On Wed, Apr 13, 2022 at 10:23:31AM +0800, Zhang Yi wrote: > 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. But if the mount/unmount messages are ratelimited, how will you know for sure if the ratelimiting mechanism elides the message? --D > Thanks, > Yi. > > >
