2022-02-15 / 0 评论 / 568 阅读 / 正在检测是否收录...


1. 概述


  1. vma;
  2. malloc;
  3. mmap;

进程地址空间中,我们常见的代码段,数据段,bss段等,实际上都是一段地址空间区域。Linux将地址空间中的区域称为Virtual Memory Area, 简称VMA,使用struct vm_area_struct来描述。


2. 数据结构

主要涉及两个结构体:struct mm_struct和struct vm_area_struct。

  • struct mm_struct


struct mm_struct {
    struct vm_area_struct *mmap;        /* list of VMAs */                              //指向VMA对象的链表头
    struct rb_root mm_rb;                                                                     //指向VMA对象的红黑树的根
    u64 vmacache_seqnum;                   /* per-thread vmacache */
    unsigned long (*get_unmapped_area) (struct file *filp,
                unsigned long addr, unsigned long len,
                unsigned long pgoff, unsigned long flags);              // 在进程地址空间中搜索有效线性地址区间的方法
    unsigned long mmap_base;        /* base of mmap area */
    unsigned long mmap_legacy_base;         /* base of mmap area in bottom-up allocations */
    /* Base adresses for compatible mmap() */
    unsigned long mmap_compat_base;
    unsigned long mmap_compat_legacy_base;
    unsigned long task_size;        /* size of task vm space */
    unsigned long highest_vm_end;        /* highest vma end address */
    pgd_t * pgd;        //指向页全局目录

     * @mm_users: The number of users including userspace.
     * Use mmget()/mmget_not_zero()/mmput() to modify. When this drops
     * to 0 (i.e. when the task exits and there are no other temporary
     * reference holders), we also release a reference on @mm_count
     * (which may then free the &struct mm_struct if @mm_count also
     * drops to 0).
    atomic_t mm_users;      //使用计数器

     * @mm_count: The number of references to &struct mm_struct
     * (@mm_users count as 1).
     * Use mmgrab()/mmdrop() to modify. When this drops to 0, the
     * &struct mm_struct is freed.
    atomic_t mm_count;      //使用计数器

    atomic_long_t nr_ptes;            /* PTE page table pages */      //进程页表数
    atomic_long_t nr_pmds;            /* PMD page table pages */
    int map_count;                /* number of VMAs */        //VMA的个数

    spinlock_t page_table_lock;        /* Protects page tables and some counters */
    struct rw_semaphore mmap_sem;

    struct list_head mmlist;        /* List of maybe swapped mm's.    These are globally strung
                         * together off init_mm.mmlist, and are protected
                         * by mmlist_lock

    unsigned long hiwater_rss;    /* High-watermark of RSS usage */
    unsigned long hiwater_vm;    /* High-water virtual memory usage */

    unsigned long total_vm;        /* Total pages mapped */    //进程地址空间的页数
    unsigned long locked_vm;    /* Pages that have PG_mlocked set */    //锁住的页数,不能换出
    unsigned long pinned_vm;    /* Refcount permanently increased */
    unsigned long data_vm;        /* VM_WRITE & ~VM_SHARED & ~VM_STACK */     //数据段内存的页数
    unsigned long exec_vm;        /* VM_EXEC & ~VM_WRITE & ~VM_STACK */         //可执行内存映射的页数
    unsigned long stack_vm;        /* VM_STACK */                                              //用户态堆栈的页数
    unsigned long def_flags;
    unsigned long start_code, end_code, start_data, end_data;       //代码段,数据段等的地址
    unsigned long start_brk, brk, start_stack;      //堆栈段的地址,start_stack表示用户态堆栈的起始地址,brk为堆的当前最后地址
    unsigned long arg_start, arg_end, env_start, env_end;  //命令行参数的地址,环境变量的地址

    unsigned long saved_auxv[AT_VECTOR_SIZE]; /* for /proc/PID/auxv */

     * Special counters, in some configurations protected by the
     * page_table_lock, in other configurations by being atomic.
    struct mm_rss_stat rss_stat;

    struct linux_binfmt *binfmt;

    cpumask_var_t cpu_vm_mask_var;

