On Saturday, 19 September 2009 5:04:31 Eric Paris wrote:

Let me start by saying I am agreeing I should pursue subtree
notification.  It's what I think everyone really wants.  It's a great
idea, and I think you might have a simple way to get close.  Clearly
these are avenues I'm willing and hoping to pursue.  Also I say it
again, I believe the interface as proposed (except maybe some of my
exclusion stuff) is flexible enough to implement any of these ideas.
Does anyone disagree?

It does seem flexible enough. However, the current interface assumes "global" 
listeners (the mask argument of fanotify_init):

  int fanotify_init(int flags, int f_flags, __u64 mask,
		    unsigned int priority);

Once subtree support is added, this parameter becomes obsolete. That's pretty 
broken for a syscall yet to be introduced.

BUT to solve one of the main problems fanotify is intending to solve it
needs a way to be the 'fscking all notifier.'  It needs to be the whole
damn system.

Think of a system after boot, with a single global namespace. Whatever you 
access by filename is reachable from the namespace root. At this point, 
nothing more global exists. A listener can watch the mount points of 
interest, and everything's fine.

What's a bit more tricky is to ensure that this listener will continue to 
receive all events from whatever else is mounted anywhere, irrespective of 
namespaces. I think we can get there.

By the way, Documentation/filesystems/sharedsubtree.txt describes how 
filesystem namespaces work.

Thanks,
Andreas