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Date:   Tue, 3 May 2022 16:53:49 +0200
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Dave Chinner <david@fromorbit.com>,
        Theodore Ts'o <tytso@mit.edu>, Karel Zak <kzak@redhat.com>,
        Christian Brauner <brauner@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        LSM <linux-security-module@vger.kernel.org>,
        Ian Kent <raven@themaw.net>,
        David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <christian@brauner.io>,
        James Bottomley <James.Bottomley@hansenpartnership.com>
Subject: Re: [RFC PATCH] getting misc stats/attributes via xattr API
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On Tue, May 03, 2022 at 05:39:46PM +0300, Amir Goldstein wrote:
> On Tue, May 3, 2022 at 3:23 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > This is a simplification of the getvalues(2) prototype and moving it to the
> > getxattr(2) interface, as suggested by Dave.
> >
> > The patch itself just adds the possibility to retrieve a single line of
> > /proc/$$/mountinfo (which was the basic requirement from which the fsinfo
> > patchset grew out of).
> >
> > But this should be able to serve Amir's per-sb iostats, as well as a host of
> > other cases where some statistic needs to be retrieved from some object.  Note:
> > a filesystem object often represents other kinds of objects (such as processes
> > in /proc) so this is not limited to fs attributes.
> >
> > This also opens up the interface to setting attributes via setxattr(2).
> >
> > After some pondering I made the namespace so:
> >
> > : - root
> > bar - an attribute
> > foo: - a folder (can contain attributes and/or folders)
> >
> > The contents of a folder is represented by a null separated list of names.
> >
> > Examples:
> >
> > $ getfattr -etext -n ":" .
> > # file: .
> > :="mnt:\000mntns:"
> >
> > $ getfattr -etext -n ":mnt:" .
> > # file: .
> > :mnt:="info"
> >
> > $ getfattr -etext -n ":mnt:info" .
> > # file: .
> > :mnt:info="21 1 254:0 / / rw,relatime - ext4 /dev/root rw\012"
> >
> > $ getfattr -etext -n ":mntns:" .
> > # file: .
> > :mntns:="21:\00022:\00024:\00025:\00023:\00026:\00027:\00028:\00029:\00030:\00031:"
> >
> > $ getfattr -etext -n ":mntns:28:" .
> > # file: .
> > :mntns:28:="info"
> >
> > Comments?
> >
> 
> I like that :)
> 
> 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.

thanks,

greg k-h