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:

quoted

quoted

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@fromorbit.com