ext2文件系统源代码之xattr.c

r囧r小猫 2022-07-28 00:09 184阅读 0赞

今天我们来看ext2的扩展属性的主要文件xattr.c，内部有扩展属性的最重要的代码实现，但是文件也真的很长，我们来开始吧。

/* 作者版权信息
     * linux/fs/ext2/xattr.c
     *
     * Copyright (C) 2001-2003 Andreas Gruenbacher <agruen@suse.de>
     * 被Harrison Xing修改过
     * Fix by Harrison Xing <harrison@mountainviewdata.com>.
     * Extended attributes for symlinks and special files added per
     *  suggestion of Luka Renko <luka.renko@hermes.si>.
     * xattr consolidation Copyright (c) 2004 James Morris <jmorris@redhat.com>,
     *  Red Hat Inc.
     *
     */
    
    
    #include <linux/buffer_head.h>
    #include <linux/module.h>
    #include <linux/init.h>
    #include <linux/slab.h>
    #include <linux/mbcache.h>
    #include <linux/quotaops.h>
    #include <linux/rwsem.h>
    #include "ext2.h"
    #include "xattr.h"
    #include "acl.h"
    /*宏定义，参数是buffer_head就诶勾踢指针，得到ext2_xattr_header类型的指针，指向buffer的头部*/
    #define HDR(bh) ((struct ext2_xattr_header *)((bh)->b_data))
    /*将指针转化为ext2_xattr_entry类型的*/
    #define ENTRY(ptr) ((struct ext2_xattr_entry *)(ptr))
    /*获得buffer的第一个项指针*/
    #define FIRST_ENTRY(bh) ENTRY(HDR(bh)+1)
    /*判断当前的项是不是最后一个*/
    #define IS_LAST_ENTRY(entry) (*(__u32 *)(entry) == 0)
    
    
    /*调试信息*/
    #ifdef EXT2_XATTR_DEBUG
    # define ea_idebug(inode, f...) do { \
    		printk(KERN_DEBUG "inode %s:%ld: ", \
    			inode->i_sb->s_id, inode->i_ino); \
    		printk(f); \
    		printk("\n"); \
    	} while (0)
    # define ea_bdebug(bh, f...) do { \
    		char b[BDEVNAME_SIZE]; \
    		printk(KERN_DEBUG "block %s:%lu: ", \
    			bdevname(bh->b_bdev, b), \
    			(unsigned long) bh->b_blocknr); \
    		printk(f); \
    		printk("\n"); \
    	} while (0)
    #else
    # define ea_idebug(f...)
    # define ea_bdebug(f...)
    #endif
    /*一些用到的函数声明*/
    static int ext2_xattr_set2(struct inode *, struct buffer_head *,
    			   struct ext2_xattr_header *);
    
    
    static int ext2_xattr_cache_insert(struct buffer_head *);
    static struct buffer_head *ext2_xattr_cache_find(struct inode *,
    						 struct ext2_xattr_header *);
    static void ext2_xattr_rehash(struct ext2_xattr_header *,
    			      struct ext2_xattr_entry *);
    /*系统存储的属性缓存*/
    static struct mb_cache *ext2_xattr_cache;
    /*属性的名称和处理函数的映射*/
    static struct xattr_handler *ext2_xattr_handler_map[] = {
    	[EXT2_XATTR_INDEX_USER]		     = &ext2_xattr_user_handler,
    #ifdef CONFIG_EXT2_FS_POSIX_ACL
    	[EXT2_XATTR_INDEX_POSIX_ACL_ACCESS]  = &ext2_xattr_acl_access_handler,
    	[EXT2_XATTR_INDEX_POSIX_ACL_DEFAULT] = &ext2_xattr_acl_default_handler,
    #endif
    	[EXT2_XATTR_INDEX_TRUSTED]	     = &ext2_xattr_trusted_handler,
    #ifdef CONFIG_EXT2_FS_SECURITY
    	[EXT2_XATTR_INDEX_SECURITY]	     = &ext2_xattr_security_handler,
    #endif
    };
    /*扩展属性的集合*/
    struct xattr_handler *ext2_xattr_handlers[] = {
    	&ext2_xattr_user_handler,
    	&ext2_xattr_trusted_handler,
    #ifdef CONFIG_EXT2_FS_POSIX_ACL
    	&ext2_xattr_acl_access_handler,
    	&ext2_xattr_acl_default_handler,
    #endif
    #ifdef CONFIG_EXT2_FS_SECURITY
    	&ext2_xattr_security_handler,
    #endif
    	NULL
    };
    /*由扩展属性在数组里的下表，获得对应的处理函数结构体，参数name_index就是下表*/
    static inline struct xattr_handler *
    ext2_xattr_handler(int name_index)
    {
    	struct xattr_handler *handler = NULL;
    	/*如果参数合法，返回对应的结构体*/
    	if (name_index > 0 && name_index < ARRAY_SIZE(ext2_xattr_handler_map))
    		/*上边刚说过的结构体*/
    		handler = ext2_xattr_handler_map[name_index];
    	return handler;
    }
    
