Linux Kernel Development有关内存管理

1 Pages

Page的概念来源为处理器Processor的部件MMU(Memory Management Unit)，MMU通过设置好的页表(通过设置CR3寄存器，指向页目录所在的物理内存)对内存进行管理，管理操作包括：

 

a) 建立线性内存地址与物理内存地址的对应关系，即pa()和va()函数；

b) 管理哪些内存页驻存(Resident)于物理内存中，而哪些内存被交换到Swap文件中；

c) 哪些内存页被映射到哪个进程的虚拟地址空间；

d) 管理哪些内存页存储磁盘上（或者文件系统中）文件的缓存；

 

数据结构， struct page

/*
 * 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 {
        /*
         * Count of ptes mapped in
         * mms, to show when page is
         * mapped & limit reverse map
         * searches.
         *
         * Used also for tail pages
         * refcounting instead of
         * _count. Tail pages cannot
         * be mapped and keeping the
         * tail page _count zero at
         * all times guarantees
         * get_page_unless_zero() will
         * never succeed on tail
         * pages.
         */
        atomic_t _mapcount;

        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
};

每个page结构体对象，代表一个物理内存页。

 

x) flags

enum pageflags {
    PG_locked,        /* Page is locked. Don't touch. */
    PG_error,
    PG_referenced,
    PG_uptodate,
    PG_dirty,
    PG_lru,
    PG_active,
    PG_slab,
    PG_owner_priv_1,    /* Owner use. If pagecache, fs may use*/
    PG_arch_1,
    PG_reserved,
    PG_private,        /* If pagecache, has fs-private data */
    PG_private_2,        /* If pagecache, has fs aux data */
    PG_writeback,        /* Page is under writeback */
#ifdef CONFIG_PAGEFLAGS_EXTENDED
    PG_head,        /* A head page */
    PG_tail,        /* A tail page */
#else
    PG_compound,        /* A compound page */
#endif
    PG_swapcache,        /* Swap page: swp_entry_t in private */
    PG_mappedtodisk,    /* Has blocks allocated on-disk */
    PG_reclaim,        /* To be reclaimed asap */
    PG_swapbacked,        /* Page is backed by RAM/swap */
    PG_unevictable,        /* Page is "unevictable"  */
#ifdef CONFIG_MMU
    PG_mlocked,        /* Page is vma mlocked */
#endif
#ifdef CONFIG_ARCH_USES_PG_UNCACHED
    PG_uncached,        /* Page has been mapped as uncached */
#endif
#ifdef CONFIG_MEMORY_FAILURE
    PG_hwpoison,        /* hardware poisoned page. Don't touch */
#endif
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
    PG_compound_lock,
#endif
    __NR_PAGEFLAGS,

    /* Filesystems */
    PG_checked = PG_owner_priv_1,

    /* Two page bits are conscripted by FS-Cache to maintain local caching
     * state.  These bits are set on pages belonging to the netfs's inodes
     * when those inodes are being locally cached.
     */
    PG_fscache = PG_private_2,    /* page backed by cache */

    /* XEN */
    PG_pinned = PG_owner_priv_1,
    PG_savepinned = PG_dirty,

    /* SLOB */
    PG_slob_free = PG_private,

    /* SLUB */
    PG_slub_frozen = PG_active,
};

 

x) _count

引用计数，代表有多少处引用到该物理内存页对象。

访问_count时，不要直接访问，调用 page_count()对象进行访问。

 

x) private, mapping

struct address_space {
    struct inode        *host;        /* owner: inode, block_device */
    struct radix_tree_root    page_tree;    /* radix tree of all pages */
    spinlock_t        tree_lock;    /* and lock protecting it */
    unsigned int        i_mmap_writable;/* count VM_SHARED mappings */
    struct prio_tree_root    i_mmap;        /* tree of private and shared mappings */
    struct list_head    i_mmap_nonlinear;/*list VM_NONLINEAR mappings */
    struct mutex        i_mmap_mutex;    /* protect tree, count, list */
    /* Protected by tree_lock together with the radix tree */
    unsigned long        nrpages;    /* number of total pages */
    pgoff_t            writeback_index;/* writeback starts here */
    const struct address_space_operations *a_ops;    /* methods */
    unsigned long        flags;        /* error bits/gfp mask */
    struct backing_dev_info *backing_dev_info; /* device readahead, etc */
    spinlock_t        private_lock;    /* for use by the address_space */
    struct list_head    private_list;    /* ditto */
    struct address_space    *assoc_mapping;    /* ditto */
} __attribute__((aligned(sizeof(long))));

 

x) virtual

虚拟地址，如果是高端内存(HighMem)，那么该物理页可能不是长久地映射到内核的内存空间中，所以该字段为NULL。

 

 

struct page

这个结构只是用来描述物理内存页，而与该页中存储的内容无关。

物理内存页的可能属主(Owner)有：

• user-space processes, 用户态的进程
• dynamically allocated kernel data， 内核态中动态分配的数据
• static kernel code， 内核静态代码
• the page cache，页缓存

 

可能程序员对为每个物理内存页都分配一个struct page而感到吃惊，“那得分配多少内存啊，多浪费啊！”。

实际上以4GB内存为例，大概需要40MB的内存来存储所有struct page的对象，相对于它能够管理的4GB物理内存，还是十分微不足道的。

 

2. Zones

“为什么不利用用户态的3GB的地址空间来映射内核的1GB地址空间映射不下的物理内存？”

因为其实，内核在工作时，是不考虑存在着用户空间的。

内核要处理的任务远比支撑用户空间要复杂。

 

为什么要使用不同的Zone?

有些硬件设备的DMA能访问的内存空间十分有即，比如x86的ISA总线，只能访问0~16M的物理内存，那么如果这段内存被随便地分配掉了，ISA设备就可能无法工作了。

ISA

 

ISA插槽是基于ISA总线（Industrial Standard Architecture，工业标准结构总线）的扩展插槽，其颜色一般为黑色，比PCI接口插槽要长些，位于主板的最下端。其工作频率为8MHz左右，为16位插槽，最大传输率16MB/sec，可插接显卡，声卡，网卡以及所谓的多功能接口卡等扩展插卡。其缺点是CPU资源占用太高，数据传输带宽太小，是已经被淘汰的插槽接口。（http://baike.baidu.com/view/13594.htm）

HighMem

对于x86体系结构，以896MB物理内存为界，大于该范围的为高端内存(High Memory)，而小于该范围的为低端内存(Low Memory)。

 

内核无法直接映射HighMem区域的物理内存，只能以暂时的方式映射其中一小部分使用，当需要使用其他的高端内存时，可以需要打破之前建立的暂时映射，而用于新的映射，就像是内存交换机制一样。