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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 12 Aug 2020 10:37:08 +0200
Message-ID: <CAJfpegsVJo9e=pHf3YGWkE16fT0QaNGhgkUdq4KUQypXaD=OgQ@mail.gmail.com>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)
To:     David Howells <dhowells@redhat.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>, Karel Zak <kzak@redhat.com>,
        Jeff Layton <jlayton@redhat.com>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>,
        Christian Brauner <christian@brauner.io>,
        Lennart Poettering <lennart@poettering.net>,
        Linux API <linux-api@vger.kernel.org>,
        Ian Kent <raven@themaw.net>,
        LSM <linux-security-module@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Aug 12, 2020 at 10:29 AM David Howells <dhowells@redhat.com> wrote:
>
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > Worried about performance?  Io-uring will allow you to do all those
> > five syscalls (or many more) with just one I/O submission.
>
> io_uring isn't going to help here.  We're talking about synchronous reads.
> AIUI, you're adding a couple more syscalls to the list and running stuff in a
> side thread to save the effort of going in and out of the kernel five times.
> But you still have to pay the set up/tear down costs on the fds and do the
> pathwalks.  io_uring doesn't magically make that cost disappear.
>
> io_uring also requires resources such as a kernel accessible ring buffer to
> make it work.
>
> You're proposing making everything else more messy just to avoid a dedicated
> syscall.  Could you please set out your reasoning for that?

a) A dedicated syscall with a complex binary API is a non-trivial
maintenance burden.

b) The awarded performance boost is not warranted for the use cases it
is designed for.

Thanks,
Miklos