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

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Date:   Tue, 3 May 2022 18:54:29 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>,
        Theodore Ts'o <tytso@....edu>, Karel Zak <kzak@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        linux-kernel <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>,
        James Bottomley <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API

On Tue, May 03, 2022 at 05:04:06PM +0200, Miklos Szeredi wrote:
> On Tue, 3 May 2022 at 16:53, Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Tue, May 03, 2022 at 05:39:46PM +0300, Amir Goldstein wrote:
> 
> > > It should be noted that while this API mandates text keys,
> > > it does not mandate text values, so for example, sb iostats could be
> > > exported as text or as binary struct, or as individual text/binary records or
> > > all of the above.
> >
> > Ugh, no, that would be a total mess.  Don't go exporting random binary
> > structs depending on the file, that's going to be completely
> > unmaintainable.  As it is, this is going to be hard enough with random
> > text fields.
> >
> > As for this format, it needs to be required to be documented in
> > Documentation/ABI/ for each entry and key type so that we have a chance
> > of knowing what is going on and tracking how things are working and
> > validating stuff.
> 
> My preference would be a single text value for each key.

Yes!  That's the only sane way to maintain apis, and is why we do that
for sysfs.

If the key isn't present, there's no value, so userspace "knows" this
automatically and parsing this is trivial.

> Contents of ":mnt:info" contradicts that, but mountinfo has a long
> established, well documented format, and nothing prevents exporting
> individual attributes with separate names as well (the getvalues(2)
> patch did exactly that).

I understand, for "legacy" things like this, that's fine, but don't add
new fields or change them over time please, that way just gets us back
to the nightmare of preserving /proc/ file apis.

thanks,

greg k-h