linux进程地址空间--vma的基本操作
在32位的系统上，线性地址空间可达到4GB，这4GB一般按照3:1的比例进行分配，也就是说用户进程享有前3GB线性地址空间，而内核独享最后1GB线性地址空间。由于虚拟内存的引入，每个进程都可拥有3GB的虚拟内存，并且用户进程之间的地址空间是互不可见、互不影响的，也就是说即使两个进程对同一个地址进行操作，也不会产生问题。在前面介绍的一些分配内存的途径中，无论是伙伴系统中分配页的函数，还是slab分配器中分配对象的函数，它们都会尽量快速地响应内核的分配请求，将相应的内存提交给内核使用，而内核对待用户空间显然不能如此。用户空间动态申请内存时往往只是获得一块线性地址的使用权，而并没有将这块线性地址区域与实际的物理内存对应上，只有当用户空间真正操作申请的内存时，才会触发一次缺页异常，这时内核才会分配实际的物理内存给用户空间。
用户进程的虚拟地址空间包含了若干区域，这些区域的分布方式是特定于体系结构的，不过所有的方式都包含下列成分：
- 可执行文件的二进制代码，也就是程序的代码段
- 存储全局变量的数据段
- 用于保存局部变量和实现函数调用的栈
- 环境变量和命令行参数
- 程序使用的动态库的代码
- 用于映射文件内容的区域
由此可以看到进程的虚拟内存空间会被分成不同的若干区域，每个区域都有其相关的属性和用途，一个合法的地址总是落在某个区域当中的，这些区域也不会重叠。在linux内核中，这样的区域被称之为虚拟内存区域(virtual memory areas),简称vma。一个vma就是一块连续的线性地址空间的抽象，它拥有自身的权限(可读，可写，可执行等等) ，每一个虚拟内存区域都由一个相关的struct vm_area_struct结构来描述
struct vm_area_struct {
struct mm_struct * vm_mm; /* 所属的内存描述符 */
unsigned long vm_start; /* vma的起始地址 */
unsigned long vm_end; /* vma的结束地址 */
/* 该vma的在一个进程的vma链表中的前驱vma和后驱vma指针，链表中的vma都是按地址来排序的*/
struct vm_area_struct *vm_next, *vm_prev;
pgprot_t vm_page_prot; /* vma的访问权限 */
unsigned long vm_flags; /* 标识集 */
struct rb_node vm_rb; /* 红黑树中对应的节点 */
/*
* For areas with an address space and backing store,
* linkage into the address_space->i_mmap prio tree, or
* linkage to the list of like vmas hanging off its node, or
* linkage of vma in the address_space->i_mmap_nonlinear list.
*/
/* shared联合体用于和address space关联 */
union {
struct {
struct list_head list;/* 用于链入非线性映射的链表 */
void *parent; /* aligns with prio_tree_node parent */
struct vm_area_struct *head;
} vm_set;
struct raw_prio_tree_node prio_tree_node;/*线性映射则链入i_mmap优先树*/
} 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.
*/
/*anno_vma_node和annon_vma用于管理源自匿名映射的共享页*/
struct list_head anon_vma_node; /* Serialized by anon_vma->lock */
struct anon_vma *anon_vma; /* Serialized by page_table_lock */
/* Function pointers to deal with this struct. */
/*该vma上的各种标准操作函数指针集*/
const struct vm_operations_struct *vm_ops;
/* Information about our backing store: */
unsigned long vm_pgoff; /* 映射文件的偏移量，以PAGE_SIZE为单位 */
struct file * vm_file; /* 映射的文件，没有则为NULL */
void * vm_private_data; /* was vm_pte (shared mem) */
unsigned long vm_truncate_count;/* truncate_count or restart_addr */
#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
};
进程的若干个vma区域都得按一定的形式组织在一起，这些vma都包含在进程的内存描述符中，也就是struct mm_struct中，这些vma在mm_struct以两种方式进行组织，一种是链表方式，对应于mm_struct中的mmap链表头，一种是红黑树方式，对应于mm_struct中的mm_rb根节点，和内核其他地方一样，链表用于遍历，红黑树用于查找。
下面以文件映射为例，来阐述文件的address_space和与其建立映射关系的vma是如何联系上的。首先来看看struct address_space中与vma相关的变量
struct address_space {
struct inode *host; /* owner: inode, block_device */
...
struct prio_tree_root i_mmap; /* tree of private and shared mappings */
struct list_head i_mmap_nonlinear; /*list VM_NONLINEAR mappings */
...
