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

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Message-ID: <98802.1597220949@warthog.procyon.org.uk>
Date:   Wed, 12 Aug 2020 09:29:09 +0100
From:   David Howells <dhowells@...hat.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     dhowells@...hat.com,
        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)

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?

David