Fat文件系统查找过程
查找一个文件是通过文件名查找的,对于fat文件系统,可以直接从vfat_lookup函数看
主体流程如下:
vfat_lookup vfat_find fat_search_long while (1) fat_get_entry(inode, &cpos, &bh, &de) fat_name_match(sbi, name, name_len, bufname, len) fat_build_inode inode = new_inode(sb); inode->i_ino = iunique(sb, MSDOS_ROOT_INO); fat_fill_inode(inode, de)
vfat_lookup调了fat_search_long查找文件,其中qname->name就是文件名,比如说我们打开file.txt,就会先查找文件,这里传入的参数qname->name就是文件名,打印出来就是file.txt。
static int vfat_find(struct inode *dir, const struct qstr *qname, struct fat_slot_info *sinfo) { unsignedint len = vfat_striptail_len(qname); if (len == 0) return -ENOENT; return fat_search_long(dir, qname->name, len, sinfo); }
fat_search_long里面有个死循环,里面先调用fat_get_entry(inode, &cpos, &bh, &de)获取目录项,也就是de变量,msdos_dir_entry这个结构体,这个就是fdt表,也是fat文件系统的目录项。
先判断文件名是否等于DELETED_FLAG,就是0xe5,如果是,代码这个是删除文件,直接continue获取下一个目录项继续解析。一般短文件名,通过fat_name_match(sbi, name, name_len, bufname, len)函数,匹配,匹配上的话,说明找到该文件,直接跳转到found,
int fat_search_long(struct inode *inode, const unsigned char *name, int name_len, struct fat_slot_info *sinfo) { struct super_block *sb = inode->i_sb; struct msdos_sb_info *sbi = MSDOS_SB(sb); struct buffer_head *bh = NULL; struct msdos_dir_entry *de; unsignedchar nr_slots; wchar_t *unicode = NULL; unsignedchar bufname[FAT_MAX_SHORT_SIZE]; loff_t cpos = 0; int err, len; err = -ENOENT; while (1) { if (fat_get_entry(inode, &cpos, &bh, &de) == -1) goto end_of_dir; parse_record: nr_slots = 0; if (de->name[0] == DELETED_FLAG) continue; if (de->attr != ATTR_EXT && (de->attr & ATTR_VOLUME)) continue; if (de->attr != ATTR_EXT && IS_FREE(de->name)) continue; if (de->attr == ATTR_EXT) { int status = fat_parse_long(inode, &cpos, &bh, &de, &unicode, &nr_slots); if (status < 0) { err = status; goto end_of_dir; } elseif (status == PARSE_INVALID) continue; elseif (status == PARSE_NOT_LONGNAME) goto parse_record; elseif (status == PARSE_EOF) goto end_of_dir; } /* Never prepend '.' to hidden files here. * That is done only for msdos mounts (and only when * 'dotsOK=yes'); if we are executing here, it is in the * context of a vfat mount. */ len = fat_parse_short(sb, de, bufname, 0); if (len == 0) continue; /* Compare shortname */ if (fat_name_match(sbi, name, name_len, bufname, len)) goto found; if (nr_slots) { void *longname = unicode + FAT_MAX_UNI_CHARS; int size = PATH_MAX - FAT_MAX_UNI_SIZE; /* Compare longname */ len = fat_uni_to_x8(sb, unicode, longname, size); if (fat_name_match(sbi, name, name_len, longname, len)) goto found; } } found: nr_slots++; /* include the de */ sinfo->slot_off = cpos - nr_slots * sizeof(*de); sinfo->nr_slots = nr_slots; sinfo->de = de; sinfo->bh = bh; sinfo->i_pos = fat_make_i_pos(sb, sinfo->bh, sinfo->de); err = 0; end_of_dir: if (unicode) __putname(unicode); return err; }
