2024-04-02  阅读(24)
原文作者:LoyenWang 原文地址: https://www.cnblogs.com/LoyenWang/p/12037658.html

背景

  • Read the fucking source code! --By 鲁迅
  • A picture is worth a thousand words. --By 高尔基

说明:

  1. Kernel版本:4.14
  2. ARM64处理器,Contex-A53,双核
  3. 使用工具:Source Insight 3.5, Visio

1. 概述

这篇文章,让我们来看看用户态进程的地址空间情况,主要会包括以下:

  • vma;
  • malloc;
  • 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。

在进行内存申请和映射时,都会去地址空间中申请一段虚拟地址区域,而这部分操作也与vma关系密切,因此本文将vma/malloc/mmap三个放到一块来进行分析。
开启探索之旅吧。

2. 数据结构

主要涉及两个结构体:struct mm_structstruct vm_area_struct

  • struct mm_struct
    用于描述与进程地址空间有关的全部信息,这个结构也包含在进程描述符中,关键字段的描述见注释。
    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 */
    #ifdef CONFIG_MMU
    	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
    #endif
    	unsigned long mmap_base;		/* base of mmap area */
    	unsigned long mmap_legacy_base;         /* base of mmap area in bottom-up allocations */
    #ifdef CONFIG_HAVE_ARCH_COMPAT_MMAP_BASES
    	/* Base adresses for compatible mmap() */
    	unsigned long mmap_compat_base;
    	unsigned long mmap_compat_legacy_base;
    #endif
    	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
    #if CONFIG_PGTABLE_LEVELS > 2
    	atomic_long_t nr_pmds;			/* PMD page table pages */
    #endif
    	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 */
    #ifdef CONFIG_MEMBARRIER
    	atomic_t membarrier_state;
    #endif
    #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
    	struct user_namespace *user_ns;
    
    	/* store ref to file /proc/<pid>/exe symlink points to */
    	struct file __rcu *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
    	/*
    	 * 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;
    #ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
    	/* See flush_tlb_batched_pending() */
    	bool tlb_flush_batched;
    #endif
    	struct uprobes_state uprobes_state;
    #ifdef CONFIG_HUGETLB_PAGE
    	atomic_long_t hugetlb_usage;
    #endif
    	struct work_struct async_put_work;
    
    #if IS_ENABLED(CONFIG_HMM)
    	/* HMM needs to track a few things per mm */
    	struct hmm *hmm;
    #endif
    } __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 */
    #endif
    #ifdef CONFIG_NUMA
    	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
    #endif
    	struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
    } __randomize_layout;

关系图来了:

202404022326146191.png

是不是有点眼熟?这个跟内核中的vmap机制很类似。

宏观的看一下进程地址空间中的各个VMA

202404022326148612.png

针对VMA的操作,有如下接口:

    /*  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

上述的操作基本上也就是针对红黑树的操作。

3. malloc

malloc大家都很熟悉,那么它是怎么与底层去交互并申请到内存的呢?

图来了:

202404022326152383.png

如图所示,malloc最终会调到底层的sys_brk函数和sys_mmap函数,在分配小内存时调用sys_brk函数,动态的调整进程地址空间中的brk位置;在分配大块内存时,调用sys_mmap函数,在堆和栈之间找到一片区域进行映射处理。

先来看sys_brk函数,通过SYSCALL_DEFINE1来定义,整体的函数调用流程如下:

202404022326154484.png

从函数的调用过程中可以看出有不少操作是针对vma的,那么结合起来的效果图如下:

202404022326156955.png

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。

完成这段申请后,只是开辟了一段区域,通常还不会立马分配物理内存,物理内存的分配会发生在访问时出现缺页异常后再处理,这个后续也会有文章来进一步分析。

4. mmap

mmap用于内存映射,也就是将一段区域映射到自己的进程地址空间中,分为两种:

  • 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;

同时,针对其他进程是否可见,又分为两种:

  • 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;

根据排列组合,就存在以下几种情况了:

  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;

常见的prot权限和flags如下:

    #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 */

mmap的操作,最终会调用到do_mmap函数,最后来一张调用图:

202404022326159786.png

202404022326163217.png

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