On Tue, Jul 27, 2021 at 01:49:39AM -0600, Andreas Dilger wrote: > > Looking at the e2label source code, it just reads the superblock, > > updates the label and writes the super block. How is that safe and > > persistent when presumably the linux kernel has an in-memory copy of the > > superblock will be written at unmount and presumable sync. > > Currently, the in-memory superblock references the device buffer cache, > which is the same cache that is accessed when reading the block > device from userspace, so they are always consistent. > > There has been some discussion about adding ioctl() calls to update > the filesystem label, UUID, and other fields from userspace in a safer way, > but nothing has been implemented in that direction yet (possibly Darrick > had some RFC patches, but they are not landed yet). As Andreas has stated, e2fsprogs programs such as e2label and tune2fs use buffered I/O to read and write the superblock, which accesses the buffer cache, which is where the kernel's copy superblock used by the file system code is located. It's not perfect; for example an updated label written by e2label might get lost when it is overwritten by a journal replay after a system crash. But for the most part, it does work. Cheers, - Ted