找到文件后,会调用fat_build_inode建立索引节点信息,因为索引节点不是一直存在的,在我们嵌入式场景中,内存通常比较小,内存回收的时候会释放掉之前建立过的索引节点,所以这里大部分情况下都是走new_inode(sb)分配新的inode,然后调iunique(sb, MSDOS_ROOT_INO)查找一个没使用过的编号填充到inode->i_ino,通过都是从1开始往后分配,inode->i_ino就像是身份证一样的信息,区别每个inode,这个我们调试一些文件系统问题的时候通常会用到它。
struct inode *fat_build_inode(struct super_block *sb, struct msdos_dir_entry *de, loff_t i_pos) { struct inode *inode; int err; fat_lock_build_inode(MSDOS_SB(sb)); inode = fat_iget(sb, i_pos); if (inode) goto out; inode = new_inode(sb); if (!inode) { inode = ERR_PTR(-ENOMEM); goto out; } inode->i_ino = iunique(sb, MSDOS_ROOT_INO); inode->i_version = 1; err = fat_fill_inode(inode, de); if (err) { iput(inode); inode = ERR_PTR(err); goto out; } fat_attach(inode, i_pos); insert_inode_hash(inode); out: fat_unlock_build_inode(MSDOS_SB(sb)); return inode; }
然后调用fat_fill_inode填充inode信息,可以看到第一个判断,如果这个目录项是目录,这里填充的一些信息,如:
MSDOS_I(inode)->i_start = fat_get_start(sbi, de); // 就是获取簇号
MSDOS_I(inode)->i_logstart = MSDOS_I(inode)->i_start;
其实就是簇号,可以参考创建目录时的处理,参考函数fat_alloc_new_dir的实现。
set_nlink的作用是设置i_nlink,它代表的是该目录的子目录数量,fat_subdirs就是计算子目录数量,一个目录至少有2个子目录,当前目录和上级目录。我的代码是4.14版本的,这里还加了个判断fat_validate_dir判断该目录是否为有效目录,据我所知,内核4.4版本是没有该函数判断的,这会导致一些问题不能及时发现。
int fat_fill_inode(struct inode *inode, struct msdos_dir_entry *de) { struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb); int error; MSDOS_I(inode)->i_pos = 0; inode->i_uid = sbi->options.fs_uid; inode->i_gid = sbi->options.fs_gid; inode->i_version++; inode->i_generation = get_seconds(); if ((de->attr & ATTR_DIR) && !IS_FREE(de->name)) { inode->i_generation &= ~1; inode->i_mode = fat_make_mode(sbi, de->attr, S_IRWXUGO); inode->i_op = sbi->dir_ops; inode->i_fop = &fat_dir_operations; MSDOS_I(inode)->i_start = fat_get_start(sbi, de); MSDOS_I(inode)->i_logstart = MSDOS_I(inode)->i_start; error = fat_calc_dir_size(inode); if (error < 0) return error; MSDOS_I(inode)->mmu_private = inode->i_size; set_nlink(inode, fat_subdirs(inode)); error = fat_validate_dir(inode); if (error < 0) return error; } else { /* not a directory */ inode->i_generation |= 1; inode->i_mode = fat_make_mode(sbi, de->attr, ((sbi->options.showexec && !is_exec(de->name + 8)) ? S_IRUGO|S_IWUGO : S_IRWXUGO)); MSDOS_I(inode)->i_start = fat_get_start(sbi, de); MSDOS_I(inode)->i_logstart = MSDOS_I(inode)->i_start; inode->i_size = le32_to_cpu(de->size); inode->i_op = &fat_file_inode_operations; inode->i_fop = &fat_file_operations; inode->i_mapping->a_ops = &fat_aops; MSDOS_I(inode)->mmu_private = inode->i_size; } if (de->attr & ATTR_SYS) { if (sbi->options.sys_immutable) inode->i_flags |= S_IMMUTABLE; } fat_save_attrs(inode, de->attr); inode->i_blocks = ((inode->i_size + (sbi->cluster_size - 1)) & ~((loff_t)sbi->cluster_size - 1)) >> 9; fat_time_fat2unix(sbi, &inode->i_mtime, de->time, de->date, 0); if (sbi->options.isvfat) { fat_time_fat2unix(sbi, &inode->i_ctime, de->ctime, de->cdate, de->ctime_cs); fat_time_fat2unix(sbi, &inode->i_atime, 0, de->adate, 0); } else inode->i_ctime = inode->i_atime = inode->i_mtime; return0; }
