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Subject: Re: [PATCH 10/18] fsinfo: Provide notification overrun handling
 support [ver #21]
From:   Ian Kent <raven@themaw.net>
To:     Miklos Szeredi <miklos@szeredi.hu>,
        David Howells <dhowells@redhat.com>
Cc:     viro@zeniv.linux.org.uk, torvalds@linux-foundation.org,
        mszeredi@redhat.com, christian@brauner.io, jannh@google.com,
        darrick.wong@oracle.com, kzak@redhat.com, jlayton@redhat.com,
        linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org
Date:   Wed, 05 Aug 2020 10:46:02 +0800
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On Wed, 2020-08-05 at 10:05 +0800, Ian Kent wrote:
> On Tue, 2020-08-04 at 15:56 +0200, Miklos Szeredi wrote:
> > On Mon, Aug 03, 2020 at 02:37:50PM +0100, David Howells wrote:
> > > Provide support for the handling of an overrun in a watch
> > > queue.  In the
> > > event that an overrun occurs, the watcher needs to be able to
> > > find
> > > out what
> > > it was that they missed.  To this end, previous patches added
> > > event
> > > counters to struct mount.
> > 
> > So this is optimizing the buffer overrun case?
> > 
> > Shoun't we just make sure that the likelyhood of overruns is low
> > and
> > if it
> > happens, just reinitialize everthing from scratch (shouldn't be
> > *that*
> > expensive).
> 
> But maybe not possible if you are using notifications for tracking
> state in user space, you need to know when the thing you have needs
> to be synced because you missed something and it's during the
> notification processing you actually have the object that may need
> to be refreshed.
> 
> > Trying to find out what was missed seems like just adding
> > complexity
> > for no good
> > reason.

Coming back to an actual use case.

What I said above is one aspect but, since I'm looking at this right
now with systemd, and I do have the legacy code to fall back to, the
"just reset everything" suggestion does make sense.

But I'm struggling to see how I can identify notification buffer
overrun in libmount, and overrun is just one possibility for lost
notifications, so I like the idea that, as a library user, I can
work out that I need to take action based on what I have in the
notifications themselves.

Ian