ext2文件系统源代码之balloc.c文件解析
2016-03-14 22:00
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前面把ext2的acl.c和acl.h文件分析过了,今天来看一个比较重要的文件,就是balloc.c,这个文件是用来做块的分配方面工作的,在文件系统是属于较低层的,可能有点困难,我努力讲解的通俗一点,大家有什么问题欢迎提问啦。
/* 作者方面的信息 * linux/fs/ext2/balloc.c * * Copyright (C) 1992, 1993, 1994, 1995 * Remy Card (card@masi.ibp.fr) * Laboratoire MASI - Institut Blaise Pascal * Universite Pierre et Marie Curie (Paris VI) * * Enhanced block allocation by Stephen Tweedie (sct@redhat.com), 1993 * Big-endian to little-endian byte-swapping/bitmaps by * David S. Miller (davem@caip.rutgers.edu), 1995 */ #include "ext2.h" #include <linux/quotaops.h> #include <linux/sched.h> #include <linux/buffer_head.h> #include <linux/capability.h> /*balloc.c 包含块的分配和销毁方面的程序*/ /*在ext2文件系统,块的管理是通过位图来实现的,一个文件系统包含若干个块组,每一个块组有一个针对数据块的位图,还有一个针对inode的位图,文件系统的组描述符在超级块的后边,每一个组描述符都有空闲块的数目记录,组描述符都会在挂载文件系统的时候读到内存里。(ext2_fill_super) */ /*很简单的宏,判断b是不是在一个以first为头指针,长度为len的内存里,相信有点c语言功底的都能看懂*/ #define in_range(b, first, len) ((b) >= (first) && (b) <= (first) + (len) - 1) /*获得组描述符,block_group代表是第几个组,bh参数如果不为空,就把buffer_head形式的组描述符放在bh里*/ struct ext2_group_desc * ext2_get_group_desc(struct super_block * sb, unsigned int block_group, struct buffer_head ** bh) { unsigned long group_desc; unsigned long offset; struct ext2_group_desc * desc; /*从super_block里获得ext2_sb_info结构体,super_block是vfs便于管理设置的统一的超级块结构体,ext2_sb_info是ext2文件系统的放在内存里的超级块结构体,super_block的s_fs_info字段就是ext2_sb_info结构体,转化还是挺方便的*/ struct ext2_sb_info *sbi = EXT2_SB(sb); /*如果参数大于组的数目,说明参数有问题,报错,返回NULL*/ if (block_group >= sbi->s_groups_count) { ext2_error (sb, "ext2_get_group_desc", "block_group >= groups_count - " "block_group = %d, groups_count = %lu", block_group, sbi->s_groups_count); return NULL; } /*EXT2_DESC_PER_BLOCK_BITS宏返回块拥有组描述符的数目转换成二进制位的位数,右移这些位就等于是除以一个块拥有组描述符的数目。我猜我这么说肯定大家不懂,还是举例子吧。比如ext2的ext2_group_desc是32字节,按照ext2的一个块有1K大小来算,一个块就有32个组描述符,所以就是5位,右移五位,除以32,块组描述符是聚集在一起的,有几个块是专门房块组描述符的,在超级块里有记录,我们知道了是第几个块组,但是接下来要知道是在块组描述符群组里的第几个块,比如我们要去第45个块组,45/32=1,所以就得到了是在第二个块里,偏移就是45%32=13,也可以与位,45&(32-1)=13*/ group_desc = block_group >> EXT2_DESC_PER_BLOCK_BITS(sb); /*这里的offset就是在块内的第几个描述符*/ offset = block_group & (EXT2_DESC_PER_BLOCK(sb) - 1); /*sbi->s_group_desc就存放着块组描述符好几个块哦,如果为空,就说明ext2出问题了,并且很严重,报错*/ if (!sbi->s_group_desc[group_desc]) { ext2_error (sb, "ext2_get_group_desc", "Group descriptor not loaded - " "block_group = %d, group_desc = %lu, desc = %lu", block_group, group_desc, offset); return NULL; } /*描述符指针指向对应的buffer_head->b_data就是组描述符所在组的第一个组描述符*/ desc = (struct ext2_group_desc *) sbi->s_group_desc[group_desc]->b_data; /*如果参数bh不为空,就赋值组描述符的buffer_head给bh*/ if (bh) *bh = sbi->s_group_desc[group_desc]; /*返回想要的组描述符*/ return desc + offset; } /*阅读给定的块组的数据块位图,如果成功,返回位图的buffer_head,失败返回NULL*/ static struct buffer_head * read_block_bitmap(struct super_block *sb, unsigned int block_group) { struct ext2_group_desc * desc; struct buffer_head * bh = NULL; /*上边的函数,刚讲过哦,根据block_group得到块组描述符*/ desc = ext2_get_group_desc (sb, block_group, NULL); /*失败的话,返回NULL*/ if (!desc) goto error_out; /*调用底层驱动函数,读取数据块位图,desc->bg_block_bitmap代表着数据块位图的块号码,返回数据块位图的buffer_head格式,sb_bread是块设备驱动和文件系统的连接函数,很重要,以后我会好好研究下它*/ bh = sb_bread(sb, le32_to_cpu(desc->bg_block_bitmap)); /*没读出来,就报错*/ if (!bh) ext2_error (sb, "read_block_bitmap", "Cannot read block bitmap - " "block_group = %d, block_bitmap = %u", block_group, le32_to_cpu(desc->bg_block_bitmap)); error_out: return bh; } /*把空闲的一部分放到保留块里边,保留块是为了防止ext2出现问题,预先保留一部分以备不测,一般是5%左右,count参数就是需要放到保留块里的块数目,需要知道,空闲块的数目也包括保留块的数目,保留块肯定也是空闲块,空闲块不一定是保留块 * Set sb->s_dirt here because the superblock was "logically" altered. We * need to recalculate its free blocks count and flush it out. */ static int reserve_blocks(struct super_block *sb, int count) { struct ext2_sb_info *sbi = EXT2_SB(sb); struct ext2_super_block *es = sbi->s_es; unsigned free_blocks; unsigned root_blocks; /*percpu_counter_read_positive函数是为了防止多处理器并发导致的读取失败,从ext2_sb_info->s_freeblocks_counter得到的是当前ext2的空闲的数据块数目*/ free_blocks = percpu_counter_read_positive(&sbi->s_freeblocks_counter); /*从ext2_super_block得到保留的数据块数目*/ root_blocks = le32_to_cpu(es->s_r_blocks_count); /*如果count超出了空闲块数目,就把count变成现有的空闲块数目*/ if (free_blocks < count) count = free_blocks; /*如果空闲块小于count+保留块,没有权限*/ if (free_blocks < root_blocks + count && !capable(CAP_SYS_RESOURCE) && sbi->s_resuid != current->fsuid && (sbi->s_resgid == 0 || !in_group_p (sbi->s_resgid))) { /*如果还有不是保留块的空闲块,就有多少转化多少,否则就不转化了*/ if (free_blocks > root_blocks) count = free_blocks - root_blocks; else return 0; } percpu_counter_mod(&sbi->s_freeblocks_counter, -count); /*标记超级块已经脏了*/ sb->s_dirt = 1; /*返回转换的块数目*/ return count; } /*释放count数目的块,把这些块加入到空闲块里边*/ static void release_blocks(struct super_block *sb, int count) { if (count) { /*空闲块的数目记录在ext2_sb_info里边*/ struct ext2_sb_info *sbi = EXT2_SB(sb); /*其实就是sbi->s_freeblocks_counter+=count,这个函数是为了防止多处理器并发出现问题*/ percpu_counter_mod(&sbi->s_freeblocks_counter, count); /*和硬盘上不一致了,需要设置脏*/ sb->s_dirt = 1; } } /*以上的都是在文件系统层次进行保留块数目的更改,接下来的函数就是在文件系统的块组的层次进行保留块数目的更改,这个函数就是把组内的count个空闲块保留起来,在组描述符*/ static int group_reserve_blocks(struct ext2_sb_info *sbi, int group_no, struct