linux-kernel - Re: [RFC PATCH] getvalues(2) prototype

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Date:   Mon, 28 Mar 2022 08:03:19 +1100
From:   Dave Chinner <david@...morbit.com>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     "miklos@...redi.hu" <miklos@...redi.hu>,
        "raven@...maw.net" <raven@...maw.net>,
        "kzak@...hat.com" <kzak@...hat.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "mszeredi@...hat.com" <mszeredi@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
        "amir73il@...il.com" <amir73il@...il.com>,
        "christian@...uner.io" <christian@...uner.io>,
        "James.Bottomley@...senpartnership.com" 
        <James.Bottomley@...senpartnership.com>
Subject: Re: [RFC PATCH] getvalues(2) prototype

On Fri, Mar 25, 2022 at 04:42:27PM +0000, Trond Myklebust wrote:
> On Fri, 2022-03-25 at 07:31 +1100, Dave Chinner wrote:
> > > and anyway the point of a
> > > hierarchical namespace is to be able to list nodes on each level. 
> > > We
> > > can use getxattr() for this purpose, just like getvalues() does in
> > > the
> > > above example.
> > 
> > Yup, and like Casey suggests, you could implement a generic
> > getvalues()-like user library on top of it so users don't even need
> > to know where and how the values are located or retrieved.
> > 
> > The other advantage of an xattr interface is that is also provides a
> > symmetrical API for -changing- values. No need for some special
> > configfs or configfd thingy for setting parameters - just change the
> > value of the parameter or mount option with a simple setxattr call.
> > That retains the simplicity of proc and sysfs attributes in that you
> > can change them just by writing a new value to the file....
> 
> The downsides are, however, that the current interface provides little
> in the way of atomicity if you want to read or write to multiple
> attributes at the same time. Something like a backup program might want
> to be able to atomically retrieve the ctime when it is backing up the
> attributes.

I assumed that batched updates were implied and understood after
my earlier comments about XFS_IOC_ATTRMULTI_BY_HANDLE as used
by xfsdump/restore for the past 20+ years.

> Also, when setting attributes, I'd like to avoid multiple syscalls when
> I'm changing multiple related attributes.
>
> IOW: Adding a batching interface that is akin to what Miklos was
> proposing would be a helpful change if we want to go down this path.

Yup, that's exactly what XFS_IOC_ATTRMULTI_BY_HANDLE provides and
I'm assuming that would also be provided by whatever formalised
generic syscall API we come up with here...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com