linux-kernel - Re: [RFC PATCH] getting misc stats/attributes via xattr API

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Date:   Wed, 4 May 2022 09:18:03 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Dave Chinner <david@...morbit.com>
Cc:     linux-fsdevel@...r.kernel.org, "Theodore Ts'o" <tytso@....edu>,
        Karel Zak <kzak@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel@...r.kernel.org,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>,
        LSM <linux-security-module@...r.kernel.org>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API

On Wed, 4 May 2022 at 00:43, Dave Chinner <david@...morbit.com> wrote:

> "values" is a very generic name - probably should end up being
> something more descriptive of the functionality is provides,
> especially if the header file is going to be dumped in
> include/linux/. I don't really have a good suggestion at the moment,
> though.

The obvious ones are stat and attr, which are taken already.   Info is
probably too generic as well.

Ideas are welcome.

>
> ....
>
> > +
> > +enum {
> > +     VAL_MNT_INFO,
> > +};
> > +
> > +static struct val_desc val_mnt_group[] = {
> > +     { VD_NAME("info"),              .idx = VAL_MNT_INFO             },
> > +     { }
> > +};
> ....
> > +
> > +
> > +static struct val_desc val_toplevel_group[] = {
> > +     { VD_NAME("mnt:"),      .get = val_mnt_get,     },
> > +     { VD_NAME("mntns:"),    .get = val_mntns_get,   },
> > +     { },
> > +};
>
> I know this is an early POC, my main question is how do you
> envisiage this table driven structure being extended down from just
> the mount into the filesystem so we can expose filesystem specific
> information that isn't covered by generic interfaces like statx?

I was thinking of adding a i_op callback.   The details are a bit
fuzzy, since the vfs and the fs would have to work together when
listing the attributes and possibly also when retrieving the attribute
itself (think mount options).

Thanks,
Miklos