1、磁盤目錄項結構
/*
* The new version of the directory entry. Since EXT2 structures are
* stored in intel byte order, and the name_len field could never be
* bigger than 255 chars, it's safe to reclaim the extra byte for the
* file_type field.
*/
struct ext2_dir_entry_2 {
__u32 inode; /* Inode number */
__u16 rec_len; /* Directory entry length */
__u8 name_len; /* Name length */
__u8 file_type;
char name[EXT2_NAME_LEN]; /* File name */
};
inode:inode節點號
rec_len:目錄項長度
name_len:實際的目錄項名稱長度(一般不等於EXT2_NAME_LEN)
file_type:文件類型編碼
name：目錄項名稱(不含'\0')
3、函數分析
* Hardlinks are often used in delicate situations. We avoid
* security-related surprises by not following symlinks on the
* newname. --KAB
*
* We don't follow them on the oldname either to be compatible
* with linux 2.0, and to avoid hard-linking to directories
* and other special files. --ADM
*/
asmlinkage long sys_link(const char * oldname, const char * newname)
{
int error;
char * from;
char * to;
from = getname(oldname);
if(IS_ERR(from))
return PTR_ERR(from);
to = getname(newname);
error = PTR_ERR(to);
if (!IS_ERR(to)) {
struct dentry *new_dentry;
struct nameidata nd, old_nd;
error = 0;
if (path_init(from, LOOKUP_POSITIVE, &old_nd))
error = path_walk(from, &old_nd);
if (error)
goto exit;
if (path_init(to, LOOKUP_PARENT, &nd))
error = path_walk(to, &nd);
if (error)
goto out;
error = -EXDEV;
if (old_nd.mnt != nd.mnt)
goto out_release;
new_dentry = lookup_create(&nd, 0);
error = PTR_ERR(new_dentry);
if (!IS_ERR(new_dentry)) {
error = vfs_link(old_nd.dentry, nd.dentry->d_inode, new_dentry);
dput(new_dentry);
}
up(&nd.dentry->d_inode->i_sem);
out_release:
path_release(&nd);
out:
path_release(&old_nd);
exit:
putname(to);
}
putname(from);
return error;
}
//作爲系統調用API和指定文件系統層的接口，處於VFS層。
int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_dentry)
{
struct inode *inode;
int error;
down(&dir->i_zombie);
error = -ENOENT;
inode = old_dentry->d_inode;
if (!inode)
goto exit_lock;
error = may_create(dir, new_dentry);//檢查是否具備創建權限
if (error)
goto exit_lock;
error = -EXDEV;
if (dir->i_dev != inode->i_dev) //硬鏈接不能跨設備，跨文件系統建立。
goto exit_lock;
/*
* A link to an append-only or immutable file cannot be created.
*/
error = -EPERM;
if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
goto exit_lock;
if (!dir->i_op || !dir->i_op->link)
goto exit_lock;
DQUOT_INIT(dir);
lock_kernel();
error = dir->i_op->link(old_dentry, dir, new_dentry);//調用ext2_link
unlock_kernel();
exit_lock:
up(&dir->i_zombie);
if (!error)
inode_dir_notify(dir, DN_CREATE);
return error;
}
//ext2文件系統的創建鏈接函數，調用ext2_add_nondir。
static int ext2_link (struct dentry * old_dentry, struct inode * dir,
struct dentry *dentry)
{
struct inode *inode = old_dentry->d_inode;
if (S_ISDIR(inode->i_mode)) //不能創建指向目錄節點的硬鏈接
return -EPERM;
if (inode->i_nlink >= EXT2_LINK_MAX)
return -EMLINK;
inode->i_ctime = CURRENT_TIME;
ext2_inc_count(inode);
atomic_inc(&inode->i_count);
return ext2_add_nondir(dentry, inode);
}
//在dentry->parent目錄中創建指向節點inode(節點號inode->i_ino)的鏈接，並將對應的內存目錄項dentry和inode建立關聯
static inline int ext2_add_nondir(struct dentry *dentry, struct inode *inode)
{
int err = ext2_add_link(dentry, inode);
if (!err) {
d_instantiate(dentry, inode);
return 0;
}
ext2_dec_count(inode);
iput(inode);
return err;
}
/*
* Parent is locked.