    
    /*ext2_xattr_get()函数，复制一个扩展属性结构体到一个给定的buffer里，或者是计算需要的buffer大小，参数buffer如果是NULL的话，就计算需要的buffer大小，当失败的时候返回负的错误编号，成功时候返回消耗的字节数目*/
    int
    ext2_xattr_get(struct inode *inode, int name_index, const char *name,
    	       void *buffer, size_t buffer_size)
    {
    	struct buffer_head *bh = NULL;
    	struct ext2_xattr_entry *entry;
    	size_t name_len, size;
    	char *end;
    	int error;
    	/*调试信息，不管了*/
    	ea_idebug(inode, "name=%d.%s, buffer=%p, buffer_size=%ld",
    		  name_index, name, buffer, (long)buffer_size);
    	/*如果要求的属性名称为NULL，说明传入参数有问题*/
    	if (name == NULL)
    		return -EINVAL;
    	/*读i_file_acl之前必须上锁*/
    	down_read(&EXT2_I(inode)->xattr_sem);
    	error = -ENODATA;
    	/*i_file_acl指向属性的文件块号，如果为空，直接返回*/
    	if (!EXT2_I(inode)->i_file_acl)
    		goto cleanup;
    	ea_idebug(inode, "reading block %d", EXT2_I(inode)->i_file_acl);
    	/*读取这个块进入内存*/
    	bh = sb_bread(inode->i_sb, EXT2_I(inode)->i_file_acl);
    	error = -EIO;
    	if (!bh)
    		goto cleanup;
    	ea_bdebug(bh, "b_count=%d, refcount=%d",
    		atomic_read(&(bh->b_count)), le32_to_cpu(HDR(bh)->h_refcount));
    	/*end指向buffer的末尾*/
    	end = bh->b_data + bh->b_size;
    	/*检查读取的缓冲区，看看这个块是不是坏块*/
    	if (HDR(bh)->h_magic != cpu_to_le32(EXT2_XATTR_MAGIC) ||
    	    HDR(bh)->h_blocks != cpu_to_le32(1)) {
    	    /*如果是坏块，报错并返回IO错误*/
    bad_block:	ext2_error(inode->i_sb, "ext2_xattr_get",
    			"inode %ld: bad block %d", inode->i_ino,
    			EXT2_I(inode)->i_file_acl);
    		error = -EIO;
    		goto cleanup;
    	}
    	/* 根据属性的名字寻找这个属性 */
    	/*先获得属性名字长度*/
    	name_len = strlen(name);
    
    
    	error = -ERANGE;
    	/*最大名字长度是255*/
    	if (name_len > 255)
    		goto cleanup;
    	/*获得属性的第一项*/
    	entry = FIRST_ENTRY(bh);
    	/*遍历属性的每一项*/
    	while (!IS_LAST_ENTRY(entry)) {
    		struct ext2_xattr_entry *next =
    			EXT2_XATTR_NEXT(entry);
    		/*检验当前项是不是合法*/
    		if ((char *)next >= end)
    			goto bad_block;
    		/*匹配，是不是我们想要的*/
    		if (name_index == entry->e_name_index &&
    		    name_len == entry->e_name_len &&
    		    memcmp(name, entry->e_name, name_len) == 0)
    			goto found;
    		entry = next;
    	}
    	/* 检查余下的项，看看有没有坏的 */
    	while (!IS_LAST_ENTRY(entry)) {
    		struct ext2_xattr_entry *next =
    			EXT2_XATTR_NEXT(entry);
    		if ((char *)next >= end)
    			goto bad_block;
    		entry = next;
    	}
    	/*创建一个新的扩展属性项，并且插入它*/
    	if (ext2_xattr_cache_insert(bh))
    		ea_idebug(inode, "cache insert failed");
    	error = -ENODATA;
    	goto cleanup;
    found:
    	/* 检查保存value的块号，否则说明这个块是坏的 */
    	if (entry->e_value_block != 0)
    		goto bad_block;
    	size = le32_to_cpu(entry->e_value_size);
    	/*检查块的大小是否合法*/
    	if (size > inode->i_sb->s_blocksize ||
    	    le16_to_cpu(entry->e_value_offs) + size > inode->i_sb->s_blocksize)
    		goto bad_block;
    	/*创建一个新的扩展属性项，并且插入到缓冲区*/
    	if (ext2_xattr_cache_insert(bh))
    		ea_idebug(inode, "cache insert failed");
    	if (buffer) {
    		error = -ERANGE;
    		if (size > buffer_size)
    			goto cleanup;
    		/* 返回属性 */
    		memcpy(buffer, bh->b_data + le16_to_cpu(entry->e_value_offs),
    			size);
    	}
    	error = size;
    
    
    cleanup:
    	/*释放资源的引用*/
    	brelse(bh);
    	up_read(&EXT2_I(inode)->xattr_sem);
    
    
    	return error;
    }
    
    
    /*ext2_xattr_list()函数，复制一系列的属性到buffer里，当buffer是NULL的时候就只计算需要的字节数，成功返回需要的字节数，失败返回错误码 */
    static int
    ext2_xattr_list(struct inode *inode, char *buffer, size_t buffer_size)
    {
    	struct buffer_head *bh = NULL;
    	struct ext2_xattr_entry *entry;
    	char *end;
    	size_t rest = buffer_size;
    	int error;
    