} __attr
与此同时，struct file和struct inode中都包含有一个struct address_space的指针，分别为f_mapping和i_mapping。struct file是一个特定于进程的数据结构，而struct inode则是一个特定于文件的数据结构。每当进程打开一个文件时，都会将file->f_mapping设置到inode->i_mapping,下图则给出了文件和与其建立映射关系的vma的联系
下面来看几个vma的基本操作函数，这些函数都是后面实现具体功能的基础
find_vma()用来寻找一个针对于指定地址的vma，该vma要么包含了指定的地址，要么位于该地址之后并且离该地址最近，或者说寻找第一个满足addr<vma_end的vma
struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
{
struct vm_area_struct *vma = NULL;
if (mm) {
/* Check the cache first. */
/* (Cache hit rate is typically around 35%.) */
vma = mm->mmap_cache; //首先尝试mmap_cache中缓存的vma
/*如果不满足下列条件中的任意一个则从红黑树中查找合适的vma
1.缓存vma不存在
2.缓存vma的结束地址小于给定的地址
3.缓存vma的起始地址大于给定的地址*/
if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {
struct rb_node * rb_node;
rb_node = mm->mm_rb.rb_node;//获取红黑树根节点
vma = NULL;
while (rb_node) {
struct vm_area_struct * vma_tmp;
vma_tmp = rb_entry(rb_node, //获取节点对应的vma
struct vm_area_struct, vm_rb);
/*首先确定vma的结束地址是否大于给定地址，如果是的话，再确定
vma的起始地址是否小于给定地址，也就是优先保证给定的地址是
处于vma的范围之内的，如果无法保证这点，则只能找到一个距离
给定地址最近的vma并且该vma的结束地址要大于给定地址*/
if (vma_tmp->vm_end > addr) {
vma = vma_tmp;
if (vma_tmp->vm_start <= addr)
break;
rb_node = rb_node->rb_left;
} else
rb_node = rb_node->rb_right;
}
if (vma)
mm->mmap_cache = vma;//将结果保存在缓存中
}
}
return vma;
}
当一个新区域被加到进程的地址空间时，内核会检查它是否可以与一个或多个现存区域合并，vma_merge()函数在可能的情况下，将一个新区域与周边区域进行合并。参数：
mm:新区域所属的进程地址空间
prev:在地址上紧接着新区域的前面一个vma
addr:新区域的起始地址
end:新区域的结束地址
vm_flags:新区域的标识集
anon_vma:新区域所属的匿名映射
file:新区域映射的文件
pgoff:新区域映射文件的偏移
policy:和NUMA相关
struct vm_area_struct *vma_merge(struct mm_struct *mm,
struct vm_area_struct *prev, unsigned long addr,
unsigned long end, unsigned long vm_flags,
struct anon_vma *anon_vma, struct file *file,
pgoff_t pgoff, struct mempolicy *policy)
{
pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
struct vm_area_struct *area, *next;
/*
* We later require that vma->vm_flags == vm_flags,
* so this tests vma->vm_flags & VM_SPECIAL, too.