    /* Architecture-specific MM context */
    mm_context_t context;

    unsigned long flags; /* Must use atomic bitops to access the bits */

    struct core_state *core_state; /* coredumping support */
    atomic_t membarrier_state;
    spinlock_t            ioctx_lock;
    struct kioctx_table __rcu    *ioctx_table;
     * "owner" points to a task that is regarded as the canonical
     * user/owner of this mm. All of the following must be true in
     * order for it to be changed:
     * current == mm->owner
     * current->mm != mm
     * new_owner->mm == mm
     * new_owner->alloc_lock is held
    struct task_struct __rcu *owner;
    struct user_namespace *user_ns;

    /* store ref to file /proc/<pid>/exe symlink points to */
    struct file __rcu *exe_file;
    struct mmu_notifier_mm *mmu_notifier_mm;
    pgtable_t pmd_huge_pte; /* protected by page_table_lock */
    struct cpumask cpumask_allocation;
     * numa_next_scan is the next time that the PTEs will be marked
     * pte_numa. NUMA hinting faults will gather statistics and migrate
     * pages to new nodes if necessary.
    unsigned long numa_next_scan;

    /* Restart point for scanning and setting pte_numa */
    unsigned long numa_scan_offset;

    /* numa_scan_seq prevents two threads setting pte_numa */
    int numa_scan_seq;
     * An operation with batched TLB flushing is going on. Anything that
     * can move process memory needs to flush the TLB when moving a
     * PROT_NONE or PROT_NUMA mapped page.
    atomic_t tlb_flush_pending;
    /* See flush_tlb_batched_pending() */
    bool tlb_flush_batched;
    struct uprobes_state uprobes_state;
    atomic_long_t hugetlb_usage;
    struct work_struct async_put_work;

    /* HMM needs to track a few things per mm */
    struct hmm *hmm;
} __randomize_layout;
  • struct vm_area_struct

用于描述进程地址空间中的一段虚拟区域,每一个VMA都对应一个struct vm_area_struct

 * This struct defines a memory VMM memory area. There is one of these
 * per VM-area/task.  A VM area is any part of the process virtual memory
 * space that has a special rule for the page-fault handlers (ie a shared
 * library, the executable area etc).
struct vm_area_struct {
    /* The first cache line has the info for VMA tree walking. */

    unsigned long vm_start;        /* Our start address within vm_mm. */       //起始地址
    unsigned long vm_end;        /* The first byte after our end address
                       within vm_mm. */         //结束地址,区间中不包含结束地址

    /* linked list of VM areas per task, sorted by address */       //按起始地址排序的链表
    struct vm_area_struct *vm_next, *vm_prev;

    struct rb_node vm_rb;       //红黑树节点

     * Largest free memory gap in bytes to the left of this VMA.
     * Either between this VMA and vma->vm_prev, or between one of the
     * VMAs below us in the VMA rbtree and its ->vm_prev. This helps
     * get_unmapped_area find a free area of the right size.
    unsigned long rb_subtree_gap;

    /* Second cache line starts here. */

    struct mm_struct *vm_mm;    /* The address space we belong to. */
    pgprot_t vm_page_prot;        /* Access permissions of this VMA. */
    unsigned long vm_flags;        /* Flags, see mm.h. */

     * For areas with an address space and backing store,
     * linkage into the address_space->i_mmap interval tree.
    struct {
        struct rb_node rb;
        unsigned long rb_subtree_last;
    } shared;

     * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
     * list, after a COW of one of the file pages.    A MAP_SHARED vma
     * can only be in the i_mmap tree.  An anonymous MAP_PRIVATE, stack
     * or brk vma (with NULL file) can only be in an anon_vma list.
    struct list_head anon_vma_chain; /* Serialized by mmap_sem &
                      * page_table_lock */
    struct anon_vma *anon_vma;    /* Serialized by page_table_lock */

    /* Function pointers to deal with this struct. */
    const struct vm_operations_struct *vm_ops;

    /* Information about our backing store: */
    unsigned long vm_pgoff;        /* Offset (within vm_file) in PAGE_SIZE
                       units */
    struct file * vm_file;        /* File we map to (can be NULL). */     //指向文件的一个打开实例
    void * vm_private_data;        /* was vm_pte (shared mem) */

    atomic_long_t swap_readahead_info;
#ifndef CONFIG_MMU
    struct vm_region *vm_region;    /* NOMMU mapping region */
    struct mempolicy *vm_policy;    /* NUMA policy for the VMA */
    struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
} __randomize_layout;