    
    	ea_idebug(inode, "buffer=%p, buffer_size=%ld",
    		  buffer, (long)buffer_size);
    	/*在读取i_file_acl之前必须上xattr_sem锁*/
    	down_read(&EXT2_I(inode)->xattr_sem);
    	error = 0;
    	/*检查i_file_acl是不是为空*/
    	if (!EXT2_I(inode)->i_file_acl)
    		goto cleanup;
    	ea_idebug(inode, "reading block %d", EXT2_I(inode)->i_file_acl);
    	/*从硬盘上读取这个属性所在的块*/
    	bh = sb_bread(inode->i_sb, EXT2_I(inode)->i_file_acl);
    	error = -EIO;
    	if (!bh)
    		goto cleanup;
    	ea_bdebug(bh, "b_count=%d, refcount=%d",
    		atomic_read(&(bh->b_count)), le32_to_cpu(HDR(bh)->h_refcount));
    	/*end指向缓冲区的末尾*/
    	end = bh->b_data + bh->b_size;
    	/*检验缓冲区是不是合法*/
    	if (HDR(bh)->h_magic != cpu_to_le32(EXT2_XATTR_MAGIC) ||
    	    HDR(bh)->h_blocks != cpu_to_le32(1)) {
    	    /*坏块，打印信息，返回IO错误*/
    bad_block:	ext2_error(inode->i_sb, "ext2_xattr_list",
    			"inode %ld: bad block %d", inode->i_ino,
    			EXT2_I(inode)->i_file_acl);
    		error = -EIO;
    		goto cleanup;
    	}
    
    
    	/* 检查得到的buffer里的数据结构是不是对的 */
    	/*第一个项*/
    	entry = FIRST_ENTRY(bh);
    	/*遍历每一个项*/
    	while (!IS_LAST_ENTRY(entry)) {
    		struct ext2_xattr_entry *next = EXT2_XATTR_NEXT(entry);
    
    
    		if ((char *)next >= end)
    			goto bad_block;
    		entry = next;
    	}
    	/*创建一个新的项并插入*/
    	if (ext2_xattr_cache_insert(bh))
    		ea_idebug(inode, "cache insert failed");
    
    
    	/* 列出所有的属性名称 */
    	for (entry = FIRST_ENTRY(bh); !IS_LAST_ENTRY(entry);
    	     entry = EXT2_XATTR_NEXT(entry)) {
    	     /*得到属性处理结构体*/
    		struct xattr_handler *handler =
    			ext2_xattr_handler(entry->e_name_index);
    
    
    		if (handler) {
    			/*调用这个结构体的函数list来列出所有的属性*/
    			size_t size = handler->list(inode, buffer, rest,
    						    entry->e_name,
    						    entry->e_name_len);
    			/*检查函数是否成功*/
    			if (buffer) {
    				if (size > rest) {
    					error = -ERANGE;
    					goto cleanup;
    				}
    				buffer += size;
    			}
    			rest -= size;
    		}
    	}
    	/*返回所占用的全部空间大小*/
    	error = buffer_size - rest;  /* total size */
    
    
    cleanup:
    	/*释放占用的全部空间*/
    	brelse(bh);
    	up_read(&EXT2_I(inode)->xattr_sem);
    
    
    	return error;
    }
    
    
    /* 这个函数是从inode调用的listxattr()函数 */
    ssize_t
    ext2_listxattr(struct dentry *dentry, char *buffer, size_t size)
    {
    	/*直接调用前边的函数*/
    	return ext2_xattr_list(dentry->d_inode, buffer, size);
    }
    
    
    /* 如果EXT2_FEATURE_COMPAT_EXT_ATTR位没有设置，就设置了 */
    static void ext2_xattr_update_super_block(struct super_block *sb)
    {
    	if (EXT2_HAS_COMPAT_FEATURE(sb, EXT2_FEATURE_COMPAT_EXT_ATTR))
    		return;
    
    
    	EXT2_SET_COMPAT_FEATURE(sb, EXT2_FEATURE_COMPAT_EXT_ATTR);
    	sb->s_dirt = 1;
    	mark_buffer_dirty(EXT2_SB(sb)->s_sbh);
    }
    
    
    /*ext2_xattr_set()函数可以创建，替换，删除一个inode的扩展属性buffer是NULL就删除，不为NUll就是替换或者是创建一个属性，flags参数的值XATTR_REPLACE和XATTR_CREATE标记处扩展属性必须存在和必须不能存在，失败的时候返回负的错误号 */
    int
    ext2_xattr_set(struct inode *inode, int name_index, const char *name,
    	       const void *value, size_t value_len, int flags)
    {
    	struct super_block *sb = inode->i_sb;
    	struct buffer_head *bh = NULL;
    	struct ext2_xattr_header *header = NULL;
    	struct ext2_xattr_entry *here, *last;
    	size_t name_len, free, min_offs = sb->s_blocksize;
    	int not_found = 1, error;
    	char *end;
    	