关于fat文件系统查找就介绍这几个函数,主要是要理解几个关键的结构体、inode,目录项这几个重要的东西在查找中的作用。
fat文件系统删除
对于文件系统删除,有2个系统调用,入口函数一个是do_unlinkat,一个是do_rmdir,它们大体流程一致,这里主要有2件事,一个是释放目录项,给fdt表文件名首字节设置成0xe5,那么查找的时候先判断de->name[0]如果是DELETED_FLAG就跳过了,不记得可以往上看回查找过程,一个是释放簇(在fat表写为0),数据区不变。
do_unlinkat vfs_unlink dir->i_op->unlink vfat_unlink vfat_find fat_remove_entries while (nr_slots && de >= (struct msdos_dir_entry *)bh->b_data) { de->name[0] = DELETED_FLAG; // 删除标志0xe5 de--; nr_slots--; } dput dentry_kill __dentry_kill iput iput_final evict op->evict_inode fat_evict_inode // 这里进入到fat文件系统层,往下就是释放簇
fat文件系统创建目录
fat_alloc_new_dir就是创建目录的函数,可以看到,创建的时候先在fat表找到一个未使用的簇号,分配出来,然后fat_clus_to_blknr函数通过该簇号计算出扇区编号,然后用sb_getblk对应的扇区读到内存上,紧接着就是填充fdt表,de[0]就是当前目录,de[1]就是上级目录,还有就是填充一些其他信息,如修改时间,簇号等,fat_zeroed_cluster就是把该簇其他扇区数据清0。
memcpy(de[0].name, MSDOS_DOT, MSDOS_NAME); memcpy(de[1].name, MSDOS_DOTDOT, MSDOS_NAME);
int fat_alloc_new_dir(struct inode *dir, struct timespec *ts) { struct super_block *sb = dir->i_sb; struct msdos_sb_info *sbi = MSDOS_SB(sb); struct buffer_head *bhs[MAX_BUF_PER_PAGE]; struct msdos_dir_entry *de; sector_t blknr; __le16 date, time; u8 time_cs; int err, cluster; err = fat_alloc_clusters(dir, &cluster, 1); if (err) goto error; blknr = fat_clus_to_blknr(sbi, cluster); bhs[0] = sb_getblk(sb, blknr); if (!bhs[0]) { err = -ENOMEM; goto error_free; } fat_time_unix2fat(sbi, ts, &time, &date, &time_cs); de = (struct msdos_dir_entry *)bhs[0]->b_data; /* filling the new directory slots ("." and ".." entries) */ memcpy(de[0].name, MSDOS_DOT, MSDOS_NAME); memcpy(de[1].name, MSDOS_DOTDOT, MSDOS_NAME); de->attr = de[1].attr = ATTR_DIR; de[0].lcase = de[1].lcase = 0; de[0].time = de[1].time = time; de[0].date = de[1].date = date; if (sbi->options.isvfat) { /* extra timestamps */ de[0].ctime = de[1].ctime = time; de[0].ctime_cs = de[1].ctime_cs = time_cs; de[0].adate = de[0].cdate = de[1].adate = de[1].cdate = date; } else { de[0].ctime = de[1].ctime = 0; de[0].ctime_cs = de[1].ctime_cs = 0; de[0].adate = de[0].cdate = de[1].adate = de[1].cdate = 0; } fat_set_start(&de[0], cluster); fat_set_start(&de[1], MSDOS_I(dir)->i_logstart); de[0].size = de[1].size = 0; memset(de + 2, 0, sb->s_blocksize - 2 * sizeof(*de)); set_buffer_uptodate(bhs[0]); mark_buffer_dirty_inode(bhs[0], dir); err = fat_zeroed_cluster(dir, blknr, 1, bhs, MAX_BUF_PER_PAGE); if (err) goto error_free; return cluster; error_free: fat_free_clusters(dir, cluster); error: return err; }
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