1
在调用mmap系统调用时，可以指定的标志(flag)参数：
/* Share changes */
/* Changes are private */
/* Mask for type of mapping */
/* Interpret addr exactly */
/* don't use a file */
#ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
/* For anonymous mmap, memory could be uninitialized */
#else
/* Don't support this flag */
#endif
MAP_SHARED
用于多个进程共享对一个文件的访问
MAP_PRIVATE
用于创建一个与数据源分离的私有映射，对区域的写入操作不影响数据源文件中的内容
MAP_FIXED
用于在指定的目标线性地址创建一个映射，不允许调整到其他地址
MAP_ANONYMOUS
用于创建与文件无关的映射，或者说没有数据源的映射
do_anonymous_page会调用alloc_zeroed_user_highpage_movable分配一个初始化为全0的内存页。
2
在vm_area_struct数据结构定义中，有一个双链表结点：anon_vma_chain
1: struct vm_area_struct {
2: ......
3: /*
4: * A file's MAP_PRIVATE vma can be in both i_mmap tree and anon_vma
5: * list, after a COW of one of the file pages. A MAP_SHARED vma
6: * can only be in the i_mmap tree. An anonymous MAP_PRIVATE, stack
7: * or brk vma (with NULL file) can only be in an anon_vma list.
8: */
9: struct list_head anon_vma_chain; /* Serialized by mmap_sem &
10: * page_table_lock */
/* Serialized by page_table_lock */
12: ......
13: }
其中，struct anon_vma定义:
1: /*
2: * The anon_vma heads a list of private "related" vmas, to scan if
3: * an anonymous page pointing to this anon_vma needs to be unmapped:
4: * the vmas on the list will be related by forking, or by splitting.
5: *
6: * Since vmas come and go as they are split and merged (particularly
7: * in mprotect), the mapping field of an anonymous page cannot point
8: * directly to a vma: instead it points to an anon_vma, on whose list
9: * the related vmas can be easily linked or unlinked.
10: *
11: * After unlinking the last vma on the list, we must garbage collect
12: * the anon_vma object itself: we're guaranteed no page can be
13: * pointing to this anon_vma once its vma list is empty.
*/
struct anon_vma {
/* Root of this anon_vma tree */
/* Serialize access to vma list */
/*
* The refcount is taken on an anon_vma when there is no
* guarantee that the vma of page tables will exist for
* the duration of the operation. A caller that takes
* the reference is responsible for clearing up the
* anon_vma if they are the last user on release
*/
25: atomic_t refcount;
26:
/*
* NOTE: the LSB of the head.next is set by
* mm_take_all_locks() _after_ taking the above lock. So the
* head must only be read/written after taking the above lock
* to be sure to see a valid next pointer. The LSB bit itself
* is serialized by a system wide lock only visible to
* mm_take_all_locks() (mm_all_locks_mutex).
*/
36: };
3
do_mmap
1: static inline unsigned long do_mmap(struct file *file, unsigned long addr,
2: unsigned long len, unsigned long prot,
3: unsigned long flag, unsigned long offset)
4: {
5: unsigned long ret = -EINVAL;
6: if ((offset + PAGE_ALIGN(len)) < offset)
7: goto out;
8: if (!(offset & ~PAGE_MASK))
9: ret = do_mmap_pgoff(file, addr, len, prot, flag, offset >> PAGE_SHIFT);
10: out:
11: return ret;
12: }
if ((offset + PAGE_ALIGN(len)) < offset)
/* to align the pointer to the (next) page boundary */
#define PAGE_ALIGN(addr) ALIGN(addr, PAGE_SIZE)
/*
* 'kernel.h' contains some often-used function prototypes etc
*/
#define __ALIGN_KERNEL(x, a) __ALIGN_KERNEL_MASK(x, (typeof(x))(a) - 1)
#define __ALIGN_KERNEL_MASK(x, mask)
即
if ((offset + (((len) + (PAGE_SIZE)) & ~(PAGE_SIZE-1))) < offset)
表示如果len太长，再进行align to page boundary操作就会溢出了，那么没有那么多的线性地址空间可以给它映射，因此失败。
if (!(offset & ~PAGE_MASK))
如果offset是位于页的边界处，则继续操作
ret = do_mmap_pgoff(file, addr, len, prot, flag, offset >> PAGE_SHIFT);
其中最后一个参数代表了映射区域在文件中的页序号。
1: /*
2: * The caller must hold down_write(¤t->mm->mmap_sem).
3: */
4:
5: unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
6: unsigned long len, unsigned long prot,
7: unsigned long flags, unsigned long pgoff)
8: {
9: struct mm_struct * mm = current->mm;
10: struct inode *inode;
11: vm_flags_t vm_flags;
12: int error;
13: unsigned long reqprot = prot;
14:
15: /*
16: * Does the application expect PROT_READ to imply PROT_EXEC?