    	ea_idebug(inode, "name=%d.%s, value=%p, value_len=%ld",
    		  name_index, name, value, (long)value_len);
    	/*参数检查*/
    	if (value == NULL)
    		value_len = 0;
    	if (name == NULL)
    		return -EINVAL;
    	/*名字长度检查*/
    	name_len = strlen(name);
    	if (name_len > 255 || value_len > sb->s_blocksize)
    		return -ERANGE;
    	/*在对文件的i_file_acl字段读写之前，必须上锁*/
    	down_write(&EXT2_I(inode)->xattr_sem);
    	/*如果i_file_acl字段不为0*/
    	if (EXT2_I(inode)->i_file_acl) {
    		/* inode已经有一个扩展属性块了，读取这个块的内容 */
    		bh = sb_bread(sb, EXT2_I(inode)->i_file_acl);
    		error = -EIO;
    		/*检查是否读取出错*/
    		if (!bh)
    			goto cleanup;
    		ea_bdebug(bh, "b_count=%d, refcount=%d",
    			atomic_read(&(bh->b_count)),
    			le32_to_cpu(HDR(bh)->h_refcount));
    		/*header指向头部*/
    		header = HDR(bh);
    		/*end指向尾部*/
    		end = bh->b_data + bh->b_size;
    		/*检查头部的数据，看这个缓冲区是不是对的*/
    		if (header->h_magic != cpu_to_le32(EXT2_XATTR_MAGIC) ||
    		    header->h_blocks != cpu_to_le32(1)) {
    bad_block:		ext2_error(sb, "ext2_xattr_set",
    				"inode %ld: bad block %d", inode->i_ino, 
    				   EXT2_I(inode)->i_file_acl);
    			error = -EIO;
    			goto cleanup;
    		}
    		/* 寻找我们想要的属性 */
    		/* 先把here指向第一个项 */
    		here = FIRST_ENTRY(bh);
    		/*遍历所有的项*/
    		while (!IS_LAST_ENTRY(here)) {
    			/*next指向下一个*/
    			struct ext2_xattr_entry *next = EXT2_XATTR_NEXT(here);
    			/*检查这个块是不是坏块，好的应该是正好不会超出缓冲区的*/
    			if ((char *)next >= end)
    				goto bad_block;
    			/*得到属性值在文件里的偏移量*/
    			if (!here->e_value_block && here->e_value_size) {
    				size_t offs = le16_to_cpu(here->e_value_offs);
    				/*min_offs指向最小的偏移量*/
    				if (offs < min_offs)
    					min_offs = offs;
    			}
    			/*判定有没有找到我们想要的项，首先要name_index一致，另外名称长度和名字也要一致*/
    			not_found = name_index - here->e_name_index;
    			if (!not_found)
    				not_found = name_len - here->e_name_len;
    			if (!not_found)
    				not_found = memcmp(name, here->e_name,name_len);
    			if (not_found <= 0)
    				break;
    			/*指向下一个项*/
    			here = next;
    		}
    		last = here;
    		/* 计算还没有遍历的，是不是有不合法的数据，还要计算min_offs */
    		while (!IS_LAST_ENTRY(last)) {
    			struct ext2_xattr_entry *next = EXT2_XATTR_NEXT(last);
    			if ((char *)next >= end)
    				goto bad_block;
    			if (!last->e_value_block && last->e_value_size) {
    				size_t offs = le16_to_cpu(last->e_value_offs);
    				if (offs < min_offs)
    					min_offs = offs;
    			}
    			last = next;
    		}
    
    
    		/* 看看是不是有多余的空间了. */
    		free = min_offs - ((char*)last - (char*)header) - sizeof(__u32);
    	} else {
    		/* 这个块是坏的，我们需要一个新的块 */
    		free = sb->s_blocksize -
    			sizeof(struct ext2_xattr_header) - sizeof(__u32);
    		here = last = NULL;  
    	}
    
    
    	if (not_found) {
    		/* 请求删除的项没找到，返回错误 */
    		error = -ENODATA;
    		if (flags & XATTR_REPLACE)
    			goto cleanup;
    		error = 0;
    		if (value == NULL)
    			goto cleanup;
    	} else {
    		/* 创建一个已经存在的项吗 */
    		error = -EEXIST;
    		if (flags & XATTR_CREATE)
    			goto cleanup;
    		if (!here->e_value_block && here->e_value_size) {
    			size_t size = le32_to_cpu(here->e_value_size);
    
    
    			if (le16_to_cpu(here->e_value_offs) + size > 
    			    sb->s_blocksize || size > sb->s_blocksize)
    				goto bad_block;
    			free += EXT2_XATTR_SIZE(size);
    		}
    		free += EXT2_XATTR_LEN(name_len);
    	}
    	error = -ENOSPC;
    	/*空间不够创建*/
    	if (free < EXT2_XATTR_LEN(name_len) + EXT2_XATTR_SIZE(value_len))
    		goto cleanup;
    
    
    	/* 设置新属性. */
    
    
    	if (header) {
    		struct mb_cache_entry *ce;
    
    
    		/*从ext2_xattr_cache获得一个缓冲区*/
    		ce = mb_cache_entry_get(ext2_xattr_cache, bh->b_bdev,
    					bh->b_blocknr);
    		/*访问前上锁*/
    		lock_buffer(bh);
    		if (header->h_refcount == cpu_to_le32(1)) {
    			ea_bdebug(bh, "modifying in-place");
    			if (ce)
    				mb_cache_entry_free(ce);
    		} else {
    			int offset;
    
    
    			if (ce)
    				mb_cache_entry_release(ce);
    			unlock_buffer(bh);
    			ea_bdebug(bh, "cloning");
    			header = kmalloc(bh->b_size, GFP_KERNEL);
    			error = -ENOMEM;
    			if (header == NULL)
    				goto cleanup;
    			/*把原来缓冲区的内容放到新分配的header里*/
    			memcpy(header, HDR(bh), bh->b_size);
    			header->h_refcount = cpu_to_le32(1);
    			/*offset是偏移，here指向项，last指向最后一个entry*/
    			offset = (char *)here - bh->b_data;
    			here = ENTRY((char *)header + offset);
    			offset = (char *)last - bh->b_data;
    			last = ENTRY((char *)header + offset);
    		}
    	} else {
    		/* 创建缓冲区，建立新的block结构体 */
    		header = kzalloc(sb->s_blocksize, GFP_KERNEL);
    		error = -ENOMEM;
    		if (header == NULL)
    			goto cleanup;
    		end = (char *)header + sb->s_blocksize;
    		header->h_magic = cpu_to_le32(EXT2_XATTR_MAGIC);
    		header->h_blocks = header->h_refcount = cpu_to_le32(1);
    		last = here = ENTRY(header+1);
    	}
    
