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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 5 Aug 2020 19:26:06 +0200
Message-ID: <CAJfpegvpjjk3JNgTfY1yOGToPLF1mXG5+UkUZqVOGDkvzEr5HQ@mail.gmail.com>
Subject: Re: [PATCH 10/18] fsinfo: Provide notification overrun handling
 support [ver #21]
To:     David Howells <dhowells@redhat.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ian Kent <raven@themaw.net>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Christian Brauner <christian@brauner.io>,
        Jann Horn <jannh@google.com>,
        "Darrick J. Wong" <darrick.wong@oracle.com>,
        Karel Zak <kzak@redhat.com>, Jeff Layton <jlayton@redhat.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org,
        LSM <linux-security-module@vger.kernel.org>,
        linux-kernel@vger.kernel.org
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On Wed, Aug 5, 2020 at 6:07 PM David Howells <dhowells@redhat.com> wrote:
>
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > Shoun't we just make sure that the likelyhood of overruns is low
>
> That's not necessarily easy.  To avoid overruns you need a bigger buffer.  The
> buffer is preallocated from unswappable kernel space.  Yes, you can increase
> the size of the buffer, but it eats out of your pipe bufferage limit.
>
> Further, it's a *general* notifications queue, not just for a specific
> purpose, but that means it might get connected to multiple sources, and doing
> something like tearing down a container might generate enough notifications to
> overrun the queue.
>
> > and if it happens, just reinitialize everthing from scratch (shouldn't be
> > *that* expensive).
>
> If you then spend time reinitialising everything, you're increasing the
> likelihood of racing with further events.  Further, there multiple expenses:
> firstly, you have to tear down and discard all the data that you've spent time
> setting up; secondly, it takes time doing all this; thirdly, it takes cpu
> cycles away from applications.
>
> The reason I put the event counters in there and made it so that fsinfo()
> could read all the mounts in a subtree and their event counters in one go is
> to make it faster for the user to find out what changed in the event that a
> notification is lost.

That's just overdesigning it, IMO.

If the protocol is extensible (as you state) then the counters can be
added as needed.  And unless the above CPU cycle wastage is actually
observed in practice, the whole thing is unnecessary.

Thanks,
Miklos