17: *
18: * (the exception is when the underlying filesystem is noexec
19: * mounted, in which case we dont add PROT_EXEC.)
20: */
21: if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
22: if (!(file && (file->f_path.mnt->mnt_flags & MNT_NOEXEC)))
23: prot |= PROT_EXEC;
24:
25: if (!len)
26: return -EINVAL;
27:
28: if (!(flags & MAP_FIXED))
29: addr = round_hint_to_min(addr);
30:
31: /* Careful about overflows.. */
32: len = PAGE_ALIGN(len);
33: if (!len)
34: return -ENOMEM;
35:
36: /* offset overflow? */
if ((pgoff + (len >> PAGE_SHIFT)) < pgoff)
return -EOVERFLOW;
39:
/* Too many mappings? */
if (mm->map_count > sysctl_max_map_count)
return -ENOMEM;
43:
/* Obtain the address to map to. we verify (or select) it and ensure
* that it represents a valid section of the address space.
*/
47: addr = get_unmapped_area(file, addr, len, pgoff, flags);
if (addr & ~PAGE_MASK)
return addr;
50:
/* Do simple checking here so the lower-level routines won't have
* to. we assume access permissions have been handled by the open
* of the memory object, so we don't do any here.
*/
55: vm_flags = calc_vm_prot_bits(prot) | calc_vm_flag_bits(flags) |
56: mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;
57:
if (flags & MAP_LOCKED)
if (!can_do_mlock())
return -EPERM;
61:
/* mlock MCL_FUTURE? */
if (vm_flags & VM_LOCKED) {
long locked, lock_limit;
65: locked = len >> PAGE_SHIFT;
66: locked += mm->locked_vm;
67: lock_limit = rlimit(RLIMIT_MEMLOCK);
68: lock_limit >>= PAGE_SHIFT;
if (locked > lock_limit && !capable(CAP_IPC_LOCK))
return -EAGAIN;
71: }
72:
73: inode = file ? file->f_path.dentry->d_inode : NULL;
74:
if (file) {
switch (flags & MAP_TYPE) {
case MAP_SHARED:
if ((prot&PROT_WRITE) && !(file->f_mode&FMODE_WRITE))
return -EACCES;
80:
/*
* Make sure we don't allow writing to an append-only
* file..
*/
if (IS_APPEND(inode) && (file->f_mode & FMODE_WRITE))
return -EACCES;
87:
/*
* Make sure there are no mandatory locks on the file.
*/
if (locks_verify_locked(inode))
return -EAGAIN;
93:
94: vm_flags |= VM_SHARED | VM_MAYSHARE;
if (!(file->f_mode & FMODE_WRITE))
96: vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
97:
/* fall through */
case MAP_PRIVATE:
if (!(file->f_mode & FMODE_READ))
return -EACCES;
if (file->f_path.mnt->mnt_flags & MNT_NOEXEC) {
if (vm_flags & VM_EXEC)
return -EPERM;
105: vm_flags &= ~VM_MAYEXEC;
106: }
107:
if (!file->f_op || !file->f_op->mmap)
return -ENODEV;
break;
111:
default:
return -EINVAL;
114: }
else {
switch (flags & MAP_TYPE) {
case MAP_SHARED:
/*
* Ignore pgoff.
*/
121: pgoff = 0;
122: vm_flags |= VM_SHARED | VM_MAYSHARE;
break;
case MAP_PRIVATE:
/*
* Set pgoff according to addr for anon_vma.
*/
128: pgoff = addr >> PAGE_SHIFT;
break;
default:
return -EINVAL;
132: }
133: }
134:
135: error = security_file_mmap(file, reqprot, prot, flags, addr, 0);
if (error)
return error;
138:
return mmap_region(file, addr, len, flags, vm_flags, pgoff);
140: }
141: EXPORT_SYMBOL(do_mmap_pgoff);
/* Obtain the address to map to. we verify (or select) it and ensure
* that it represents a valid section of the address space.
*/
addr = get_unmapped_area(file, addr, len, pgoff, flags);
if (addr & ~PAGE_MASK)
return addr;
get_unmapped_area函数用于查找到一个可以安放请求的这么长的一个vma的线性地址范围，返回这个范围的起始地址。如果这个起始地址不是从页对齐处开始的，代表找到的这个地址是不符合要求的，因此也不再往下走了，直接返回。
但是是问题是，如果直接返回了，那么调用都会不会不做检查，直接认为内核已经完成了mmap的操作，而尝试去读写这块还没有与文件建立起关联的内存区域呢，会发生什么不可知的事？
【根据http://www.cnblogs.com/long123king/p/3502170.html中的思想，当进程真正需要访问页时，会触发Page Fault，那么这一步关键是设置好相应的Page Fault handler以及相应struct的指针成员】