Re: [PATCH 1/5] fsnotify/vfsmount: add fsnotify fields to struct vfsmount

[Andreas Gruenbacher <agruen@suse.de>]

On Friday 04 December 2009 16:39:25 Christoph Hellwig wrote:
> What's the rationale for adding them?

The idea is to provide a mechanism to watch for "all events", but in a 
namespace aware way: for that, a process registers interest in all mount 
points it can reach.  With the previous hack, a global "I want it all" flag, 
per-namespace listeners were not possible.

Root can use bind mounts onto themselves (e.g., mount --bind /foo/bar 
/foo/bar) to watch arbitrary directories with the per-mount-point mechanism.  
Mount as user will open up additional use cases.

It would be nice to be able to register interest in all events below a 
directory which is not a mount point, too.  The problem with that is that the 
number of cached inodes below a given directory could be huge -- and we would 
have to visit them all atomically in order to "mark" them, which is not 
feasible.  The per-mount-point mechanism will eventually give us *almost* the 
same, and so I believe it's good enough.

Thanks,
Andreas