*/
if (vm_flags & VM_SPECIAL)
return NULL;
if (prev)//指定了先驱vma，则获取先驱vma的后驱vma
next = prev->vm_next;
else //否则指定mm的vma链表中的第一个元素为后驱vma
next = mm->mmap;
area = next;
/*后驱节点存在，并且后驱vma的结束地址和给定区域的结束地址相同，
也就是说两者有重叠，那么调整后驱vma*/
if (next && next->vm_end == end) /* cases 6, 7, 8 */
next = next->vm_next;
/*
* 先判断给定的区域能否和前驱vma进行合并，需要判断如下的几个方面:
1.前驱vma必须存在
2.前驱vma的结束地址正好等于给定区域的起始地址
3.两者的struct mempolicy中的相关属性要相同，这项检查只对NUMA架构有意义
4.其他相关项必须匹配，包括两者的vm_flags，是否映射同一个文件等等
*/
if (prev && prev->vm_end == addr &&
mpol_equal(vma_policy(prev), policy) &&
can_vma_merge_after(prev, vm_flags,
anon_vma, file, pgoff)) {
/*
*确定可以和前驱vma合并后再判断是否能和后驱vma合并，判断方式和前面一样，
不过这里多了一项检查，在给定区域能和前驱、后驱vma合并的情况下还要检查
前驱、后驱vma的匿名映射可以合并
*/
if (next && end == next->vm_start &&
mpol_equal(policy, vma_policy(next)) &&
can_vma_merge_before(next, vm_flags,
anon_vma, file, pgoff+pglen) &&
is_mergeable_anon_vma(prev->anon_vma,
next->anon_vma)) {
/* cases 1, 6 */
vma_adjust(prev, prev->vm_start,
next->vm_end, prev->vm_pgoff, NULL);
} else /* cases 2, 5, 7 */
vma_adjust(prev, prev->vm_start,
end, prev->vm_pgoff, NULL);
return prev;
}
/*
* Can this new request be merged in front of next?
*/
/*如果前面的步骤失败，那么则从后驱vma开始进行和上面类似的步骤*/
if (next && end == next->vm_start &&
mpol_equal(policy, vma_policy(next)) &&
can_vma_merge_before(next, vm_flags,
anon_vma, file, pgoff+pglen)) {
if (prev && addr < prev->vm_end) /* case 4 */
vma_adjust(prev, prev->vm_start,
addr, prev->vm_pgoff, NULL);
else /* cases 3, 8 */
vma_adjust(area, addr, next->vm_end,
next->vm_pgoff - pglen, NULL);
return area;
}
return NULL;
}
vma_adjust会执行具体的合并调整操作
void vma_adjust(struct vm_area_struct *vma, unsigned long start,
unsigned long end, pgoff_t pgoff, struct vm_area_struct *insert)
{
struct mm_struct *mm = vma->vm_mm;
struct vm_area_struct *next = vma->vm_next;
struct vm_area_struct *importer = NULL;
struct address_space *mapping = NULL;
struct prio_tree_root *root = NULL;
struct file *file = vma->vm_file;
struct anon_vma *anon_vma = NULL;
long adjust_next = 0;
int remove_next = 0;
if (next && !insert) {
/*指定的范围已经跨越了整个后驱vma，并且有可能超过后驱vma*/
if (end >= next->vm_end) {
/*
* vma expands, overlapping all the next, and
* perhaps the one after too (mprotect case 6).
*/
again: remove_next = 1 + (end > next->vm_end);//确定是否超过了后驱vma
end = next->vm_end;
anon_vma = next->anon_vma;
importer = vma;
} else if (end > next->vm_start) {/*指定的区域和后驱vma部分重合*/
/*
* vma expands, overlapping part of the next:
* mprotect case 5 shifting the boundary up.
*/
adjust_next = (end - next->vm_start) >> PAGE_SHIFT;
anon_vma = next->anon_vma;
importer = vma;
} else if (end < vma->vm_end) {/*指定的区域没到达后驱vma的结束处*/
/*
* vma shrinks, and !insert tells it's not
* split_vma inserting another: so it must be
* mprotect case 4 shifting the boundary down.
*/
adjust_next = - ((vma->vm_end - end) >> PAGE_SHIFT);
anon_vma = next->anon_vma;
importer = next;
}
}
if (file) {//如果有映射文件
mapping = file->f_mapping;//获取文件对应的address_space
if (!(vma->vm_flags & VM_NONLINEAR))
root = &mapping->i_mmap;
spin_lock(&mapping->i_mmap_lock);
if (importer &&
vma->vm_truncate_count != next->vm_truncate_count) {
/*
* unmap_mapping_range might be in progress:
* ensure that the expanding vma is rescanned.
*/
importer->vm_truncate_count = 0;
}
/*如果指定了待插入的vma，则根据vma是否以非线性的方式映射文件来选择是将
vma插入file对应的address_space的优先树(对应线性映射)还是双向链表(非线性映射)*/
if (insert) {
insert->vm_truncate_count = vma->vm_truncate_count;
/*
* Put into prio_tree now, so instantiated pages
* are visible to arm/parisc __flush_dcache_page
* throughout; but we cannot insert into address
* space until vma start or end is updated.
