On 2022/4/13 9:35, Theodore Ts'o wrote: > On Tue, Apr 12, 2022 at 12:01:37PM -0400, Gabriel Krisman Bertazi wrote: >> Zhang Yi <yi.zhang@xxxxxxxxxx> writes: >> >>> Now that we have kernel message at mount time, system administrator > > "Now that we have...." is a bit misleading, since (at least to an > English speaker) that this is something that was recently added, and > that's not the case. > >>> could acquire the mount time, device and options easily. But we don't >>> have corresponding unmounting message at umount time, so we cannot know >>> if someone umount a filesystem easily. Some of the modern filesystems >>> (e.g. xfs) have the umounting kernel message, so add one for ext4 >>> filesystem for convenience. >>> >>> EXT4-fs (sdb): mounted filesystem with ordered data mode. Quota mode: none. >>> EXT4-fs (sdb): unmounting filesystem. >> >> I don't think sysadmins should be relying on the kernel log for this, >> since the information can easily be overwritten by new messages there. >> Is there a reason why you can't just monitor /proc/self/mountinfo? > > You're right that it can be dangerous for sysadmins to be relying on > the kernel log for mount and umount notifications --- but it depends > on what they think it means, and the potential pitfalls are there for > both the mount and unmount messages. The problem of course, is that > bind mounts, and mount name spaces, so if the question is whether a > file system is available at a particular mount point, then using the > kernel log is definitely not going to be reliable. > > But if the goal is to determine whether a particular device is safe to > run fsck or otherwise access directly, or for the purposes of > debugging the kernel and looking at the logs to understand when the > device is being accessed by the kernel and when the file system is > done with the device, I can see how it might be useful. > Yes, I understand that the kernel log is not reliable, and /proc/self/mountinfo neither. Our goal is simple, As Ted said, just add a method to help sysadmins to know whether a particular ext4 device is really doing unmount procedure, it could be helpful for us to debug kernel and locate kernel bug. Thanks, Yi.
