Hi Amir, > On Nov 6, 2024, at 11:40 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote: > > On Thu, Oct 31, 2024 at 2:52 AM Song Liu <songliubraving@xxxxxxxx> wrote: >> >> Hi Jeff, >> >>> On Oct 30, 2024, at 5:23 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote: >> >> [...] >> >>>> If the subtree is all in the same file system, we can attach fanotify to >>>> the whole file system, and then use some dget_parent() and follow_up() >>>> to walk up the directory tree in the fastpath handler. However, if there >>>> are other mount points in the subtree, we will need more logic to handle >>>> these mount points. >>>> >>> >>> My 2 cents... >>> >>> I'd just confine it to a single vfsmount. If you want to monitor in >>> several submounts, then you need to add new fanotify watches. >>> > > Agreed. > >>> Alternately, maybe there is some way to designate that an entire >>> vfsmount is a child of a watched (or ignored) directory? >>> >>>> @Christian, I would like to know your thoughts on this (walking up the >>>> directory tree in fanotify fastpath handler). It can be expensive for >>>> very very deep subtree. >>>> >>> >>> I'm not Christian, but I'll make the case for it. It's basically a >>> bunch of pointer chasing. That's probably not "cheap", but if you can >>> do it under RCU it might not be too awful. It might still suck with >>> really deep paths, but this is a sample module. It's not expected that >>> everyone will want to use it anyway. >> >> Thanks for the suggestion! I will try to do it under RCU. >> > > That's the cost of doing a subtree filter. > Not sure how it could be avoided? [...] >> >>> >>> Then, when you get a dentry-based event, you just walk the d_parent >>> pointers back up to the root of the vfsmount. If one of them matches >>> the dentry in your fd then you're done. If you hit the root of the >>> vfsmount, then you're also done (and know that it's not a child of that >>> dentry). Thanks for your feedback. I will update the set based on Jeff's suggestion. Song