*/
__vma_link_file(insert);
}
}
/*
* When changing only vma->vm_end, we don't really need
* anon_vma lock.
*/
if (vma->anon_vma && (insert || importer || start != vma->vm_start))
anon_vma = vma->anon_vma;
if (anon_vma) {
spin_lock(&anon_vma->lock);
/*
* Easily overlooked: when mprotect shifts the boundary,
* make sure the expanding vma has anon_vma set if the
* shrinking vma had, to cover any anon pages imported.
*/
if (importer && !importer->anon_vma) {
importer->anon_vma = anon_vma;
__anon_vma_link(importer);//将importer插入importer的anon_vma匿名映射链表中
}
}
if (root) {
flush_dcache_mmap_lock(mapping);
vma_prio_tree_remove(vma, root);
if (adjust_next)
vma_prio_tree_remove(next, root);
}
/*调整vma的相关量*/
vma->vm_start = start;
vma->vm_end = end;
vma->vm_pgoff = pgoff;
if (adjust_next) {//调整后驱vma的相关量
next->vm_start += adjust_next << PAGE_SHIFT;
next->vm_pgoff += adjust_next;
}
if (root) {
if (adjust_next)//如果后驱vma被调整了，则重新插入到优先树中
vma_prio_tree_insert(next, root);
vma_prio_tree_insert(vma, root);//将vma插入到优先树中
flush_dcache_mmap_unlock(mapping);
}
if (remove_next) {//给定区域与后驱vma有重合
/*
* vma_merge has merged next into vma, and needs
* us to remove next before dropping the locks.
*/
__vma_unlink(mm, next, vma);//将后驱vma从红黑树中删除
if (file)//将后驱vma从文件对应的address space中删除
__remove_shared_vm_struct(next, file, mapping);
if (next->anon_vma)//将后驱vma从匿名映射链表中删除
__anon_vma_merge(vma, next);
} else if (insert) {
/*
* split_vma has split insert from vma, and needs
* us to insert it before dropping the locks
* (it may either follow vma or precede it).
*/
__insert_vm_struct(mm, insert);//将待插入的vma插入mm的红黑树，双向链表以及
//匿名映射链表
}
if (anon_vma)
spin_unlock(&anon_vma->lock);
if (mapping)
spin_unlock(&mapping->i_mmap_lock);
if (remove_next) {
if (file) {
fput(file);
if (next->vm_flags & VM_EXECUTABLE)
removed_exe_file_vma(mm);
}
mm->map_count--;
mpol_put(vma_policy(next));
kmem_cache_free(vm_area_cachep, next);
/*
* In mprotect's case 6 (see comments on vma_merge),
* we must remove another next too. It would clutter
* up the code too much to do both in one go.
*/
if (remove_next == 2) {//还有待删除的区域
next = vma->vm_next;
goto again;
}
}
validate_mm(mm);
}
insert_vm_struct()函数用于插入一块新区域
int insert_vm_struct(struct mm_struct * mm, struct vm_area_struct * vma)
{
struct vm_area_struct * __vma, * prev;
struct rb_node ** rb_link, * rb_parent;
/*
* The vm_pgoff of a purely anonymous vma should be irrelevant
* until its first write fault, when page's anon_vma and index
* are set. But now set the vm_pgoff it will almost certainly
* end up with (unless mremap moves it elsewhere before that
* first wfault), so /proc/pid/maps tells a consistent story.
*
* By setting it to reflect the virtual start address of the
* vma, merges and splits can happen in a seamless way, just
* using the existing file pgoff checks and manipulations.
* Similarly in do_mmap_pgoff and in do_brk.