/*  VMA的查找 */
/* Look up the first VMA which satisfies  addr < vm_end,  NULL if none. */
extern struct vm_area_struct * find_vma(struct mm_struct * mm, unsigned long addr); //查找第一个满足addr < vm_end的VMA块
extern struct vm_area_struct * find_vma_prev(struct mm_struct * mm, unsigned long addr,
                         struct vm_area_struct **pprev); //与find_vma功能类似,不同之处在于还会返回VMA链接的前一个VMA;
 static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * mm, unsigned long start_addr, unsigned long end_addr); //查找与start_addr~end_addr区域有交集的VMA

 /* VMA的插入 */
 extern int insert_vm_struct(struct mm_struct *, struct vm_area_struct *); //插入VMA到红黑树中和链表中

 /* VMA的合并 */
 extern struct vm_area_struct *vma_merge(struct mm_struct *,
    struct vm_area_struct *prev, unsigned long addr, unsigned long end,
    unsigned long vm_flags, struct anon_vma *, struct file *, pgoff_t,
    struct mempolicy *, struct vm_userfaultfd_ctx); //将VMA与附近的VMA进行融合操作

 /* VMA的拆分 */
 extern int split_vma(struct mm_struct *, struct vm_area_struct *,
    unsigned long addr, int new_below); //将VMA以addr为界线分成两个VMA


3. malloc









整个过程看起来就比较清晰和简单了,每个进程都用struct mm_struct来描述自身的进程地址空间,这些空间都是一些vma区域,通过一个红黑树和链表来管理。因此针对malloc的处理,会去动态的调整brk的位置,具体的大小则由struct vm_area_struct结构中的vm_start ~ vm_end来指定。在实际过程中,会根据请求分配区域是否与现有vma重叠的情况来进行处理,或者重新申请一个vma来描述这段区域,并最终插入到红黑树和链表中。


4. mmap


  • 文件映射: 将文件区域映射到进程空间,文件存放在存储设备上;
  • 匿名映射:没有文件对应的区域映射,内容存放在物理内存上;


  • 私有映射:将数据源拷贝副本,不影响其他进程;
  • 共享映射:共享的进程都能看到;


  1. 私有匿名映射: 通常分配大块内存时使用,堆,栈,bss段等;
  2. 共享匿名映射:常用于父子进程间通信,在内存文件系统中创建/dev/zero设备;
  3. 私有文件映射:常用的比如动态库加载,代码段,数据段等;
  4. 共享文件映射:常用于进程间通信,文件读写等;


#define PROT_READ    0x1        /* page can be read */
#define PROT_WRITE    0x2        /* page can be written */
#define PROT_EXEC    0x4        /* page can be executed */
#define PROT_SEM    0x8        /* page may be used for atomic ops */
#define PROT_NONE    0x0        /* page can not be accessed */
#define PROT_GROWSDOWN    0x01000000    /* mprotect flag: extend change to start of growsdown vma */
#define PROT_GROWSUP    0x02000000    /* mprotect flag: extend change to end of growsup vma */

#define MAP_SHARED    0x01        /* Share changes */
#define MAP_PRIVATE    0x02        /* Changes are private */
#define MAP_TYPE    0x0f        /* Mask for type of mapping */
#define MAP_FIXED    0x10        /* Interpret addr exactly */
#define MAP_ANONYMOUS    0x20        /* don't use a file */

#define MAP_GROWSDOWN    0x0100        /* stack-like segment */
#define MAP_DENYWRITE    0x0800        /* ETXTBSY */
#define MAP_EXECUTABLE    0x1000        /* mark it as an executable */
#define MAP_LOCKED    0x2000        /* pages are locked */
#define MAP_NORESERVE    0x4000        /* don't check for reservations */
#define MAP_POPULATE    0x8000        /* populate (prefault) pagetables */
#define MAP_NONBLOCK    0x10000        /* do not block on IO */
#define MAP_STACK    0x20000        /* give out an address that is best suited for process/thread stacks */
#define MAP_HUGETLB    0x40000        /* create a huge page mapping */




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