Linux文件系统(六)---三大缓冲区之 目录缓冲区dcache

在文件系统中，有三大缓冲为了提升效率：inode缓冲区、dentry缓冲区、块缓冲。

 (内核：2.4.37)

为什么这个缓冲区会存在，不好意思，我说了废话，当然和前面一样的，为了提升效率，例如我们写一个.c的helloworld文件，简单的过程是编辑，编译，执行。。。那么这个过程都是需要找到所在的文件位置的，如果每次都从根开始找并且还有构造相应的目录项对象，是很费时的，所以将目录项一般也都是缓存起来的~~~

Ps：dentry结构

 67 struct dentry {
 68         atomic_t d_count;
 69         unsigned int d_flags;
 70         struct inode  * d_inode;        /* Where the name belongs to - NULL is negative */
 71         struct dentry * d_parent;       /* parent directory */
 72         struct list_head d_hash;        /* lookup hash list */
 73         struct list_head d_lru;         /* d_count = 0 LRU list */
 74         struct list_head d_child;       /* child of parent list */
 75         struct list_head d_subdirs;     /* our children */
 76         struct list_head d_alias;       /* inode alias list */
 77         int d_mounted;
 78         struct qstr d_name;
 79         unsigned long d_time;           /* used by d_revalidate */
 80         struct dentry_operations  *d_op;
 81         struct super_block * d_sb;      /* The root of the dentry tree */
 82         unsigned long d_vfs_flags;
 83         void * d_fsdata;                /* fs-specific data */
 84         unsigned char d_iname[DNAME_INLINE_LEN]; /* small names */
 85 };

和前面的一样，这个也涉及到几个相应的链表来管理，那么看看/fs/dcache.c中哪些链表被定义了。

 52 static struct list_head *dentry_hashtable;
 53 static LIST_HEAD(dentry_unused);

哈希链表：从中能够快速获取与给定的文件名和目录名对应的目录项对象。

“未使用”链表：所有未使用 目录项对象都存放在一个LRU的双向链表。LRU链表的首元素和尾元素的地址存放在变量dentry_unused中的next 域和prev域中。目录项对象的d_lru域包含的指针指向该链表中相邻目录的对象。

简单的看一下dcache初始化过程：

1181 static void __init dcache_init(unsigned long mempages)
1182 {
1183         struct list_head *d;
1184         unsigned long order;
1185         unsigned int nr_hash;
1186         int i;
1187 
1188         /* 
1189          * A constructor could be added for stable state like the lists,
1190          * but it is probably not worth it because of the cache nature
1191          * of the dcache. 
1192          * If fragmentation is too bad then the SLAB_HWCACHE_ALIGN
1193          * flag could be removed here, to hint to the allocator that
1194          * it should not try to get multiple page regions.  
1195          */
1196         dentry_cache = kmem_cache_create("dentry_cache",
1197                                          sizeof(struct dentry),
1198                                          0,
1199                                          SLAB_HWCACHE_ALIGN,
1200                                          NULL, NULL);
1201         if (!dentry_cache)
1202                 panic("Cannot create dentry cache");
1203 
1204 #if PAGE_SHIFT < 13
1205         mempages >>= (13 - PAGE_SHIFT);
1206 #endif
1207         mempages *= sizeof(struct list_head);
1208         for (order = 0; ((1UL << order) << PAGE_SHIFT) < mempages; order++)
1209                 ;
1210 
1211         do {
1212                 unsigned long tmp;
1213 
1214                 nr_hash = (1UL << order) * PAGE_SIZE /
1215                         sizeof(struct list_head);
1216                 d_hash_mask = (nr_hash - 1);
1217 
1218                 tmp = nr_hash;
1219                 d_hash_shift = 0;
1220                 while ((tmp >>= 1UL) != 0UL)
1221                         d_hash_shift++;
1222 
1223                 dentry_hashtable = (struct list_head *)
1224                         __get_free_pages(GFP_ATOMIC, order);
1225         } while (dentry_hashtable == NULL && --order >= 0);
1226 
1227         printk(KERN_INFO "Dentry cache hash table entries: %d (order: %ld, %ld bytes)\n",
1228                         nr_hash, order, (PAGE_SIZE << order));
1229 
1230         if (!dentry_hashtable)
1231                 panic("Failed to allocate dcache hash table\n");
1232 
1233         d = dentry_hashtable;
1234         i = nr_hash;
1235         do {
1236                 INIT_LIST_HEAD(d);
1237                 d++;
1238                 i--;
1239         } while (i);
1240 }

