On Tue, Nov 19, 2024 at 08:15:29AM +0000, Windl, Ulrich wrote: > > Reads of ext superblocks can race with updates. If libblkid observes a > [Windl, Ulrich] > > I really wonder: > > Can one single block be inconsistent when read, considering that the > block on disk is not inconsistent? That would mean that the block > buffer you are reading is being modified by another process. AFAIK > the basic UNIX semantic guarantee that a block is read atomically; > if it's not, something is severely broken, and I don't think that > O_DIRECT fixes that. Yes, this can happen if the file system is mounted. The reason for this is that the kernel updates metadata blocks via the block buffer cache, with the jbd2 (journaled block layer v2) subsystem managing the atomic updates. The jbd2 layer will block buffer cache writebacks until the changes are committed in a jbd2 transaction. So the version on disk is guaranteed to be consistent. However, a buffer cache read does not have any consistency guarantees, and if the file system is being actively modified, it is possible that you could a superblock where the checksum hasn't yet been updated. The O_DIRECT read isn't a magic bullet. For example, if you have a scratch file system which is guaranteed not to survive a Kubernetes or Borg container getting aborted, you might decide to format the file system without a jbd2 journal, since that would be more efficient, and by definition you don't care about the contents of the file system after a crash. So there are millions of ext4 file systems in hyperscale computing environments that are created without a journal; and in that case, O_DIRECT will not be sufficient for guaranteeing a consistent read of the superblock. In the long term, I'll probably be adding an ioctl which will allow userspace to read the superblock consistently for a mounted file system. We actually already have ioctls, EXT4_IOC_GETFSUUID and FS_IOC_GETFSLABEL which will allow userspace to fetch the UUID and Label for a mounted file system. So eventually, I'll probably end up adding EXT4_IOC_GET_SUPERBLOCK. Let me know if this is something that util-linux would very much want. Note: this does require figuring out (a) whether the file system is mounted, and (b) if so, where is it mounted. So if blkid wants to use this, it would need to have something like the function ext2fs_check_mount_point[1]. Cheers, - Ted [1] https://github.com/tytso/e2fsprogs/blob/950a0d69c82b585aba30118f01bf80151deffe8c/lib/ext2fs/ismounted.c#L363
