如何删除 ext2 文件系统中的文件
How removing a file in ext2 file system work
我正在学习EXT2文件系统。我对删除文件对 EXT2 的工作方式感到困惑。我的理解是，删除后，它实际上并没有删除索引节点，而是将一些元数据标记为未使用。我的问题是，它在删除时修改了哪些元数据，文件系统如何知道文件已被删除？谢谢。
在Linux中，这是围绕fs/ext2/inode.c文件的ext2_delete_inode函数实现的：
http://lxr.free-electrons.com/source/fs/ext2/inode.c?v=2.6.32#L59
56 /*
57 * Called at the last iput() if i_nlink is zero.
58 */
59 void ext2_delete_inode (struct inode * inode)
60 {
61 truncate_inode_pages(&inode->i_data, 0);
..
65 EXT2_I(inode)->i_dtime = get_seconds();
66 mark_inode_dirty(inode);
67 ext2_write_inode(inode, inode_needs_sync(inode));
68
69 inode->i_size = 0;
70 if (inode->i_blocks)
71 ext2_truncate (inode);
72 ext2_free_inode (inode);
73
74 return;
..
77 }
因此，它从 truncate_inode_pages, sets dtime (deletion time) and marks inode as dirty - I_DIRTY which is combination of (I_DIRTY_SYNC | I_DIRTY_DATASYNC | I_DIRTY_PAGES) 中的页面缓存中删除页面：
1601 * I_DIRTY_SYNC Inode is dirty, but doesn't have to be written on
1602 * fdatasync(). i_atime is the usual cause.
1603 * I_DIRTY_DATASYNC Data-related inode changes pending. We keep track of
1604 * these changes separately from I_DIRTY_SYNC so that we
1605 * don't have to write inode on fdatasync() when only
1606 * mtime has changed in it.
1607 * I_DIRTY_PAGES Inode has dirty pages. Inode itself may be clean.
然后写入修改后的 inode，将 大小更改为零 ，截断所有从 inode 链接到 ext2_truncate() 的块（实际将数据块标记为空闲是在那里完成的): http://lxr.free-electrons.com/source/fs/ext2/inode.c?v=2.6.32#L1025
1025 void ext2_truncate(struct inode *inode)
1026 {
..
1059 n = ext2_block_to_path(inode, iblock, offsets, NULL);
99 /* ext2_block_to_path - parse the block number into array of offsets
105 * To store the locations of file's data ext2 uses a data structure common
106 * for UNIX filesystems - tree of pointers anchored in the inode, with
107 * data blocks at leaves and indirect blocks in intermediate nodes.
108 * This function translates the block number into path in that tree -
109 * return value is the path length and @offsets[n] is the offset of
110 * pointer to (n+1)th node in the nth one. If @block is out of range
111 * (negative or too large) warning is printed and zero returned. */
1069 if (n == 1) {
1070 ext2_free_data(inode, i_data+offsets[0],
1071 i_data + EXT2_NDIR_BLOCKS);
1072 goto do_indirects;
1073 }
..
1082 ext2_free_branches(inode, &nr, &nr+1, (chain+n-1) - partial);
..
1084 /* Clear the ends of indirect blocks on the shared branch */
1085 while (partial > chain) {
1086 ext2_free_branches(inode,
1087 partial->p + 1,
1088 (__le32*)partial->bh->b_data+addr_per_block,
1089 (chain+n-1) - partial);
..
1094 do_indirects:
1095 /* Kill the remaining (whole) subtrees */
1096 switch (offsets[0]) {
1097 default:
1098 nr = i_data[EXT2_IND_BLOCK];
1099 if (nr) {
1100 i_data[EXT2_IND_BLOCK] = 0;
1101 mark_inode_dirty(inode);
1102 ext2_free_branches(inode, &nr, &nr+1, 1);
1103 }
1104 case EXT2_IND_BLOCK:
1105 nr = i_data[EXT2_DIND_BLOCK];
1106 if (nr) {
1107 i_data[EXT2_DIND_BLOCK] = 0;
1108 mark_inode_dirty(inode);
1109 ext2_free_branches(inode, &nr, &nr+1, 2);
1110 }
1111 case EXT2_DIND_BLOCK:
1112 nr = i_data[EXT2_TIND_BLOCK];
1113 if (nr) {
1114 i_data[EXT2_TIND_BLOCK] = 0;
1115 mark_inode_dirty(inode);
1116 ext2_free_branches(inode, &nr, &nr+1, 3);
1117 }
1118 case EXT2_TIND_BLOCK:
1119 ;
1120 }
（为什么EXT2_TIND_BLOCK没有被清除？）
然后我们可以释放内核内存中的inode结构。
how does the file system know that the file is deleted?
检查在 ext2_iget 函数中：http://lxr.free-electrons.com/source/fs/ext2/inode.c?v=2.6.32#L1251
1251 /* We now have enough fields to check if the inode was active or not.
