在Linux中，struct vm_area_struct表示的虚拟地址是给进程使用的，在虚拟地址中，给定一个地址，试图找到结束地址高于addr的第一个区间经常用到。find_vma()它扫描当前进程地址空间的vm_area_struct结构所形成的红黑树，试图找到结束地址高于addr的第一个区间;1578 /* Look up the first VMA which satisfies addr < vm_end, NULL if none. */1579 struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)1580 {1581 struct vm_area_struct *vma = NULL;1582 1583 if (mm) {1584 /* Check the cache first. */1585 /* (Cache hit rate is typically around 35%.) */1586 vma = mm->mmap_cache;1587 if (!(vma && vma->vm_end > addr && vma->vm_start <= addr)) {1588 struct rb_node * rb_node;1589 1590 rb_node = mm->mm_rb.rb_node;1591 vma = NULL;1592 1593 while (rb_node) {1594 struct vm_area_struct * vma_tmp;1595 1596 vma_tmp = rb_entry(rb_node,1597 struct vm_area_struct, vm_rb);1598 1599 if (vma_tmp->vm_end > addr) {1600 vma = vma_tmp;1601 if (vma_tmp->vm_start <= addr)1602 break;1603 rb_node = rb_node->rb_left;1604 } else1605 rb_node = rb_node->rb_right;1606 }1607 if (vma)1608 mm->mmap_cache = vma;1609 }1610 }1611 return vma;1612 }代码通过首先查看mm中缓存的vma是否满足要求，如果满足，则返回。正如注释所说，有35％的命中率。如果缓存的vma不满足要求，则通过遍历红黑树来查找。rb_entry跟list_entry功能一样。要注意的是这一句：vma_tmp->vm_end > addr因为vm_area_struct的vm_start是包函在此vm_area_struct中的，而vm_end则不包函在些vm_area_struct。函数find_vma_prepare（）与find_vma（）基本相同，它扫描当前进程地址空间的vm_area_struct结构所形成的红黑树，试图找到结束地址高于addr的第一个区间：350 static struct vm_area_struct * 351 find_vma_prepare(struct mm_struct *mm, unsigned long addr, 352 struct vm_area_struct **pprev, struct rb_node ***rb_link, 353 struct rb_node ** rb_parent) 354 { 355 struct vm_area_struct * vma; 356 struct rb_node ** __rb_link, * __rb_parent, * rb_prev; 357 358 __rb_link = &mm->mm_rb.rb_node; 359 rb_prev = __rb_parent = NULL; 360 vma = NULL; 361 362 while (*__rb_link) { 363 struct vm_area_struct *vma_tmp; 364 365 __rb_parent = *__rb_link; 366 vma_tmp = rb_entry(__rb_parent, struct vm_area_struct, vm_rb); 367 368 if (vma_tmp->vm_end > addr) { 369 vma = vma_tmp; 370 if (vma_tmp->vm_start <= addr) 371 break; 372 __rb_link = &__rb_parent->rb_left; 373 } else { 374 rb_prev = __rb_parent; 375 __rb_link = &__rb_parent->rb_right; 376 } 377 } 378 379 *pprev = NULL; 380 if (rb_prev) 381 *pprev = rb_entry(rb_prev, struct vm_area_struct, vm_rb); 382 *rb_link = __rb_link; 383 *rb_parent = __rb_parent; 384 return vma; 385 }还有一个与此相关的函数是insert_vm_struct()，这是把vma插入到mm的VMA链表和mm的红黑树中，如果映射了文件，也进行了与文件相关的操作。insert_vm_struct():2283 int insert_vm_struct(struct mm_struct * mm, struct vm_area_struct * vma)2284 {2285 struct vm_area_struct * __vma, * prev;2286 struct rb_node ** rb_link, * rb_parent;2287 2288 /*2289 * The vm_pgoff of a purely anonymous vma should be irrelevant2290 * until its first write fault, when page's anon_vma and index2291 * are set. But now set the vm_pgoff it will almost certainly2292 * end up with (unless mremap moves it elsewhere before that2293 * first wfault), so /proc/pid/maps tells a consistent story.2294 *2295 * By setting it to reflect the virtual start address of the2296 * vma, merges and splits can happen in a seamless way, just2297 * using the existing file pgoff checks and manipulations.2298 * Similarly in do_mmap_pgoff and in do_brk.2299 */2300 if (!vma->vm_file) {2301 BUG_ON(vma->anon_vma);2302 vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT;2303 }2304 __vma = find_vma_prepare(mm,vma->vm_start,&prev,&rb_link,&rb_parent);2305 if (__vma && __vma->vm_start < vma->vm_end)2306 return -ENOMEM;2307 if ((vma->vm_flags & VM_ACCOUNT) &&2308 security_vm_enough_memory_mm(mm, vma_pages(vma)))2309 return -ENOMEM;2310 vma_link(mm, vma, prev, rb_link, rb_parent);2311 return 0;2312 }而struct vm_struct表示的虚拟地址是给内核使用的，它们对应的物理页面都可以是不连续的。struct vm_struct表示的地址空间范围为什么不是3G~4G呢？原来，3G ~ (3G + 896M)范围的地址是用来映射连续的物理页面的，这个范围的虚拟地址和对应的实际物理地址有着简单的对应关系，即对应0~896M的物理地址空间，而(3G + 896M) ~ (3G + 896M + 8M)是安全保护区域（例如，所有指向这8M地址空间的指针都是非法的），因此struct vm_struct使用(3G + 896M + 8M) ~ 4G地址空间来映射非连续的物理页面。
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