d_alloc

回顾do_lookup里面的情景：
比如说要查找"/mnt/tmp1"里面的tmp1分量, __d_lookup在内存里面没有查找到。
这时需要分配一个dentry的数据结构。分配的函数就是d_alloc。

函数参数：
struct dentry * parent，指向的是"/mnt”的dentry
qstr *name，保存有"tmp1"分量的hash值，len=3，name=”tmp1".

/**
 * d_alloc    -    allocate a dcache entry
 * @parent: parent of entry to allocate
 * @name: qstr of the name
 *
 * Allocates a dentry. It returns %NULL if there is insufficient memory
 * available. On a success the dentry is returned. The name passed in is
 * copied and the copy passed in may be reused after this call.
 */

struct dentry *d_alloc(struct dentry * parent, const struct qstr *name)
{
    struct dentry *dentry;
    char *dname;

    /* 分配一块内存
    dentry结构里面有一个d_iname[DNAME_INLINE_LEN_MIN]数组 */

    dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);
    if (!dentry)
        return NULL;

    /* 如果路径分量name->name字符串太长，超过了DNAME_INLINE_LEN_MIN（128字节），那么dentry->d_iname就不能用来保存name->name了。需要另外分配一块内存来 */

    if (name->len > DNAME_INLINE_LEN-1) {
        dname = kmalloc(name->len + 1, GFP_KERNEL);
        if (!dname) {
            kmem_cache_free(dentry_cache, dentry);
            return NULL;
        }
    } else {
        dname = dentry->d_iname;
    }
    dentry->d_name.name = dname;

    /* 将qstr *name的内容copy过来 */

    dentry->d_name.len = name->len;
    dentry->d_name.hash = name->hash;
    memcpy(dname, name->name, name->len);
    dname[name->len] = 0;

    /* dentry刚刚分配的状态是DCACHE_UNHASHED，因为它还没有链入dentry_hashtable[dentry->d_name.hash]的hash链，等dentry在文件系统的lookup函数中填充完毕后，会将dentry通过d_hash链入hashtdentry_able[dentry->d_name.hash]，链入以后才取消DCACHE_UNHASHED标志
比如procfs的lookup函数proc_root_lookup

调用流程：
proc_root_lookup->
    proc_lookup->
        proc_lookup_de

在proc_lookup_de函数里面，通过d_add将dentry链入hashtdentry_able[dentry->d_name.hash]，并取消dentry的DCACHE_UNHASHED标志
    if (inode) {
        dentry->d_op = &proc_dentry_operations;
        d_add(dentry, inode);
        return NULL;
    }


*/

    atomic_set(&dentry->d_count, 1);
    dentry->d_flags = DCACHE_UNHASHED;
    spin_lock_init(&dentry->d_lock);
    dentry->d_inode = NULL;
    dentry->d_parent = NULL;
    dentry->d_sb = NULL;
    dentry->d_op = NULL;
    dentry->d_fsdata = NULL;
    dentry->d_mounted = 0;
    INIT_HLIST_NODE(&dentry->d_hash);
    INIT_LIST_HEAD(&dentry->d_lru);
    INIT_LIST_HEAD(&dentry->d_subdirs);
    INIT_LIST_HEAD(&dentry->d_alias);

    /* dentry在分配时已经设置其parent entry */

    if (parent) {
        dentry->d_parent = dget(parent);
        dentry->d_sb = parent->d_sb;
    } else {
        INIT_LIST_HEAD(&dentry->d_u.d_child);
    }

    /* dentry在分配时还要通过d_u.d_child链入parent 的d_subdirs中 */
    spin_lock(&dcache_lock);
    if (parent)
        list_add(&dentry->d_u.d_child, &parent->d_subdirs);
    dentry_stat.nr_dentry++;
    spin_unlock(&dcache_lock);

    return dentry;
}