ext2_group_desc *desc, struct buffer_head *bh, int count) { unsigned free_blocks; /*如果这个块组已经没有空闲块了*/ if (!desc->bg_free_blocks_count) return 0; /*防止并发操作的自旋锁,访问摸一个组的时候必须是互斥的*/ spin_lock(sb_bgl_lock(sbi, group_no)); /*ext2_group_desc里的是小端字节序,转化成内存计算的字节序,得到组内空闲块的数目*/ free_blocks = le16_to_cpu(desc->bg_free_blocks_count); /*如果空闲块的数目小于要求保留的,就设置保留的数目位空闲块的数目,就是把所有的空闲块都保留起来*/ if (free_blocks < count) count = free_blocks; /*修改空闲块数目,把count个块保留起来,由于组描述符没有保留块数目字段,所以直接把空闲块数目减去count*/ desc->bg_free_blocks_count = cpu_to_le16(free_blocks - count); spin_unlock(sb_bgl_lock(sbi, group_no)); /*这个bh已经脏了*/ mark_buffer_dirty(bh); return count; } /*组内释放一些块,并把这些块加入到组内的空闲块内,基本上就是只修改组描述符*/ static void group_release_blocks(struct super_block *sb, int group_no, struct ext2_group_desc *desc, struct buffer_head *bh, int count) { if (count) { /*通过super_block得到内存内统计ext2文件系统的ext2_sb_info结构体*/ struct ext2_sb_info *sbi = EXT2_SB(sb); unsigned free_blocks; /*确保对于某一个组的访问是互斥的*/ spin_lock(sb_bgl_lock(sbi, group_no)); /*这个组的空闲块的数目*/ free_blocks = le16_to_cpu(desc->bg_free_blocks_count); /*组空闲块数目加上参数count*/ desc->bg_free_blocks_count = cpu_to_le16(free_blocks + count); spin_unlock(sb_bgl_lock(sbi, group_no)); /*标记超级块为脏*/ sb->s_dirt = 1; /*由于统计空闲块的组描述符是在buffer_head上存放的,所以这个buffer_head也要标记为脏*/ mark_buffer_dirty(bh); } } /* 把从block号块开始的count个块释放掉,并且修改quota字段和i_blocks字段 */ void ext2_free_blocks (struct inode * inode, unsigned long block, unsigned long count) { struct buffer_head *bitmap_bh = NULL; struct buffer_head * bh2; unsigned long block_group; unsigned long bit; unsigned long i; unsigned long overflow; /*从inode的i_sb字段得到vfs层对于超级块的结构体super_block*/ struct super_block * sb = inode->i_sb; /*内存对于ext2的统计信息的结构体*/ struct ext2_sb_info * sbi = EXT2_SB(sb); struct ext2_group_desc * desc; /*再从ext2_sb_info得到ext2真正的超级块结构体ext2_super_block*/ struct ext2_super_block * es = sbi->s_es; unsigned freed = 0, group_freed; /*es->s_first_data_block代表第一个数据块在的块号,如果block小于它,说明传入的参数错了*/ if (block < le32_to_cpu(es->s_first_data_block) || /*如果block + count < block说明超出了unsigned long的最大值类型*/ block + count < block || /*如果block + count大于数据块的数目*/ block + count > le32_to_cpu(es->s_blocks_count)) { /*报严重错误,并返回*/ ext2_error (sb, "ext2_free_blocks", "Freeing blocks not in datazone - " "block = %lu, count = %lu", block, count); goto error_return; } ext2_debug ("freeing block(s) %lu-%lu\n", block, block + count - 1); do_more: overflow = 0; /*传来的block减去es->s_first_data_block就得到了要删除的第一个块是文件系统的第几个块,因为ext2是小端字节序,所以要转成内存计算的字节序,然后再除以每一个组有的数据块的数目,就得到了要删除的第一个块在的块组号,EXT2_BLOCKS_PER_GROUP(sb)就是从super_block获得每一个块组拥有的块的数目*/ block_group = (block - le32_to_cpu(es->s_first_data_block)) / EXT2_BLOCKS_PER_GROUP(sb); /*和上边唯一的区别就是把/变成了%,得到的就是要删除的第一个块在组内的偏移,即数这个组内的第几个块*/ bit = (block - le32_to_cpu(es->s_first_data_block)) % EXT2_BLOCKS_PER_GROUP(sb); /*检验我们要删除的块是不是超过了组的边界限*/ if (bit + count > EXT2_BLOCKS_PER_GROUP(sb)) { /*overflow得到超出这个组的块的数目*/ overflow = bit + count - EXT2_BLOCKS_PER_GROUP(sb); /*然后count减去overflow,所以bit+count就是都在这个组内的了*/ count -= overflow; } /*因为bitmap_bh是NULL,所以这里不用管这个释放函数*/ brelse(bitmap_bh); /*前边讲解过这个函数,就是得到组的块位图的buffer_head*/ bitmap_bh = read_block_bitmap(sb, block_group); if (!bitmap_bh) goto error_return; /*得到组描述符,前边讲过*/ desc = ext2_get_group_desc (sb, block_group, &bh2); if (!desc) goto error_return; /*如果组描述符的数据块位图和inode位图也在删除的范围之内*/ if (in_range (le32_to_cpu(desc->bg_block_bitmap), block, count) || in_range (le32_to_cpu(desc->bg_inode_bitmap), block, count) || /*如果inode表区也在要删除的范围之内*/ in_range (block, le32_to_cpu(desc->bg_inode_table), sbi->s_itb_per_group) || in_range (block + count - 1, le32_to_cpu(desc->bg_inode_table), sbi->s_itb_per_group)) /*删除的数据有关键性数据,不可以删除,就报错*/ ext2_error (sb, "ext2_free_blocks", "Freeing blocks in system zones - " "Block = %lu, count = %lu", block, count); /*先把要删除的块多对应的位图置位为0*/ for (i = 0, group_freed = 0; i < count; i++) { /*从数据位图上吧对应的数据块的位置为0,如果已经是0,说明出错啦*/ if (!ext2_clear_bit_atomic(sb_bgl_lock(sbi, block_group), bit + i, bitmap_bh->b_data)) { ext2_error(sb, __FUNCTION__, "bit already cleared for block %lu", block + i); } else { /*group_freed标示组的空闲块数目,每删除一个快,空闲块就多一个*/ group_freed++; } } /*位图的buffer_head已经和硬盘上的不一致了,置为脏*/ mark_buffer_dirty(bitmap_bh); /*如果超级块的标记位上标记写过立即同步,就同步buffer*/ if (sb->s_flags & MS_SYNCHRONOUS) sync_dirty_buffer(bitmap_bh); /*上边刚讲过,这个函数就是把释放过的块数目在块组描述符上边记录*/ group_release_blocks(sb, block_group, desc, bh2, group_freed); /*freed是统计一共释放的块数目,group_freed是这个组释放的数目*/ freed += group_freed; /*如果还有没删除完的,就是超出这个块组的,跳转到do_more继续*/ if (overflow) { /*代表着开始块的block+=count*/ block += count; count = overflow; goto do_more; } error_return: /*减少位图缓冲区的引用计数*/ brelse(bitmap_bh); /*上边刚说过这个函数,在超级块上增加freed个空闲块*/ release_blocks(sb, freed); DQUOT_FREE_BLOCK(inode, freed); } /*在组内找到一个空闲块,并获取它,goal参数就是想得到的块在组内的偏移,map是块位图,size是这个组的大小*/ static int grab_block(spinlock_t *lock, char *map, unsigned size, int goal) { int k; char *p, *r; /*首先测试位图的第goal位,如果是0的话,就说明得到了这个块,跳转*/ if (!ext2_test_bit(goal, map)) goto got_it; repeat: /*如果goal参数不为0,说明有设置希望得到的块在组内的偏移*/ if (goal) { /*goal块已经被占有了,所以我们只可以去向前或者是向后寻找空闲的块了,但是不可以超出块组的边界,前后各32个,加起来就是64个块*/ k = (goal + 63) & ~63; /*在map位图上边从goal开始寻找在k个的范围内另一个为0的位*/ goal = ext2_find_next_zero_bit(map, k, goal); if (goal < k) goto got_it; } /*既然往前没有找到,就试图分配连续的8个块*/ p = map + (goal >> 3); /*在p指针开始的地方寻找0*/ r = memscan(p, 0, (size - goal + 7) >> 3); /*左移三位就是第几个位,原先是代表第几个字节*/ k = (r - map) << 3; if (k < size) { /* 成功找到空闲的字节,就是8个空闲块*/ for (goal = k; goal && !ext2_test_bit (goal - 1, map); goal--) ; goto got_it; } /*没有找到8个连续的空闲0位,就扫描整个位图,只寻找一个为0的位*/ k = ext2_find_next_zero_bit ((u32 *)map, size, goal); /*如果找到了,就返回为0的那个位的位数*/ if (k < size) { goal = k; goto got_it; } /*失败返回-1*/ return -1; got_it: /*如果goal位是0,就得到goal块位图,返回,如果是1的话,就继续寻找*/ if (ext2_set_bit_atomic(lock, goal, (void *) map)) goto repeat; return goal; } /*很重要的函数,使用一个goal来帮助分配块,如果goal块是空闲块,或者是在goal的前后32个块之内有空闲块,那个快就会被分配,否则的话就会向前寻找空闲的块,开始的时候都会寻找空闲的字节,一次性分配8个块,如果这失败了的话,就直接寻找为0的位,不去寻找连续的8个空闲块,就像上边那个函数的实现一样 */ int ext2_new_block(struct inode *inode, unsigned long goal, u32 *prealloc_count, u32 *prealloc_block, int *err) { /*数据块位图的缓冲区*/ struct buffer_head *bitmap_bh = NULL; /*块组缓冲区*/ struct buffer_head *gdp_bh; /* bh2 */ struct ext2_group_desc *desc; int group_no; /* i */ int ret_block; /* j */ int group_idx; /* k */ int target_block; /* tmp */ int block = 0; /*几乎任何函数都有这三个结构体。。。*/ struct super_block *sb = inode->i_sb; struct ext2_sb_info *sbi = EXT2_SB(sb); struct ext2_super_block *es = sbi->s_es; unsigned group_size = EXT2_BLOCKS_PER_GROUP(sb); /*预分配的块数目*/ unsigned prealloc_goal = es->s_prealloc_blocks; unsigned group_alloc = 0, es_alloc, dq_alloc; int nr_scanned_groups; /*在ext2文件系统里,有一个叫做预分配制度的,就是之前分配多一点,可以减少磁盘读写,先预分配块减一,如果预分配块没了,就先设置prealloc_goal为7,就是再重新分配8个块的意思*/ if (!prealloc_goal--) prealloc_goal = EXT2_DEFAULT_PREALLOC_BLOCKS - 1; if (!prealloc_count || *prealloc_count) prealloc_goal = 0; /*配额检查,如果超出了,就返回错误*/ if (DQUOT_ALLOC_BLOCK(inode, 1)) { *err = -EDQUOT; goto out; } /*如果用户预分配块已经超出了用户配额就--,直到不超为止*/ while (prealloc_goal && DQUOT_PREALLOC_BLOCK(inode, prealloc_goal)) prealloc_goal--; /*dq_alloc存放要分配的块数*/ dq_alloc = prealloc_goal + 1; /*reserve_blocks我们之前讲过,获取一定的保留块,如果返回0说明没有空间了给保留块了,预分配就不行了,因为预分配的块数就是以保留块的形式保存的*/ es_alloc = reserve_blocks(sb, dq_alloc); if (!es_alloc) { *err = -ENOSPC; goto out_dquot; } ext2_debug ("goal=%lu.\n", goal); /*参数检查,如果要分配的目标块小于第一个数据块或是大于数据块的总数,就把目标块设置为第一个数据块号*/ if (goal < le32_to_cpu(es->s_first_data_block) || goal >= le32_to_cpu(es->s_blocks_count)) goal = le32_to_cpu(es->s_first_data_block); /*这个和之前的一样,相信大家都能看懂了,group_no就是要分配的块在的组号*/ group_no = (goal - le32_to_cpu(es->s_first_data_block)) / group_size; /*之前讲过的函数,获得组描述符*/ desc = ext2_get_group_desc (sb, group_no, &gdp_bh); if (!desc) { /*io出错了*/ goto io_error; } /*在这个组内预分配块,group_alloc得到的就是可以预分配的块的数目*/ group_alloc = group_reserve_blocks(sbi, group_no, desc, gdp_bh, es_alloc); /*如果有空间预分配*/ if (group_alloc) { /*老算法了,减去第一个开始的数据块取余组内的数据块数目,就得到在组内的偏移*/ ret_block = ((goal - le32_to_cpu(es->s_first_data_block)) % group_size); /*先释放buffer_head,然后获得组内的数据位图,这个函数之前讲过的,此处就不再赘述了*/ brelse(bitmap_bh); bitmap_bh = read_block_bitmap(sb, group_no); if (!bitmap_bh) goto io_error; ext2_debug("goal is at %d:%d.\n", group_no, ret_block); /*通过grab_block函数在组内寻找一个块,grab_block函数刚讲过,就在上边,不懂的可以倒回去再看看哦*/ ret_block = grab_block(sb_bgl_lock(sbi, group_no), bitmap_bh->b_data, group_size, ret_block); /*如果分配ok了,跳转*/ if (ret_block >= 0) goto got_block; /*如果没找到,就在组内释放刚刚申请的group_alloc个块,并把赋值为0*/ group_release_blocks(sb, group_no, desc, gdp_bh, group_alloc); group_alloc = 0; } ext2_debug ("Bit not found in block group %d.\n", group_no); /*运行到这里,说明这个组内没有预分配的空间或者是分配失败了,就只能去其他的组看一看了,nr_scanned_groups先设置为0*/ nr_scanned_groups = 0; retry: /*遍历文件系统的组,从group_no开始,尽量使得分配的空间在一起或者是挨得近一点*/ for (group_idx = 0; !group_alloc && group_idx < sbi->s_groups_count; group_idx++) { /*之前的看过了,下一个*/ group_no++; /*如果到了最后一组,就从第一组再开始*/ if (group_no >= sbi->s_groups_count) group_no = 0; /*获得这一组的组描述符*/ desc = ext2_get_group_desc(sb, group_no, &gdp_bh); if (!desc) goto io_error; /*在这一组内预分配,group_alloc得到分配到的块数目*/ group_alloc = group_reserve_blocks(sbi, group_no, desc, gdp_bh, es_alloc); } /*如果group_alloc是0,说明遍历整个文件系统都没找到,直接返回吧*/ if (!group_alloc) { *err = -ENOSPC; goto out_release; } /*释放之前的缓冲区,读取当前找到预分配空间的组的数据块位图*/ brelse(bitmap_bh); bitmap_bh = read_block_bitmap(sb, group_no); if (!bitmap_bh) goto io_error; /*grab_block函数上边讲过,从这个组的块位图上寻找可以分配的块,由于不是之前的块了,所以goal就是0*/ ret_block = grab_block(sb_bgl_lock(sbi, group_no), bitmap_bh->b_data, group_size, 0); if (ret_block < 0) { /*获取失败,在此,说明这一个空闲块被占用了,我们可以无限等待 */ nr_scanned_groups++; /*如果已经把整个文件系统的组遍历了两遍了还没分配好,就直接返回错误,说明是出现问题了*/ if (nr_scanned_groups > 2 * sbi->s_groups_count) { ext2_error(sb, "ext2_new_block", "corrupted free blocks counters"); goto io_error; } /*说明这个块被占用了,我们继续等待,释放刚刚想分配的块,赋值group_alloc为0*/ group_release_blocks(sb, group_no, desc, gdp_bh, group_alloc); group_alloc = 0; goto retry; } got_block: /*运行到此,说明终于分配到块了*/ ext2_debug("using block group %d(%d)\n", group_no, desc->bg_free_blocks_count); /*获得分配到的块在文件系统范围内的块号*/ target_block = ret_block + group_no * group_size + le32_to_cpu(es->s_first_data_block); /*检验target_block是不是合法*/ if (target_block == le32_to_cpu(desc->bg_block_bitmap) || target_block == le32_to_cpu(desc->bg_inode_bitmap) || in_range(target_block, le32_to_cpu(desc->bg_inode_table), sbi->s_itb_per_group)) ext2_error (sb, "ext2_new_block", "Allocating block in system zone - " "block = %u", target_block); if (target_block >= le32_to_cpu(es->s_blocks_count)) { ext2_error (sb, "ext2_new_block", "block(%d) >= blocks count(%d) - " "block_group = %d, es == %p ", ret_block, le32_to_cpu(es->s_blocks_count), group_no, es); goto io_error; } block = target_block; /* OK, we _had_ allocated something */ ext2_debug("found bit %d\n", ret_block); /*分配ok,这些变量值要修改下*/ dq_alloc--; es_alloc--; group_alloc--; /*设置位图,先上锁,防止其他进程访问*/ write_lock(&EXT2_I(inode)->i_meta_lock); /*如果之前这个文件的预分配块为0,并且已经分配到块了*/ if (group_alloc && !*prealloc_count) { unsigned n; /*循环把预分配的位图位设置为1*/ for (n = 0; n < group_alloc && ++ret_block < group_size; n++) { if (ext2_set_bit_atomic(sb_bgl_lock(sbi, group_no), ret_block, (void*) bitmap_bh->b_data)) break; } /*预分配数目和开始块赋值*/ *prealloc_block = block + 1; *prealloc_count = n; /*这些值修改*/ es_alloc -= n; dq_alloc -= n; group_alloc -= n; } write_unlock(&EXT2_I(inode)->i_meta_lock); /*这个数据块标记为脏*/ mark_buffer_dirty(bitmap_bh); /*MS_SYNCHRONOUS标志代表着一有修改立即写入磁盘*/ if (sb->s_flags & MS_SYNCHRONOUS) sync_dirty_buffer(bitmap_bh); ext2_debug ("allocating block %d. ", block); *err = 0; out_release: /*没用上的块就放回去*/ group_release_blocks(sb, group_no, desc, gdp_bh, group_alloc); release_blocks(sb, es_alloc); out_dquot: /*配额修改*/ DQUOT_FREE_BLOCK(inode, dq_alloc); out: /*释放位图缓冲区*/ brelse(bitmap_bh); return block; io_error: *err = -EIO; goto out_release; } #ifdef EXT2FS_DEBUG /*如果配置了EXT2FS_DEBUG宏才会有这个ext2_count_free函数*/ static int nibblemap[] = {4, 3, 3, 2, 3, 2, 2, 1, 3, 2, 2, 1, 2, 1, 1, 0}; /* 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15*/ /*ext2文件系统计算空闲块的数目,map是数据块位图,numchars是块的大小,按字节来算*/ unsigned long ext2_count_free (struct buffer_head * map, unsigned int numchars) { unsigned int i; unsigned long sum = 0; /*如果数据块位图为空,直接返回*/ if (!map) return (0); /*遍历每一个字节,这里巧妙利用了一个数组来计算字节内0的数目,避免了一连串的if判断*/ for (i = 0; i < numchars; i++) sum += nibblemap[map->b_data[i] & 0xf] + nibblemap[(map->b_data[i] >> 4) & 0xf]; return (sum); } #endif /* EXT2FS_DEBUG */ /*ext2文件系统计算空闲块的数目*/ unsigned long ext2_count_free_blocks (struct super_block * sb) { struct ext2_group_desc * desc; unsigned long desc_count = 0; int i; /*如果定义了EXT2FS_DEBUG宏,就采用下边的方式*/ #ifdef EXT2FS_DEBUG unsigned long bitmap_count, x; struct ext2_super_block *es; es = EXT2_SB(sb)->s_es; desc_count = 0; bitmap_count = 0; desc = NULL; /*遍历所有的组*/ for (i = 0; i < EXT2_SB(sb)->s_groups_count; i++) { /*获得当前组的组描述符*/ struct buffer_head *bitmap_bh; desc = ext2_get_group_desc (sb, i, NULL); if (!desc) continue; /*获得统计的组的空闲块数目*/ desc_count += le16_to_cpu(desc->bg_free_blocks_count); /*获得数据块的位图*/ bitmap_bh = read_block_bitmap(sb, i); if (!bitmap_bh) continue; /*上边的函数,真正意义上遍历位图获得空闲的位图*/ x = ext2_count_free(bitmap_bh, sb->s_blocksize); printk ("group %d: stored = %d, counted = %lu\n", i, le16_to_cpu(desc->bg_free_blocks_count), x); bitmap_count += x; brelse(bitmap_bh); } /*打印获得的信息,理论上这两种方式获得的应该是一样的,但是凡是有意外,可能运行的时候出现了一点问题*/ printk("ext2_count_free_blocks: stored = %lu, computed = %lu, %lu\n", (long)le32_to_cpu(es->s_free_blocks_count), desc_count, bitmap_count); return bitmap_count; #else /*没定义这个宏,就采用遍历每一个组描述符,来统计*/ /*遍历每一个组*/ for (i = 0; i < EXT2_SB(sb)->s_groups_count; i++) { /*先获得组的组描述符*/ desc = ext2_get_group_desc (sb, i, NULL); if (!desc) continue; /*从组描述符里获得组的空闲块数目,并加入统计变量*/ desc_count += le16_to_cpu(desc->bg_free_blocks_count); } return desc_count; #endif } /*检验一个块是不是正在使用中,参数block是块号码,map是数据块位图*/ static inline int block_in_use(unsigned long block, struct super_block *sb, unsigned char *map) { /*(block - le32_to_cpu(EXT2_SB(sb)->s_es->s_first_data_block)) % EXT2_BLOCKS_PER_GROUP(sb)得到的是这个块在组内的偏移,然后得到这个位的值*/ return ext2_test_bit ((block - le32_to_cpu(EXT2_SB(sb)->s_es->s_first_data_block)) % EXT2_BLOCKS_PER_GROUP(sb), map); } /*根据代码的意思来看,就是检验a是不是b的幂*/ static inline int test_root(int a, int b) { int num = b; while (a > num) num *= b; return num == a; } /*这个函数是用来查找组号是3或者5或者7的幂的组号,在ext2文件系统里边357的幂的组号会放超级块,这个函数是用来被下面的函数调用的*/ static int ext2_group_sparse(int group) { if (group <= 1) return 1; return (test_root(group, 3) || test_root(group, 5) || test_root(group, 7)); } /**判断这个组的被超级块使用的块的数目 */ int ext2_bg_has_super(struct super_block *sb, int group) { /*EXT2_HAS_RO_COMPAT_FEATURE宏是判断ext2_super_block结构体的s_feature_ro_compat字段的第一位是不是1,然后再判断组的号码*/ if (EXT2_HAS_RO_COMPAT_FEATURE(sb,EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER)&& !ext2_group_sparse(group)) return 0; return 1; } /**得出这个组被组描述符占用的块的数目 * * Return the number of blocks used by the group descriptor table * (primary or backup) in this group. In the future there may be a * different number of descriptor blocks in each group. */ unsigned long ext2_bg_num_gdb(struct super_block *sb, int group) { /*如果EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER位没有置位并且组号不是357幂,就说明没有*/ if (EXT2_HAS_RO_COMPAT_FEATURE(sb,EXT2_FEATURE_RO_COMPAT_SPARSE_SUPER)&& !ext2_group_sparse(group)) return 0; /*从超级块返回组描述符的块数目*/ return EXT2_SB(sb)->s_gdb_count; }
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