On Mon, Jul 30, Theodore Tso wrote: > On Mon, Jul 30, 2007 at 06:13:35PM +0200, Jan Blunck wrote: > > Introduce white-out support to ext2. > > > > Known Bugs: > > - Needs a reserved inode number for white-outs > > You picked different reserved inodes for the ext2 and ext3 > filesystems. That's good for a NACK right there. The codepoints > (i.e., reserved inode numbers, feature bit masks, etc.) for ext2, > ext3, and ext4 MUST not overlap. After all, someone might use tune2fs > -j to convert an ext2 filesystem to ext3, and is it's REALLY BAD that > you're using a reserved inode of 7 for ext2, and 9 for ext3. Ouch, right. > Also, I note that you have created a new INCOMPAT feature flag support > for whiteouts. That's really unfortunate; we try to avoid introducing > incompatible feature flags unless absolutely necessary; note that even > adding a COMPAT feature flag means that you need a new version of > e2fsprogs if you want e2fsck to be willing to touch that filesystem. > > So --- if you're looking for a way to add whiteout support to > ext2/ext3 without needing a feature bit, here's how. We allocate a > new inode flag in struct ext3_inode.i_flags: > > #define EXT2_WHTOUT_FL 0x00040000 > > We also allocate a new field in the ext2 superblock to store the > "whiteout inode". (Please coordinate with me so it's a superblock > field not in use by ext3/ext4, and so it's reserved so that no one > else uses it.) The superblock field, call it s_whtout_ino, stores the > inode number for the "white out inode". > > When you create a new whiteout file, the code checks sb->s_whtout_ino, > and if it is zero, it allocates a new inode, and creates it as a > zero-length regular file (i_mode |= S_IFREG) with the EXT2_WHTOUT_FL > flag set in the inode, and then store the inode number in > sb->s_whtout_ino. If sb->s_whtout_ino is non-zero, you must read in > the inode and make sure that the EXT2_WHTOUT_FL is set. If it is not, > then allocate a new whiteout inode as described previously. Then link > the inode into the directory as before. > > When reading an inode, if the EXT2_WHTOUT_FL flag is set, then set the > in-memory mode of the inode to be S_IFWHT. > > That's pretty much about it. For cleanliness sake, it would be good > if ext2_delete_inode clears sb->s_whtout_ino if the last whiteout link > has been deleted, but it's strictly speaking not necessary. If you do > it this way, the filesystem is completely backwards compatible; the > whiteout files will just appear to links to a normal zero-lenth file. Ok, this is pretty similar to the way I implemented this for tmpfs. The problem is that the union mount code is explicitly checking if the filesystem is supporting whiteout. I used to use a new filesystem flag (FS_WHITEOUT) for this but thought that disk filesystem like ext2/3/4 will have problem with that if you mount an old image. So I guess I still need a feature flag. > I wouldn't bother with setting the directory type field to be DT_WHT, > given that they will never be returned to userspace anyway. At the moment I still rely on this for the current readdir implementation. Viro already said that he doesn't want to see this (the readdir changes) in the kernel but in userspace. Thanks, Jan - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