*/
if (!vma->vm_file) {
BUG_ON(vma->anon_vma);
vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT;
}
/*__vma用来保存和vma->start对应的vma(与find_vma()一样)，同时获取以下信息:
1.prev用来保存对应的前驱vma
2.rb_link保存该vma区域插入对应的红黑树节点
3.rb_parent保存该vma区域对应的父节点*/
__vma = find_vma_prepare(mm,vma->vm_start,&prev,&rb_link,&rb_parent);
if (__vma && __vma->vm_start < vma->vm_end)
return -ENOMEM;
if ((vma->vm_flags & VM_ACCOUNT) &&
security_vm_enough_memory_mm(mm, vma_pages(vma)))
return -ENOMEM;
vma_link(mm, vma, prev, rb_link, rb_parent);//将vma关联到所有的数据结构中
return 0;
}
static void vma_link(struct mm_struct *mm, struct vm_area_struct *vma,
struct vm_area_struct *prev, struct rb_node **rb_link,
struct rb_node *rb_parent)
{
struct address_space *mapping = NULL;
if (vma->vm_file)//如果存在文件映射则获取文件对应的地址空间
mapping = vma->vm_file->f_mapping;
if (mapping) {
spin_lock(&mapping->i_mmap_lock);
vma->vm_truncate_count = mapping->truncate_count;
}
anon_vma_lock(vma);
/*将vma插入到相应的数据结构中--双向链表，红黑树和匿名映射链表*/
__vma_link(mm, vma, prev, rb_link, rb_parent);
__vma_link_file(vma);//将vma插入到文件地址空间的相应数据结构中
anon_vma_unlock(vma);
if (mapping)
spin_unlock(&mapping->i_mmap_lock);
mm->map_count++;
validate_mm(mm);
}
在创建新的vma区域之前先要寻找一块足够大小的空闲区域，该项工作由get_unmapped_area()函数完成，而实际的工作将会由mm_struct中定义的辅助函数来完成。根据进程虚拟地址空间的布局，会选择使用不同的映射函数，在这里考虑大多数系统上采用的标准函数arch_get_unmapped_area();
unsigned long
arch_get_unmapped_area(struct file *filp, unsigned long addr,
unsigned long len, unsigned long pgoff, unsigned long flags)
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma;
unsigned long start_addr;
if (len > TASK_SIZE)
return -ENOMEM;
if (flags & MAP_FIXED)
return addr;
if (addr) {
addr = PAGE_ALIGN(addr);//将地址按页对齐
vma = find_vma(mm, addr);//获取一个vma，该vma可能包含了addr也可能在addr后面并且离addr最近
/*这里确定是否有一块适合的空闲区域，先要保证addr+len不会
超过进程地址空间的最大允许范围，然后如果前面vma获取成功的话则要保证
vma位于addr的后面并且addr+len不会延伸到该vma的区域*/
if (TASK_SIZE - len >= addr &&
(!vma || addr + len <= vma->vm_start))
return addr;
}
/*前面获取不成功的话则要调整起始地址了，根据情况选择缓存的空闲区域地址
或者TASK_UNMAPPED_BASE=TASK_SIZE/3*/
if (len > mm->cached_hole_size) {
start_addr = addr = mm->free_area_cache;
} else {
start_addr = addr = TASK_UNMAPPED_BASE;
mm->cached_hole_size = 0;
}
full_search:
/*从addr开始遍历用户地址空间*/
for (vma = find_vma(mm, addr); ; vma = vma->vm_next) {
/* At this point: (!vma || addr < vma->vm_end). */
if (TASK_SIZE - len < addr) {//这里判断是否已经遍历到了用户地址空间的末端
/*
* Start a new search - just in case we missed
* some holes.
*/
//如果上次不是从TAKS_UNMAPPED_BASE开始遍历的，则尝试从TASK_UNMAPPED_BASE开始遍历
if (start_addr != TASK_UNMAPPED_BASE) {
addr = TASK_UNMAPPED_BASE;
start_addr = addr;
mm->cached_hole_size = 0;
goto full_search;
}
return -ENOMEM;
}
if (!vma || addr + len <= vma->vm_start) {//判断是否有空闲区域
/*
*找到空闲区域的话则记住我们搜索的结束处，以便下次搜索
*/
mm->free_area_cache = addr + len;
return addr;
}
/*该空闲区域不符合大小要求，但是如果这个空闲区域大于之前保存的最大值的话
则将这个空闲区域保存，这样便于前面确定从哪里开始搜索*/
if (addr + mm->cached_hole_size < vma->vm_start)
mm->cached_hole_size = vma->vm_start - addr;
addr = vma->vm_end;
}
}