1252 * This is needed because nfsd might try to access dead inodes
1253 * the test is that same one that e2fsck uses
1254 * NeilBrown 1999oct15
1255 */
1256 if (inode->i_nlink == 0 && (inode->i_mode == 0 || ei->i_dtime)) {
1257 /* this inode is deleted */
1258 brelse (bh);
1259 ret = -ESTALE;
1260 goto bad_inode;
1261 }
所以，删除的inode是没有传入链接的inode（它在任何目录中都没有提到i_nlink）并且具有零模式或非零删除时间。
我正在学习EXT2文件系统。我对删除文件对 EXT2 的工作方式感到困惑。我的理解是，删除后，它实际上并没有删除索引节点，而是将一些元数据标记为未使用。我的问题是，它在删除时修改了哪些元数据，文件系统如何知道文件已被删除？谢谢。
在Linux中，这是围绕fs/ext2/inode.c文件的ext2_delete_inode函数实现的：
http://lxr.free-electrons.com/source/fs/ext2/inode.c?v=2.6.32#L59
56 /*
57 * Called at the last iput() if i_nlink is zero.
58 */
59 void ext2_delete_inode (struct inode * inode)
60 {
61 truncate_inode_pages(&inode->i_data, 0);
..
65 EXT2_I(inode)->i_dtime = get_seconds();
66 mark_inode_dirty(inode);
67 ext2_write_inode(inode, inode_needs_sync(inode));
68
69 inode->i_size = 0;
70 if (inode->i_blocks)
71 ext2_truncate (inode);
72 ext2_free_inode (inode);
73
74 return;
..
77 }
因此，它从 truncate_inode_pages, sets dtime (deletion time) and marks inode as dirty - I_DIRTY which is combination of (I_DIRTY_SYNC | I_DIRTY_DATASYNC | I_DIRTY_PAGES) 中的页面缓存中删除页面：
1601 * I_DIRTY_SYNC Inode is dirty, but doesn't have to be written on
1602 * fdatasync(). i_atime is the usual cause.
1603 * I_DIRTY_DATASYNC Data-related inode changes pending. We keep track of
1604 * these changes separately from I_DIRTY_SYNC so that we
1605 * don't have to write inode on fdatasync() when only
1606 * mtime has changed in it.
1607 * I_DIRTY_PAGES Inode has dirty pages. Inode itself may be clean.
然后写入修改后的 inode，将 大小更改为零 ，截断所有从 inode 链接到 ext2_truncate() 的块（实际将数据块标记为空闲是在那里完成的): http://lxr.free-electrons.com/source/fs/ext2/inode.c?v=2.6.32#L1025
1025 void ext2_truncate(struct inode *inode)
1026 {
..
1059 n = ext2_block_to_path(inode, iblock, offsets, NULL);
99 /* ext2_block_to_path - parse the block number into array of offsets
105 * To store the locations of file's data ext2 uses a data structure common
106 * for UNIX filesystems - tree of pointers anchored in the inode, with
107 * data blocks at leaves and indirect blocks in intermediate nodes.
108 * This function translates the block number into path in that tree -
109 * return value is the path length and @offsets[n] is the offset of
110 * pointer to (n+1)th node in the nth one. If @block is out of range
111 * (negative or too large) warning is printed and zero returned. */
1069 if (n == 1) {
1070 ext2_free_data(inode, i_data+offsets[0],
1071 i_data + EXT2_NDIR_BLOCKS);
1072 goto do_indirects;
1073 }
..
1082 ext2_free_branches(inode, &nr, &nr+1, (chain+n-1) - partial);
..
1084 /* Clear the ends of indirect blocks on the shared branch */
1085 while (partial > chain) {
1086 ext2_free_branches(inode,
1087 partial->p + 1,
1088 (__le32*)partial->bh->b_data+addr_per_block,
1089 (chain+n-1) - partial);
..
1094 do_indirects:
1095 /* Kill the remaining (whole) subtrees */
1096 switch (offsets[0]) {
1097 default:
1098 nr = i_data[EXT2_IND_BLOCK];
1099 if (nr) {
1100 i_data[EXT2_IND_BLOCK] = 0;
1101 mark_inode_dirty(inode);
1102 ext2_free_branches(inode, &nr, &nr+1, 1);
1103 }
1104 case EXT2_IND_BLOCK:
1105 nr = i_data[EXT2_DIND_BLOCK];
1106 if (nr) {
1107 i_data[EXT2_DIND_BLOCK] = 0;
1108 mark_inode_dirty(inode);
1109 ext2_free_branches(inode, &nr, &nr+1, 2);
1110 }
1111 case EXT2_DIND_BLOCK:
1112 nr = i_data[EXT2_TIND_BLOCK];
1113 if (nr) {
1114 i_data[EXT2_TIND_BLOCK] = 0;
1115 mark_inode_dirty(inode);
1116 ext2_free_branches(inode, &nr, &nr+1, 3);
1117 }
1118 case EXT2_TIND_BLOCK:
1119 ;
1120 }
（为什么EXT2_TIND_BLOCK没有被清除？）
然后我们可以释放内核内存中的inode结构。
how does the file system know that the file is deleted?
检查在 ext2_iget 函数中：http://lxr.free-electrons.com/source/fs/ext2/inode.c?v=2.6.32#L1251
1251 /* We now have enough fields to check if the inode was active or not.
1252 * This is needed because nfsd might try to access dead inodes
1253 * the test is that same one that e2fsck uses
1254 * NeilBrown 1999oct15
1255 */
1256 if (inode->i_nlink == 0 && (inode->i_mode == 0 || ei->i_dtime)) {
1257 /* this inode is deleted */
1258 brelse (bh);
1259 ret = -ESTALE;
1260 goto bad_inode;
1261 }
所以，删除的inode是没有传入链接的inode（它在任何目录中都没有提到i_nlink）并且具有零模式或非零删除时间。