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Date:   Wed, 05 Aug 2020 10:46:02 +0800
From:   Ian Kent <raven@...maw.net>
To:     Miklos Szeredi <miklos@...redi.hu>,
        David Howells <dhowells@...hat.com>
Cc:     viro@...iv.linux.org.uk, torvalds@...ux-foundation.org,
        mszeredi@...hat.com, christian@...uner.io, jannh@...gle.com,
        darrick.wong@...cle.com, kzak@...hat.com, jlayton@...hat.com,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/18] fsinfo: Provide notification overrun handling
 support [ver #21]

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

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