On Thu, 2016-09-15 at 12:12 +0800, Ian Kent wrote: > On Wed, 2016-09-14 at 21:08 -0500, Eric W. Biederman wrote: > > Ian Kent <ra...@themaw.net> writes: > > > > > On Wed, 2016-09-14 at 12:28 -0500, Eric W. Biederman wrote: > > > > Ian Kent <ra...@themaw.net> writes: > > > > > > > > > If an automount mount is clone(2)ed into a file system that is > > > > > propagation private, when it later expires in the originating > > > > > namespace subsequent calls to autofs ->d_automount() for that > > > > > dentry in the original namespace will return ELOOP until the > > > > > mount is manually umounted in the cloned namespace. > > > > > > > > > > In the same way, if an autofs mount is triggered by automount(8) > > > > > running within a container the dentry will be seen as mounted in > > > > > the root init namespace and calls to ->d_automount() in that namespace > > > > > will return ELOOP until the mount is umounted within the container. > > > > > > > > > > Also, have_submounts() can return an incorect result when a mount > > > > > exists in a namespace other than the one being checked. > > > > > > > > Overall this appears to be a fairly reasonable set of changes. It does > > > > increase the expense when an actual mount point is encountered, but if > > > > these are the desired some increase in cost when a dentry is a > > > > mountpoint is unavoidable. > > > > > > > > May I ask the motiviation for this set of changes? Reading through the > > > > changes I don't grasp why we want to change the behavior of autofs. > > > > What problem is being solved? What are the benefits? > > > > > > LOL, it's all too easy for me to give a patch description that I think > > > explains > > > a problem I need to solve without realizing it isn't clear to others what > > > the > > > problem is, sorry about that. > > > > > > For quite a while now, and not that frequently but consistently, I've been > > > getting reports of people using autofs getting ELOOP errors and not being > > > able > > > to mount automounts. > > > > > > This has been due to the cloning of autofs file systems (that have active > > > automounts at the time of the clone) by other systems. > > > > > > An unshare, as one example, can easily result in the cloning of an autofs > > > file > > > system that has active mounts which shows this problem. > > > > > > Once an active mount that has been cloned is expired in the namespace that > > > performed the unshare it can't be (auto)mounted again in the the > > > originating > > > namespace because the mounted check in the autofs module will think it is > > > already mounted. > > > > > > I'm not sure this is a clear description either, hopefully it is enough to > > > demonstrate the type of problem I'm typing to solve. > > > > So to rephrase the problem is that an autofs instance can stop working > > properly from the perspective of the mount namespace it is mounted in > > if the autofs instance is shared between multiple mount namespaces. The > > problem is that mounts and unmounts do not always propogate between > > mount namespaces. This lack of symmetric mount/unmount behavior > > leads to mountpoints that become unusable. > > That's right. > > It's also worth considering that symmetric mount propagation is usually not > the > behaviour needed either and things like LXC and Docker are set propagation > slave > because of problems caused by propagation back to the parent namespace. > > So a mount can be triggered within a container, mounted by the automount > daemon > in the parent namespace, and propagated to the child and similarly for > expires, > which is the common use case now. > > > > > Which leads to the question what is the expected new behavior with your > > patchset applied. New mounts can be added in the parent mount namespace > > (because the test is local). Does your change also allow the > > autofs mountpoints to be used in the other mount namespaces that share > > the autofs instance if everything becomes unmounted? > > The problem occurs when the subordinate namespace doesn't deal with these > propagated mounts properly, although they can obviously be used by the > subordinate namespace. > > > > > Or is it expected that other mount namespaces that share an autofs > > instance will get changes in their mounts via mount propagation and if > > mount propagation is insufficient they are on their own. > > Namespaces that receive updates via mount propagation from a parent will > continue to function as they do now. > > Mounts that don't get updates via mount propagation will retain the mount to > use > if they need to, as they would without this change, but the originating > namespace will also continue to function as expected. > > The child namespace needs cleanup its mounts on exit, which it had to do prior > to this change also. > > > > > I believe this is a question of how do notifications of the desire for > > an automount work after your change, and are those notifications > > consistent with your desired and/or expected behavior. > > It sounds like you might be assuming the service receiving these cloned mounts > actually wants to use them or is expecting them to behave like automount > mounts. > But that's not what I've seen and is not the way these cloned mounts behave > without the change. > > However, as has probably occurred to you by now, there is a semantic change > with > this for namespaces that don't receive mount propogation. > > If a mount request is triggered by an access in the subordinate namespace for > a > dentry that is already mounted in the parent namespace it will silently fail > (in > that a mount won't appear in the subordinate namespace) rather than getting an > ELOOP error as it would now.
My mistake, sorry, looking at this again this case will still fail with ELOOP as it does now. Ian