    
    	/* 修改属性. */
    	if (not_found) {
    		/* 插入新的属性名称 */
    		/* 得到对应名称的项的字节大小size，rest是余下的大小 */
    		size_t size = EXT2_XATTR_LEN(name_len);
    		size_t rest = (char *)last - (char *)here;
    		/*为新的属性腾出位置*/
    		memmove((char *)here + size, here, rest);
    		/*新的地方先初始化为0*/
    		memset(here, 0, size);
    		/*赋值*/
    		here->e_name_index = name_index;
    		here->e_name_len = name_len;
    		memcpy(here->e_name, name, name_len);
    	} else {
    		/* 如果属性的值就存在本块内 */
    		if (!here->e_value_block && here->e_value_size) {
    			/*指向第一个值*/
    			char *first_val = (char *)header + min_offs;
    			size_t offs = le16_to_cpu(here->e_value_offs);
    			char *val = (char *)header + offs;
    			size_t size = EXT2_XATTR_SIZE(
    				le32_to_cpu(here->e_value_size));
    			/*如果新旧属性值长度一样*/
    			if (size == EXT2_XATTR_SIZE(value_len)) {
    				/* 直接替换*/
    				here->e_value_size = cpu_to_le32(value_len);
    				memset(val + size - EXT2_XATTR_PAD, 0,
    				       EXT2_XATTR_PAD); /* Clear pad bytes. */
    				memcpy(val, value, value_len);
    				goto skip_replace;
    			}
    
    
    			/* 新旧属性长度不一样，移除旧的 */
    			memmove(first_val + size, first_val, val - first_val);
    			memset(first_val, 0, size);
    			here->e_value_offs = 0;
    			min_offs += size;
    
    
    			/* 还要调整所有的偏移 */
    			last = ENTRY(header+1);
    			while (!IS_LAST_ENTRY(last)) {
    				size_t o = le16_to_cpu(last->e_value_offs);
    				if (!last->e_value_block && o < offs)
    					last->e_value_offs =
    						cpu_to_le16(o + size);
    				last = EXT2_XATTR_NEXT(last);
    			}
    		}
    		if (value == NULL) {
    			/* 移除原有的属性名 */
    			size_t size = EXT2_XATTR_LEN(name_len);
    			last = ENTRY((char *)last - size);
    			memmove(here, (char*)here + size,
    				(char*)last - (char*)here);
    			memset(last, 0, size);
    		}
    	}
    
    
    	if (value != NULL) {
    		/* 插入新的值 */
    		here->e_value_size = cpu_to_le32(value_len);
    		/*值不是空值的话*/
    		if (value_len) {
    			/*修改*/
    			size_t size = EXT2_XATTR_SIZE(value_len);
    			char *val = (char *)header + min_offs - size;
    			here->e_value_offs =
    				cpu_to_le16((char *)val - (char *)header);
    			memset(val + size - EXT2_XATTR_PAD, 0,
    			       EXT2_XATTR_PAD); /* Clear the pad bytes. */
    			memcpy(val, value, value_len);
    		}
    	}
    
    
    skip_replace:
    	if (IS_LAST_ENTRY(ENTRY(header+1))) {
    		/* 这个块不是空的. */
    		if (bh && header == HDR(bh))
    			unlock_buffer(bh);  /* we were modifying in-place. */
    		error = ext2_xattr_set2(inode, bh, NULL);
    	} else {
    		ext2_xattr_rehash(header, here);
    		if (bh && header == HDR(bh))
    			unlock_buffer(bh);  /* we were modifying in-place. */
    		error = ext2_xattr_set2(inode, bh, header);
    	}
    
    
    cleanup:
    	/*释放对buffer_head的引用*/
    	brelse(bh);
    	if (!(bh && header == HDR(bh)))
    		kfree(header);
    	/*释放锁*/
    	up_write(&EXT2_I(inode)->xattr_sem);
    
    
    	return error;
    }
    
    
    /*ext2_xattr_set()函数的下一半，更新文件系统*/
    static int
    ext2_xattr_set2(struct inode *inode, struct buffer_head *old_bh,
    		struct ext2_xattr_header *header)
    {
    	struct super_block *sb = inode->i_sb;
    	struct buffer_head *new_bh = NULL;
    	int error;
    
    
    	if (header) {
    		/*在缓存里寻找header的项*/
    		new_bh = ext2_xattr_cache_find(inode, header);
    		/*如果找到了*/
    		if (new_bh) {
    			/*同一个*/
    			if (new_bh == old_bh) {
    				ea_bdebug(new_bh, "keeping this block");
    			} else {
    				/* The old block is released after updating
    				   the inode.  */
    				ea_bdebug(new_bh, "reusing block");
    
