linux-kernel - Re: file metadata via fs API

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Date:   Thu, 13 Aug 2020 11:44:54 +0800
From:   Ian Kent <raven@...maw.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Whitehouse <swhiteho@...hat.com>
Cc:     David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>, Karel Zak <kzak@...hat.com>,
        Jeff Layton <jlayton@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        Christian Brauner <christian@...uner.io>,
        Lennart Poettering <lennart@...ttering.net>,
        Linux API <linux-api@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API

On Wed, 2020-08-12 at 12:50 -0700, Linus Torvalds wrote:
> On Wed, Aug 12, 2020 at 12:34 PM Steven Whitehouse <
> swhiteho@...hat.com> wrote:
> > The point of this is to give us the ability to monitor mounts from
> > userspace.
> 
> We haven't had that before, I don't see why it's suddenly such a big
> deal.

Because there's a trend occurring in user space where there are
frequent and persistent mount changes that cause high overhead.

I've seen the number of problems building up over the last few months
that are essentially the same problem that I wanted to resolve. And
that's related to side effects of autofs using a large number of
mounts.

The problems are real.

> 
> The notification side I understand. Polling /proc files is not the
> answer.

Yep, that's one aspect, getting the information about a mount without
reading the entire mount table seems like the sensible thing to do to
allow for a more efficient notification mechanism.

> 
> But the whole "let's design this crazy subsystem for it" seems way
> overkill. I don't see anybody caring that deeply.
> 
> It really smells like "do it because we can, not because we must".
> 
> Who the hell cares about monitoring mounts at a kHz frequencies? If
> this is for MIS use, you want a nice GUI and not wasting CPU time
> polling.

That part of the problem still remains.

The kernel sending a continuous stream of wake ups under load does
also introduce a resource problem but that's probably something to
handle in user space.

> 
> I'm starting to ignore the pull requests from David Howells, because
> by now they have had the same pattern for a couple of years now:
> esoteric new interfaces that seem overdesigned for corner-cases that
> I'm not seeing people clamoring for.
> 
> I need (a) proof this is actualyl something real users care about and
> (b) way more open discussion and implementation from multiple
> parties.
> 
> Because right now it looks like a small in-cabal of a couple of
> people
> who have wild ideas but I'm not seeing the wider use of it.
> 
> Convince me otherwise. AGAIN. This is the exact same issue I had with
> the notification queues that I really wanted actual use-cases for,
> and
> feedback from actual outside users.
> 
> I really think this is engineering for its own sake, rather than
> responding to actual user concerns.
> 
>                Linus