對於一個文件路徑的分量,如果其不爲'.'和'..'則屬於普通文件名,普通文件名的解析由do_lookup()函數來處理
static int do_lookup(struct nameidata *nd, struct qstr *name,
struct path *path)
{
struct vfsmount *mnt = nd->path.mnt;
struct dentry *dentry = __d_lookup(nd->path.dentry, name);//查找name對應的dentry
if (!dentry)//dentry不存在,跳轉至need_lookup
goto need_lookup;
/*如果底層文件系統中定義了d_revalidate函數,則要判斷目錄項是否有效,以保證一致性,
該函數是針對於網絡文件系統存在的*/
if (dentry->d_op && dentry->d_op->d_revalidate)
goto need_revalidate;
done:
path->mnt = mnt;
path->dentry = dentry;
/*這裏由於path往下走了一層,因此要調用__follow_mount()判斷dentry對應的目錄下是否掛載了其他的文件系統,
以保證對應的mnt是正確的*/
__follow_mount(path);
return 0;
need_lookup:
/*沒有找到name對應的dentry,則要創建新的dentry並從磁盤中讀取數據保存在dentry中*/
dentry = real_lookup(nd->path.dentry, name, nd);
if (IS_ERR(dentry))
goto fail;
goto done;
need_revalidate:
dentry = do_revalidate(dentry, nd);
if (!dentry)
goto need_lookup;
if (IS_ERR(dentry))
goto fail;
goto done;
fail:
return PTR_ERR(dentry);
}
可以想象,搜索一個文件(目錄)時,首先肯定要在dentry緩存中查找,當緩存中查找不到對應的dentry時,才需要從磁盤中查找,並新建一個dentry,將磁盤中的數據保存到其中。找到了目標dentry後,就將相應的信息保存到path中,這裏因爲路徑向下進了一層,因此要判斷下層目錄是否有新的文件系統掛載的問題,和上文討論的類似,因此要通過__follow_mount()函數判斷是否有文件系統掛載在該目錄下,另外,對於網絡文件系統,還要通過文件系統中定義的d_revalidate()函數來判斷該dentry是否有效以保證一致性。
先來看看在dentry緩存中查找的過程
struct dentry * __d_lookup(struct dentry * parent, struct qstr * name)
{
unsigned int len = name->len;
unsigned int hash = name->hash;
const unsigned char *str = name->name;
struct hlist_head *head = d_hash(parent,hash);//通過parent的地址和hash(hash是name的哈希值)進行定位
struct dentry *found = NULL;
struct hlist_node *node;
struct dentry *dentry;
rcu_read_lock();
hlist_for_each_entry_rcu(dentry, node, head, d_hash) {//掃描head對應的碰撞溢出表
struct qstr *qstr;
if (dentry->d_name.hash != hash)//name的hash值不相符,則放棄該dentry
continue;
if (dentry->d_parent != parent)//父目錄不一樣,則放棄該dentry
continue;
spin_lock(&dentry->d_lock);
/*
* Recheck the dentry after taking the lock - d_move may have
* changed things. Don't bother checking the hash because we're
* about to compare the whole name anyway.
*/
if (dentry->d_parent != parent)
goto next;
/* non-existing due to RCU? */
if (d_unhashed(dentry))
goto next;
/*
* It is safe to compare names since d_move() cannot
* change the qstr (protected by d_lock).
*/
/*當確保了父目錄和文件名的哈希值與目標dentry的一致性後,接下來就只用匹配文件名了*/
qstr = &dentry->d_name;//取當前dentry的文件名
/*如果父目錄文件系統定義了比較文件名的方法,則調用之*/
if (parent->d_op && parent->d_op->d_compare) {
if (parent->d_op->d_compare(parent, qstr, name))
goto next;
} else {//如果沒定義
if (qstr->len != len)//先確定長度是否相等
goto next;
if (memcmp(qstr->name, str, len))//再比較內存
goto next;
}
atomic_inc(&dentry->d_count);
found = dentry; //這裏表明找到了目標dentry
spin_unlock(&dentry->d_lock);
break;
next:
spin_unlock(&dentry->d_lock);
}
rcu_read_unlock();
return found;
}
d_hash()函數將父目錄dentry的地址和所要查找的文件名的哈希值組合起來,重新構建一個哈希值,並根據其定位到
dentry_hashtable哈希表中,dentry_hashtable是dentry緩存的一部分,所有的dentry都會保存在dentry_hashtable中,這樣一來,就得到了一個哈希表的溢出鏈表的表頭,即代碼中的head變量。下面的工作就是掃描這個鏈表,並從中查找真正的目標。
如果在dentry_hashtable中沒能找到目標dentry,則通過real_lookup()函數從磁盤中查找
static struct dentry * real_lookup(struct dentry * parent, struct qstr * name, struct nameidata *nd)
{
struct dentry * result;
struct inode *dir = parent->d_inode;
mutex_lock(&dir->i_mutex);
/*
* First re-do the cached lookup just in case it was created
* while we waited for the directory semaphore..
*
* FIXME! This could use version numbering or similar to
* avoid unnecessary cache lookups.
*
* The "dcache_lock" is purely to protect the RCU list walker
* from concurrent renames at this point (we mustn't get false
* negatives from the RCU list walk here, unlike the optimistic
* fast walk).
*
* so doing d_lookup() (with seqlock), instead of lockfree __d_lookup
*/
//再從磁盤讀取前再進行一次d_lookup嘗試,因爲之前等待互斥鎖時有可能已經創建了相應的dentry
result = d_lookup(parent, name);
/*下面進行dentry的創建*/
if (!result) {
struct dentry *dentry;
/* Don't create child dentry for a dead directory. */
result = ERR_PTR(-ENOENT);
if (IS_DEADDIR(dir))
goto out_unlock;
/*分配一個dentry並進行相應的初始化*/
dentry = d_alloc(parent, name);
result = ERR_PTR(-ENOMEM);
if (dentry) {
/*調用特定於文件系統的lookup函數從磁盤中讀取數據並將dentry添入散列表*/
result = dir->i_op->lookup(dir, dentry, nd);
if (result)
dput(dentry);
else
result = dentry;
}
out_unlock:
mutex_unlock(&dir->i_mutex);
return result;
}
/*
* Uhhuh! Nasty case: the cache was re-populated while
* we waited on the semaphore. Need to revalidate.
*/
mutex_unlock(&dir->i_mutex);
if (result->d_op && result->d_op->d_revalidate) {
result = do_revalidate(result, nd);
if (!result)
result = ERR_PTR(-ENOENT);
}
return result;
}