t       span_seqlock;
#endif
    struct free_area    free_area[MAX_ORDER];/*标识出管理区中的空闲页框块*/

#ifndef CONFIG_SPARSEMEM
    /*
     * Flags for a pageblock_nr_pages block. See pageblock-flags.h.
     * In SPARSEMEM, this map is stored in struct mem_section
     */
    unsigned long       *pageblock_flags;
#endif /* CONFIG_SPARSEMEM */


    ZONE_PADDING(_pad1_)

    /* Fields commonly accessed by the page reclaim scanner */
    spinlock_t      lru_lock;
    struct zone_lru {
        struct list_head list;
    } lru[NR_LRU_LISTS];

    struct zone_reclaim_stat reclaim_stat; /*页面回收的状态*/

    /*管理区回收页框时使用的计数器，记录到上一次回收，一共扫过的页框数*/
    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;


    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;   /*进程等待队列的散列表，这些进程正在等待管理区中的某页*/
    unsigned long       wait_table_hash_nr_entries;   /*散列表数组的大小*/
    unsigned long       wait_table_bits;              /*散列表数组的大小对2取log的结果*/

    /*
     * Discontig memory support fields.
     */
    struct pglist_data  *zone_pgdat;              /*管理区所属节点*/
    /* zone_start_pfn == zone_start_paddr >> PAGE_SHIFT */
    unsigned long       zone_start_pfn; /*管理区的起始页框号*/

    /*
     * 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;  /*管理区的大小,包括洞*/
    unsigned long       present_pages;  /*管理区的大小，不包括洞*/

    /*
     * rarely used fields:
     */
    const char      *name; /*指向管理区的名称，为"DMA","NORMAL"或"HighMem"*/
}

 

物理内存中的每个页都会有一个与之关联的struct page结构来对其进行描述和跟踪，其结构描述如下

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 */
        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_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
};

 

Node,Zone和Page的关系可以用下图来描述

 

至此，已经对内存管理中的这三个关键数据结构有了一个感性的认识，在后面将会结合具体的代码来逐步深入，选择的代码版本为2.6.32.59~