linux-kernel - Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

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Date:   Wed, 12 Aug 2020 10:37:08 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Ian Kent <raven@...maw.net>,
        LSM <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: file metadata via fs API (was: [GIT PULL] Filesystem Information)

On Wed, Aug 12, 2020 at 10:29 AM David Howells <dhowells@...hat.com> wrote:
>
> Miklos Szeredi <miklos@...redi.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