触碰虚拟文件系统

5.6 触碰虚拟文件系统

回到start_kernel中，下面我们该第一次接触文件系统了，582行执行vfs_caches_init_early：

 

void __init vfs_caches_init_early(void) {        dcache_init_early();        inode_init_early(); }

 

vfs_caches_init_early调用两个函数dcache_init_early和inode_init_early。内核第一次触碰文件系统的主要目的就是初始化VFS的两个重要数据结构dcache和inode的缓存。

dcache_init_early来自fs/dcache.c：

 

2287static void __init dcache_init_early(void) 2288{ 2289        int loop; 2290 2291        /* If hashes are distributed across NUMA nodes, defer 2292         * hash allocation until vmalloc space is available. 2293         */ 2294        if (hashdist) 2295                return; 2296 2297        dentry_hashtable = 2298                alloc_large_system_hash("Dentry cache", 2299                                        sizeof(struct hlist_head), 2300                                        dhash_entries, 2301                                        13, 2302                                        HASH_EARLY, 2303                                        &d_hash_shift, 2304                                        &d_hash_mask, 2305                                        0); 2306 2307        for (loop = 0; loop < (1 << d_hash_shift); loop++) 2308                INIT_HLIST_HEAD(&dentry_hashtable[loop]); 2309}

 

inode_init_early来自fs/inode.c：

 

1539void __init inode_init_early(void) 1540{ 1541        int loop; 1542 1543        /* If hashes are distributed across NUMA nodes, defer 1544         * hash allocation until vmalloc space is available. 1545         */ 1546        if (hashdist) 1547                return; 1548 1549        inode_hashtable = 1550                alloc_large_system_hash("Inode-cache", 1551                                        sizeof(struct hlist_head), 1552                                        ihash_entries, 1553                                        14, 1554                                        HASH_EARLY, 1555                                        &i_hash_shift, 1556                                        &i_hash_mask, 1557                                        0); 1558 1559        for (loop = 0; loop < (1 << i_hash_shift); loop++) 1560                INIT_HLIST_HEAD(&inode_hashtable[loop]); 1561}

 

两个函数都是调用alloc_large_system_hash函数分别为dentry_hashtable和inode_hashtable分配空间。至于这两个散列的用途，请参考博客“把Linux中的VFS对象串联起来”http://blog.youkuaiyun.com/yunsongice/archive/2010/06/21/5683859.aspx