From: Andreas Gruenbacher <agruen@suse.de>
Organization: SUSE Labs
To: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 1/5] fsnotify/vfsmount: add fsnotify fields to struct vfsmount
Date: Thu, 10 Dec 2009 17:31:42 +0100
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Cc: Eric Paris <eparis@redhat.com>, linux-kernel@vger.kernel.org,
       linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk
References: <20091203055315.21918.83562.stgit@paris.rdu.redhat.com> <20091204153925.GA17062@infradead.org>
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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
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