8. vma操作概述

在32位系统中，每个用户进程可以拥有3GB大小的虚拟地址空间，通常要远大于物理内存，那么如何管理这些虚拟地址空间呢？用户进程通常会多次调用malloc()或使用mmap()接口映射文件到用户空间来进行读写等操作，这些操作都会要求在虚拟地址空间中分配内存块，这些内存块基本上都是离散的。malloc()是用户态常用的分配内存的接口API函数。mmap()是用户态常用的用于建立文件映射或匿名映射的函数。这些进程地址空间在内核中使用struct vm_area_struct数据结构来描述，简称VMA，也被称为进程地址空间或进程线性区。由于这些地址空间归属于各个用户进程，所以在用户进程的struct mm_struct数据结构中也有相应的成员，用于对这些VMA进行管理。

VMA数据结构定义在mm_types.h（include/linux/mm_types.h）


/** 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. *//*指定VMA在进程地址空间的起始地址*/unsigned long vm_start;     /* Our start address within vm_mm. *//*指定VMA在进程地址空间的结束地址*/unsigned long vm_end;       /* The first byte after our end addresswithin vm_mm. *//* linked list of VM areas per task, sorted by address *//*进程的vma都连接成一个链表*/struct vm_area_struct *vm_next, *vm_prev;/*vma作为一个节点加入红黑树中，每个进程的struct mm_struct数据结构中都有这样一棵红黑树mm->mm_rb*/struct rb_node vm_rb;/** 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. *//*指向该vma所属的进程struct mm_struct数据结构*/struct mm_struct *vm_mm;    /* The address space we belong to. *//*vma的访问权限*/pgprot_t vm_page_prot;      /* Access permissions of this VMA. *//*描述该vma的一组标志位*/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.*//*anon_vam_chain和anon_vma：用于管理RMAP反向映射*/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. *//*指向许多方法的集合，这些方法用于在vma中执行各种操作，通常用于文件映射*/const struct vm_operations_struct *vm_ops;/* Information about our backing store: *//*指定文件映射的偏移量，这个变量的单位不是Byte，而是页面的大小(PAGE_SIZE)*/unsigned long vm_pgoff;     /* Offset (within vm_file) in PAGE_SIZEunits, *not* PAGE_CACHE_SIZE *//*指向file的实例，描述一个被映射的文件*/struct file * vm_file;      /* File we map to (can be NULL). */void * vm_private_data;     /* was vm_pte (shared mem) */#ifndef CONFIG_MMUstruct vm_region *vm_region;    /* NOMMU mapping region */
#endif
#ifdef CONFIG_NUMAstruct mempolicy *vm_policy;    /* NUMA policy for the VMA */
#endif
};

struct mm_struct数据结构是描述进程内存管理的核心数据结构，该数据结构也提供了管理VMA所需的信息，这些信息概况如下：

VMA按照起始地址以递增的方式插入mm_struct->mmap链表中。当进程拥有大量的VMA时，扫描链表和查找特定的VMA是非常低效的操作。

例如在云计算的机器中，所有内核中通常要靠红黑树来协助，以便提高查找速度。

include/linux/mm_types.h

//每个VMA都要连接到mm_struct中的链表和红黑树中，以方便查找。

struct mm_struct {

/*mmap形成一个单链表，进程中所有的vma都连接到这个链表中，链表头时mm_struct->mmap*/

struct vm_area_struct *mmap; /* list of VMAs */

/*mm_rb是红黑树的根节点，每个进程有一棵VMA的红黑树*/

struct rb_root mm_rb;

u32 vmacache_seqnum; /* per-thread vmacache */

#ifdef CONFIG_MMU

unsigned long (*get_unmapped_area) (struct file *filp,

unsigned long addr, unsigned long len,

unsigned long pgoff, unsigned long flags);

#endif

unsigned long mmap_base; /* base of mmap area */

unsigned long mmap_legacy_base; /* base of mmap area in bottom-up allocations */

unsigned long task_size; /* size of task vm space */

unsigned long highest_vm_end; /* highest vma end address */

pgd_t * pgd;

atomic_t mm_users; /* How many users with user space? */

atomic_t mm_count; /* How many references to "struct mm_struct" (users count as 1) */

atomic_long_t nr_ptes; /* PTE page table pages */

atomic_long_t nr_pmds; /* PMD page table pages */

int map_count; /* number of VMAs */

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 */

unsigned long locked_vm; /* Pages that have PG_mlocked set */

unsigned long pinned_vm; /* Refcount permanently increased */

unsigned long shared_vm; /* Shared pages (files) */

unsigned long exec_vm; /* VM_EXEC & ~VM_WRITE */

unsigned long stack_vm; /* VM_GROWSUP/DOWN */

unsigned long def_flags;

unsigned long start_code, end_code, start_data, end_data;

unsigned long start_brk, brk, start_stack;

unsigned long arg_start, arg_end, env_start, env_end;

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 */

#ifdef CONFIG_AIO

spinlock_t ioctx_lock;

struct kioctx_table __rcu *ioctx_table;

#endif

#ifdef CONFIG_MEMCG

* "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;

#endif

/* store ref to file /proc/<pid>/exe symlink points to */

struct file *exe_file;

#ifdef CONFIG_MMU_NOTIFIER

struct mmu_notifier_mm *mmu_notifier_mm;

#endif

#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && !USE_SPLIT_PMD_PTLOCKS

pgtable_t pmd_huge_pte; /* protected by page_table_lock */

#endif

#ifdef CONFIG_CPUMASK_OFFSTACK

struct cpumask cpumask_allocation;

#endif

#ifdef CONFIG_NUMA_BALANCING

* 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;

#endif

#if defined(CONFIG_NUMA_BALANCING) || defined(CONFIG_COMPACTION)

* 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.

bool tlb_flush_pending;

#endif

struct uprobes_state uprobes_state;

#ifdef CONFIG_X86_INTEL_MPX

/* address of the bounds directory */

void __user *bd_addr;

#endif

};

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