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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 12 Aug 2020 09:55:53 +0200
Message-ID: <CAJfpegt=cQ159kEH9zCYVHV7R_08jwMxF0jKrSUV5E=uBg4Lzw@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 2:05 AM David Howells <dhowells@redhat.com> wrote:

> >   {
> >      int fd, attrfd;
> >
> >      fd = open(path, O_PATH);
> >      attrfd = openat(fd, name, O_ALT);
> >      close(fd);
> >      read(attrfd, value, size);
> >      close(attrfd);
> >   }
>
> Please don't go down this path.  You're proposing five syscalls - including
> creating two file descriptors - to do what fsinfo() does in one.

So what?  People argued against readfile() for exactly the opposite of
reasons, even though that's a lot less specialized than fsinfo().

Worried about performance?  Io-uring will allow you to do all those
five syscalls (or many more) with just one I/O submission.

Thanks,
Miklos