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

On Thu, Dec 10, 2009 at 10:01 PM, Andreas Gruenbacher <agruen@xxxxxxx> wrote:
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.


As another dimension of the same problem, it would be nice to
register interest in events generated by only a particular set of processes.
As a special case of this, can I register only for events generated
by myself and all my child processes. Is it already covered in some way?

There are situations where this could be very useful. One such case could be
somebody wanting to audit any random application. Collecting data for the whole
system and then filtering might be possible, but maynot be very efficient.
It might be good to add this while we are at it.

Let me know if I am missing something.

-Niraj
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