    
    				error = -EDQUOT;
    				if (DQUOT_ALLOC_BLOCK(inode, 1)) {
    					unlock_buffer(new_bh);
    					goto cleanup;
    				}
    				HDR(new_bh)->h_refcount = cpu_to_le32(1 +
    					le32_to_cpu(HDR(new_bh)->h_refcount));
    				ea_bdebug(new_bh, "refcount now=%d",
    					le32_to_cpu(HDR(new_bh)->h_refcount));
    			}
    			unlock_buffer(new_bh);
    		} else if (old_bh && header == HDR(old_bh)) {
    			/* Keep this block. No need to lock the block as we
    			   don't need to change the reference count. */
    			new_bh = old_bh;
    			get_bh(new_bh);
    			ext2_xattr_cache_insert(new_bh);
    		} else {
    			/* We need to allocate a new block */
    			int goal = le32_to_cpu(EXT2_SB(sb)->s_es->
    						           s_first_data_block) +
    				   EXT2_I(inode)->i_block_group *
    				   EXT2_BLOCKS_PER_GROUP(sb);
    			int block = ext2_new_block(inode, goal,
    						   NULL, NULL, &error);
    			if (error)
    				goto cleanup;
    			ea_idebug(inode, "creating block %d", block);
    
    
    			new_bh = sb_getblk(sb, block);
    			if (!new_bh) {
    				ext2_free_blocks(inode, block, 1);
    				error = -EIO;
    				goto cleanup;
    			}
    			lock_buffer(new_bh);
    			memcpy(new_bh->b_data, header, new_bh->b_size);
    			set_buffer_uptodate(new_bh);
    			unlock_buffer(new_bh);
    			ext2_xattr_cache_insert(new_bh);
    			
    			ext2_xattr_update_super_block(sb);
    		}
    		/*new_bh脏了*/
    		mark_buffer_dirty(new_bh);
    		if (IS_SYNC(inode)) {
    			sync_dirty_buffer(new_bh);
    			error = -EIO;
    			if (buffer_req(new_bh) && !buffer_uptodate(new_bh))
    				goto cleanup;
    		}
    	}
    
    
    	/*更新inode的i_file_acl字段，修改时间等*/
    	EXT2_I(inode)->i_file_acl = new_bh ? new_bh->b_blocknr : 0;
    	inode->i_ctime = CURRENT_TIME_SEC;
    	/*是否需要同步*/
    	if (IS_SYNC(inode)) {
    		error = ext2_sync_inode (inode);
    		if (error && error != -ENOSPC) {
    			if (new_bh && new_bh != old_bh)
    				DQUOT_FREE_BLOCK(inode, 1);
    			goto cleanup;
    		}
    	} else
    		mark_inode_dirty(inode);
    
    
    	error = 0;
    	/*如果存在原有的块，并且我们已经不用了就释放*/
    	if (old_bh && old_bh != new_bh) {
    		struct mb_cache_entry *ce;
    
    
    		/*找到他在缓存里的位置*/
    		ce = mb_cache_entry_get(ext2_xattr_cache, old_bh->b_bdev,
    					old_bh->b_blocknr);
    		lock_buffer(old_bh);
    		if (HDR(old_bh)->h_refcount == cpu_to_le32(1)) {
    			/* 如果引用仅仅有一个，释放 */
    			if (ce)
    				mb_cache_entry_free(ce);
    			ea_bdebug(old_bh, "freeing");
    			ext2_free_blocks(inode, old_bh->b_blocknr, 1);
    			/* We let our caller release old_bh, so we
    			 * need to duplicate the buffer before. */
    			get_bh(old_bh);
    			bforget(old_bh);
    		} else {
    			/* 减少引用计数 */
    			HDR(old_bh)->h_refcount = cpu_to_le32(
    				le32_to_cpu(HDR(old_bh)->h_refcount) - 1);
    			if (ce)
    				mb_cache_entry_release(ce);
    			DQUOT_FREE_BLOCK(inode, 1);
    			mark_buffer_dirty(old_bh);
    			ea_bdebug(old_bh, "refcount now=%d",
    				le32_to_cpu(HDR(old_bh)->h_refcount));
    		}
    		unlock_buffer(old_bh);
    	}
    
    
    cleanup:
    	brelse(new_bh);
    