*/
int ext2_add_link (struct dentry *dentry, struct inode *inode)
{
struct inode *dir = dentry->d_parent->d_inode;
const char *name = dentry->d_name.name;
int namelen = dentry->d_name.len;
unsigned reclen = EXT2_DIR_REC_LEN(namelen);//根據創建目錄項名稱長度計算出目錄項長度reclen
unsigned short rec_len, name_len;
struct page *page = NULL;
ext2_dirent * de;
unsigned long npages = dir_pages(dir);//根據目錄長度，推算出目錄文件的頁面數
unsigned long n;
char *kaddr;
unsigned from, to;
int err;
/* We take care of directory expansion in the same loop */
for (n = 0; n <= npages; n++) {//循環檢測每一頁，尋找目錄項空位
page = ext2_get_page(dir, n);
err = PTR_ERR(page);
if (IS_ERR(page))
goto out;
kaddr = page_address(page);//本頁起始地址：頁結構指針轉換爲線性地址
de = (ext2_dirent *)kaddr;//當前目錄項地址
kaddr += PAGE_CACHE_SIZE - reclen;//kaddr=本頁最後一個目錄項地址（頁起始地址+頁長-新目錄項長度）
while ((char *)de <= kaddr) { //頁內循環檢測每個目錄項，一旦下個檢測地址高於kaddr，那麼說明本頁沒有適合的位置，必須看下頁
err = -EEXIST;
if (ext2_match (namelen, name, de))//目錄中存在同名的目錄項，則退出，返回-EEXIST
goto out_page;
name_len = EXT2_DIR_REC_LEN(de->name_len);//name_len=當前目錄項需要的長度
rec_len = le16_to_cpu(de->rec_len); //rec_len=當前目錄項實際佔用的長度
if (!de->inode && rec_len >= reclen)//情況1：如果當前目錄項空閒，且長度合適，那麼可以放置新的目錄項，轉向got_it。
goto got_it;
if (rec_len >= name_len + reclen)//情況2：如果當前目錄項長度>=當前目錄項需要長度+新目錄項需要長度，那麼可以從中分出尾部部分放置新目錄項
goto got_it;
de = (ext2_dirent *) ((char *) de + rec_len);//否則看下個目錄項
}
ext2_put_page(page);
}
BUG();
return -EINVAL;
got_it:
from = (char*)de - (char*)page_address(page);
to = from + rec_len;
lock_page(page);
err = page->mapping->a_ops->prepare_write(NULL, page, from, to);
if (err)
goto out_unlock;
if (de->inode) {//針對情況2
ext2_dirent *de1 = (ext2_dirent *) ((char *) de + name_len);//新目錄項的放置位置de1在當前目錄項後邊，即當前目錄項位置de+當前目錄項實際需要的長度name_len
de1->rec_len = cpu_to_le16(rec_len - name_len);//新目錄項的實際佔用長度de1->rec_len=當前目錄項實際佔用長度rec_len - 當前目錄項需要的長度name_len
de->rec_len = cpu_to_le16(name_len);//修改當前目錄項實際佔用長度rec_len爲需要的長度name_len
de = de1;//de指向新目錄項的位置
}
de->name_len = namelen;
memcpy (de->name, name, namelen);//拷貝目錄項名稱字符串
de->inode = cpu_to_le32(inode->i_ino);//關聯i節點號
ext2_set_de_type (de, inode);
err = ext2_commit_chunk(page, from, to);//提交頁面的修改部分(相關bh置爲髒),如果塊設備有MS_SYNCHRONOUS標誌或者目錄文件有S_SYNC標誌，則立刻同步的磁盤。
dir->i_mtime = dir->i_ctime = CURRENT_TIME;//修改目錄節點的時間戳。
mark_inode_dirty(dir);//目錄文件節點置爲髒
/* OFFSET_CACHE */
out_unlock:
UnlockPage(page);
out_page:
ext2_put_page(page);
out:
return err;
}