007-2虚拟地址空间布局

二、虚拟地址空间布局

ARM64处理器不支持64位完全虚拟地址。在ARM64结构的linux内核中，内核虚拟地址和用户虚拟地址都是48位，并没有占用前面的16位。所有进程共享内核虚拟地址:ffff 0 0 0 - ffff ffff ffff ffff。每个进程拥有独立的用户空间：0 0 0 0 -- 0 ffff ffff ffff 。同一个进程底下的线程组共享用户的虚拟地址，内核线程不具备用户态的虚拟地址空间。

1、用户虚拟地址的划分

进程的用户虚拟空间的起始地址是0， 长度是TASK_SIZE，由每种处理器架构定义自己的宏TASK_SIZE。ARM64架构定义的宏如下：

D:\linux-4.1.2\Linux-4.12\arch\arm64\include\asm\memory.h

32位用户空间程序：TASK_SIZE == TASK_SIZE_32 == 0x100000000等于4GB

64位用户空间程序：TASK_SIZE == TASK_SIZE_64,即2^VA_BITS字节。一般情况是编译的时候配置VA_BITS的值。

 aston@ubuntu$ grep -rn --colour 'CONFIG_ARM64_VA_BITS' . --include=*
./arch/arm64/configs/defconfig:74:CONFIG_ARM64_VA_BITS_48=y

aston@$ grep -rnw --colour 'CONFIG_ARM64_VA_BITS' . --include=*
./arch/arm64/include/asm/memory.h:66:#define VA_BITS            (CONFIG_ARM64_VA_BITS)
./arch/arm64/Makefile:90:            (0xffffffff & (-1 << ($(CONFIG_ARM64_VA_BITS) - 32))) \
./arch/arm64/Makefile:91:            + (1 << ($(CONFIG_ARM64_VA_BITS) - 32 - 3)) \
aston@ubuntu:/mnt/hgfs/share/025-linux-4.12/Linux-4.12$

2、内核地址空间布局

3、内存描述结构

struct mm_struct {
	struct vm_area_struct *mmap;/* 虚拟内存区域链表，每个进程都有list of VMAs */
	struct rb_root mm_rb;//虚拟内存区域的红黑树
	u32 vmacache_seqnum;/* per-thread vmacache */
#ifdef CONFIG_MMU //在内存映射区域找到一个没有映射的区域
	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;/*内存映射区域的起始地址 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;/*用户虚拟地址空间的长度 size of task vm space */
	unsigned long highest_vm_end;		/* highest vma end address */
	pgd_t * pgd;//指向页全局目录，也就是一级页表

	/**
	 * @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;//共享一个用户虚拟地址空间的线程的数量，也就是线程组包含的线程的数量

	/**
	 * @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;//内存描述符的引用计数

	atomic_long_t nr_ptes;			/* PTE page table pages */
#if CONFIG_PGTABLE_LEVELS > 2
	atomic_long_t nr_pmds;			/* PMD page table pages */
#endif
	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;	/* 进程所拥有的最大页框数 High-watermark of RSS usage */
	unsigned long hiwater_vm;	/* 进程线性区中最大页数 High-water virtual memory usage */

	unsigned long total_vm;		/* 进程地址空间的大小 Total pages mapped */
	unsigned long locked_vm;	/* 锁住而不能换出的页的个数 Pages that have PG_mlocked set */
	unsigned long pinned_vm;	/* Refcount permanently increased */
	unsigned long data_vm;		/* VM_WRITE & ~VM_SHARED & ~VM_STACK */
	unsigned long exec_vm;		/* VM_EXEC & ~VM_WRITE & ~VM_STACK */
	unsigned long stack_vm;		/* VM_STACK */
	unsigned long def_flags;
	/*代码段的起始地址和结束地址 数据段的起始和结束地址*/
	unsigned long start_code, end_code, start_data, end_data;
	/*堆的起始地址和结束地址，栈的起始地址*/
	unsigned long start_brk, brk, start_stack;
	/*参数字符串的起始地址和结束地址，环境变量的起始地址和结束地址*/
	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;

	/* 处理器的特定内存管理上下文 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
	struct user_namespace *user_ns;

	/* store ref to file /proc//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
#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_HUGETLB_PAGE
	atomic_long_t hugetlb_usage;
#endif
	struct work_struct async_put_work;
};