    
    	return error;
    }
    
    
    /* ext2_xattr_delete_inode()函数释放与inode相关的属性资源 */
    void
    ext2_xattr_delete_inode(struct inode *inode)
    {
    	struct buffer_head *bh = NULL;
    	struct mb_cache_entry *ce;
    	/*读i_file_acl之前都要上这个锁*/
    	down_write(&EXT2_I(inode)->xattr_sem);
    	if (!EXT2_I(inode)->i_file_acl)
    		goto cleanup;
    	/*读这个属性所在的块*/
    	bh = sb_bread(inode->i_sb, EXT2_I(inode)->i_file_acl);
    	/*如果读取出现错误*/
    	if (!bh) {
    		ext2_error(inode->i_sb, "ext2_xattr_delete_inode",
    			"inode %ld: block %d read error", inode->i_ino,
    			EXT2_I(inode)->i_file_acl);
    		goto cleanup;
    	}
    	ea_bdebug(bh, "b_count=%d", atomic_read(&(bh->b_count)));
    	/*检验读取出来的buffer_head是不是有问题，是不是ext2文件系统的属性*/
    	if (HDR(bh)->h_magic != cpu_to_le32(EXT2_XATTR_MAGIC) ||
    	    HDR(bh)->h_blocks != cpu_to_le32(1)) {
    		ext2_error(inode->i_sb, "ext2_xattr_delete_inode",
    			"inode %ld: bad block %d", inode->i_ino,
    			EXT2_I(inode)->i_file_acl);
    		goto cleanup;
    	}
    	/*在ext2_xattr_cache缓存里寻找并删除*/
    	ce = mb_cache_entry_get(ext2_xattr_cache, bh->b_bdev, bh->b_blocknr);
    	lock_buffer(bh);
    	/*引用计数为1就删除*/
    	if (HDR(bh)->h_refcount == cpu_to_le32(1)) {
    		if (ce)
    			mb_cache_entry_free(ce);
    		ext2_free_blocks(inode, EXT2_I(inode)->i_file_acl, 1);
    		get_bh(bh);
    		bforget(bh);
    		unlock_buffer(bh);
    	} else {
    		/*递减引用计数*/
    		HDR(bh)->h_refcount = cpu_to_le32(
    			le32_to_cpu(HDR(bh)->h_refcount) - 1);
    		if (ce)
    			mb_cache_entry_release(ce);
    		ea_bdebug(bh, "refcount now=%d",
    			le32_to_cpu(HDR(bh)->h_refcount));
    		unlock_buffer(bh);
    		mark_buffer_dirty(bh);
    		if (IS_SYNC(inode))
    			sync_dirty_buffer(bh);
    		DQUOT_FREE_BLOCK(inode, 1);
    	}
    	EXT2_I(inode)->i_file_acl = 0;
    
    
    cleanup:
    	brelse(bh);
    	up_write(&EXT2_I(inode)->xattr_sem);
    }
    
    
    /* ext2_xattr_put_super()当文件系统被卸载的时候调用 */
    void
    ext2_xattr_put_super(struct super_block *sb)
    {
    	mb_cache_shrink(sb->s_bdev);
    }
    
    
    
    
    /* ext2_xattr_cache_insert()函数在属性缓存里创建一个新的扩展属性项，不管它是不是已经在ext2_xattr_cache缓存里，成返回0 */
    static int
    ext2_xattr_cache_insert(struct buffer_head *bh)
    {
    	/*插入缓存是需要hash来便于查找*/
    	__u32 hash = le32_to_cpu(HDR(bh)->h_hash);
    	struct mb_cache_entry *ce;
    	int error;
    	/*在ext2_xattr_cache分配缓存*/
    	ce = mb_cache_entry_alloc(ext2_xattr_cache);
    	if (!ce)
    		return -ENOMEM;
    	/*把这个ce插入*/
    	error = mb_cache_entry_insert(ce, bh->b_bdev, bh->b_blocknr, &hash);
    	/*插入失败，释放缓存*/
    	if (error) {
    		mb_cache_entry_free(ce);
    		if (error == -EBUSY) {
    			ea_bdebug(bh, "already in cache (%d cache entries)",
    				atomic_read(&ext2_xattr_cache->c_entry_count));
    			error = 0;
    		}
    	} else {
    		ea_bdebug(bh, "inserting [%x] (%d cache entries)", (int)hash,
    			  atomic_read(&ext2_xattr_cache->c_entry_count));
    		mb_cache_entry_release(ce);
    	}
    	return error;
    }
    
    
    /* ext2_xattr_cmp()函数比较两个扩展属性块，当这两个一样时返回0，不一样返回1，有错误返回负数 */
    static int
    ext2_xattr_cmp(struct ext2_xattr_header *header1,
    	       struct ext2_xattr_header *header2)
    {
    	struct ext2_xattr_entry *entry1, *entry2;
    	/*得到这两个缓冲区的head*/
    	entry1 = ENTRY(header1+1);
    	entry2 = ENTRY(header2+1);
    	/*遍历开始*/
    	while (!IS_LAST_ENTRY(entry1)) {
    		/*第一个没有到结尾第二个到结尾了，说明不一样*/
    		if (IS_LAST_ENTRY(entry2))
    			return 1;
    		/*比较*/
    		if (entry1->e_hash != entry2->e_hash ||
    		    entry1->e_name_index != entry2->e_name_index ||
    		    entry1->e_name_len != entry2->e_name_len ||
    		    entry1->e_value_size != entry2->e_value_size ||
    		    memcmp(entry1->e_name, entry2->e_name, entry1->e_name_len))
    			return 1;
    		if (entry1->e_value_block != 0 || entry2->e_value_block != 0)
    			return -EIO;
    		/*比较value*/
    		if (memcmp((char *)header1 + le16_to_cpu(entry1->e_value_offs),
    			   (char *)header2 + le16_to_cpu(entry2->e_value_offs),
    			   le32_to_cpu(entry1->e_value_size)))
    			return 1;
    		/*指向下一个*/
    		entry1 = EXT2_XATTR_NEXT(entry1);
    		entry2 = EXT2_XATTR_NEXT(entry2);
    	}
    	/*不一样*/
    	if (!IS_LAST_ENTRY(entry2))
    		return 1;
    	return 0;
    }
    