上面代码就是相当于分配cache空间，并将hash表什么的都初始化了~~~

下面看一下怎么分配一个目录项对象，涉及函数d_alloc：

580 /**
581  * d_alloc      -       allocate a dcache entry
582  * @parent: parent of entry to allocate
583  * @name: qstr of the name
584  *
585  * Allocates a dentry. It returns %NULL if there is insufficient memory
586  * available. On a success the dentry is returned. The name passed in is
587  * copied and the copy passed in may be reused after this call.
588  */
589  
590 struct dentry * d_alloc(struct dentry * parent, const struct qstr *name)
591 {
592         char * str;
593         struct dentry *dentry;
594 
595         dentry = kmem_cache_alloc(dentry_cache, GFP_KERNEL);   /* 分配一个dentry空间 */
596         if (!dentry)
597                 return NULL;
598 
599         if (name->len > DNAME_INLINE_LEN-1) {
600                 str = kmalloc(NAME_ALLOC_LEN(name->len), GFP_KERNEL);
601                 if (!str) {
602                         kmem_cache_free(dentry_cache, dentry); 
603                         return NULL;
604                 }
605         } else
606                 str = dentry->d_iname; 
607         /* 复制name */
608         memcpy(str, name->name, name->len);
609         str[name->len] = 0;
610         /* 下面根据dentr的字段进行赋值，具体的字段意义见：http://blog.csdn.net/shanshanpt/article/details/38943731 */
611         atomic_set(&dentry->d_count, 1);
612         dentry->d_vfs_flags = 0;
613         dentry->d_flags = 0;
614         dentry->d_inode = NULL;
615         dentry->d_parent = NULL;
616         dentry->d_sb = NULL;
617         dentry->d_name.name = str;
618         dentry->d_name.len = name->len;
619         dentry->d_name.hash = name->hash;
620         dentry->d_op = NULL;
621         dentry->d_fsdata = NULL;
622         dentry->d_mounted = 0;
623         INIT_LIST_HEAD(&dentry->d_hash);
624         INIT_LIST_HEAD(&dentry->d_lru);
625         INIT_LIST_HEAD(&dentry->d_subdirs);
626         INIT_LIST_HEAD(&dentry->d_alias);
627         if (parent) {
628                 dentry->d_parent = dget(parent);
629                 dentry->d_sb = parent->d_sb;
630         } else
631                 INIT_LIST_HEAD(&dentry->d_child);
632 
633         spin_lock(&dcache_lock);
634         if (parent)
635                 list_add(&dentry->d_child, &parent->d_subdirs);
636         dentry_stat.nr_dentry++;
637         spin_unlock(&dcache_lock);
638 
639         return dentry;
640 }
641 

下面看看怎么去寻找一个目录，涉及函数d_lookup：

698 /**
699  * d_lookup - search for a dentry
700  * @parent: parent dentry
701  * @name: qstr of name we wish to find
702  *
703  * Searches the children of the parent dentry for the name in question. If
704  * the dentry is found its reference count is incremented and the dentry
705  * is returned. The caller must use d_put to free the entry when it has
706  * finished using it. %NULL is returned on failure.
707  */
708  
709 struct dentry * d_lookup(struct dentry * parent, struct qstr * name)
710 {
711         unsigned int len = name->len;
712         unsigned int hash = name->hash;
713         const unsigned char *str = name->name;
714         struct list_head *head = d_hash(parent,hash); /* 通过hash值计算得到目录项缓冲区位置的head */
715         struct list_head *tmp;
716 
717         spin_lock(&dcache_lock);
718         tmp = head->next;
719         for (;;) {   /* 下面循环找到对应的dentry */
720                 struct dentry * dentry = list_entry(tmp, struct dentry, d_hash);
721                 if (tmp == head)
722                         break;
723                 tmp = tmp->next;
724                 if (dentry->d_name.hash != hash)
725                         continue;
726                 if (dentry->d_parent != parent)
727                         continue;
728                 if (parent->d_op && parent->d_op->d_compare) {
729                         if (parent->d_op->d_compare(parent, &dentry->d_name, name))
730                                 continue;
731                 } else {
732                         if (dentry->d_name.len != len)
733                                 continue;
734                         if (memcmp(dentry->d_name.name, str, len))
735                                 continue;
736                 }
737                 __dget_locked(dentry);
738                 dentry->d_vfs_flags |= DCACHE_REFERENCED;   /* 找到，那么添加引用就OK */
739                 spin_unlock(&dcache_lock);
740                 return dentry;     /* 返回找到的dentry。里面有我们需要的信息例如inode */
741         }
742         spin_unlock(&dcache_lock);
743         return NULL;
744 }

其他的代码暂时就不看了，以后总结。。。