ext2文件系统源代码之xattr.c

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);
}