    
    /*ext2_xattr_cache_find()函数，寻找一个标记的扩展属性块。成功的话返回找到的块的buffer_head，失败返回NULL*/
    static struct buffer_head *
    ext2_xattr_cache_find(struct inode *inode, struct ext2_xattr_header *header)
    {
    	__u32 hash = le32_to_cpu(header->h_hash);
    	struct mb_cache_entry *ce;
    	/*没有共享在hash里*/
    	if (!header->h_hash)
    		return NULL;  
    	ea_idebug(inode, "looking for cached blocks [%x]", (int)hash);
    again:
    	/*在ext2_xattr_cache缓存里一个一个的找*/
    	ce = mb_cache_entry_find_first(ext2_xattr_cache, 0,
    				       inode->i_sb->s_bdev, hash);
    	/*寻找的循环*/
    	while (ce) {
    		struct buffer_head *bh;
    		/*检验得到的是不是合法的*/
    		if (IS_ERR(ce)) {
    			if (PTR_ERR(ce) == -EAGAIN)
    				goto again;
    			break;
    		}
    		/*读取这一块进入内存*/
    		bh = sb_bread(inode->i_sb, ce->e_block);
    		/*如果IO读取出错*/
    		if (!bh) {
    			ext2_error(inode->i_sb, "ext2_xattr_cache_find",
    				"inode %ld: block %ld read error",
    				inode->i_ino, (unsigned long) ce->e_block);
    		} else {
    			/*开始读取buffer_head*/
    			lock_buffer(bh);
    			/*一个缓冲区被引用太多次*/
    			if (le32_to_cpu(HDR(bh)->h_refcount) >
    				   EXT2_XATTR_REFCOUNT_MAX) {
    				ea_idebug(inode, "block %ld refcount %d>%d",
    					  (unsigned long) ce->e_block,
    					  le32_to_cpu(HDR(bh)->h_refcount),
    					  EXT2_XATTR_REFCOUNT_MAX);
    				/*匹配*/
    			} else if (!ext2_xattr_cmp(header, HDR(bh))) {
    				ea_bdebug(bh, "b_count=%d",
    					  atomic_read(&(bh->b_count)));
    				mb_cache_entry_release(ce);
    				return bh;
    			}
    			unlock_buffer(bh);
    			brelse(bh);
    		}
    		/*继续找*/
    		ce = mb_cache_entry_find_next(ce, 0, inode->i_sb->s_bdev, hash);
    	}
    	return NULL;
    }
    
    
    #define NAME_HASH_SHIFT 5
    #define VALUE_HASH_SHIFT 16
    
    
    /*ext2_xattr_hash_entry()计算这个扩展属性的hash值 */
    static inline void ext2_xattr_hash_entry(struct ext2_xattr_header *header,
    					 struct ext2_xattr_entry *entry)
    {
    	__u32 hash = 0;
    	char *name = entry->e_name;
    	int n;
    
    
    	for (n=0; n < entry->e_name_len; n++) {
    		hash = (hash << NAME_HASH_SHIFT) ^
    		       (hash >> (8*sizeof(hash) - NAME_HASH_SHIFT)) ^
    		       *name++;
    	}
    
    
    	if (entry->e_value_block == 0 && entry->e_value_size != 0) {
    		__le32 *value = (__le32 *)((char *)header +
    			le16_to_cpu(entry->e_value_offs));
    		for (n = (le32_to_cpu(entry->e_value_size) +
    		     EXT2_XATTR_ROUND) >> EXT2_XATTR_PAD_BITS; n; n--) {
    			hash = (hash << VALUE_HASH_SHIFT) ^
    			       (hash >> (8*sizeof(hash) - VALUE_HASH_SHIFT)) ^
    			       le32_to_cpu(*value++);
    		}
    	}
    	entry->e_hash = cpu_to_le32(hash);
    }
    
    
    #undef NAME_HASH_SHIFT
    #undef VALUE_HASH_SHIFT
    
    
    #define BLOCK_HASH_SHIFT 16
    
    
    /*
     * ext2_xattr_rehash()重新计算hash值
     */
    static void ext2_xattr_rehash(struct ext2_xattr_header *header,
    			      struct ext2_xattr_entry *entry)
    {
    	struct ext2_xattr_entry *here;
    	__u32 hash = 0;
    	
    	ext2_xattr_hash_entry(header, entry);
    	here = ENTRY(header+1);
    	while (!IS_LAST_ENTRY(here)) {
    		if (!here->e_hash) {
    			/* Block is not shared if an entry's hash value == 0 */
    			hash = 0;
    			break;
    		}
    		hash = (hash << BLOCK_HASH_SHIFT) ^
    		       (hash >> (8*sizeof(hash) - BLOCK_HASH_SHIFT)) ^
    		       le32_to_cpu(here->e_hash);
    		here = EXT2_XATTR_NEXT(here);
    	}
    	header->h_hash = cpu_to_le32(hash);
    }
    
    
    #undef BLOCK_HASH_SHIFT
    /*ext2属性初始化*/
    int __init
    init_ext2_xattr(void)
    {
    	/*创建缓冲区*/
    	ext2_xattr_cache = mb_cache_create("ext2_xattr", NULL,
    		sizeof(struct mb_cache_entry) +
    		sizeof(((struct mb_cache_entry *) 0)->e_indexes[0]), 1, 6);
    	if (!ext2_xattr_cache)
    		return -ENOMEM;
    	return 0;
    }
    /*ext2属性退出销毁*/
    void
    exit_ext2_xattr(void)
    {
    	mb_cache_destroy(ext2_xattr_cache);
    }