Re: [PATCH] btrfs: sysfs: export dev stats in devinfo directory

[David Sterba <dsterba@suse.cz>]

On Wed, Jun 09, 2021 at 05:55:05PM -0700, Omar Sandoval wrote:
> > > The ioctl returns ENODEV is !dev_stats_valid, maybe this file should do
> > > the same? It seems a little awkward to have a flag that means that the
> > > rest of the file is meaningless.
> > 
> > You mean returning -ENODEV when reading the stats file? Or return 0 but
> > the contents is something like 'stats invalid' or similar.
> 
> I'd vote for returning -ENODEV when reading the stats file, but I think
> either one is fine.

Hm so I think this should reflect how the sysfs files are used. They all
contain textual information, and errors are returned when eg. there are
no permissions.

In a shell script it's IMHO more convenient to do

stats=$(cat $devicepath/stats)

and then validate contents of $stats rather then catching the error
value and deciding based on that what happend. Not to say that this
would also print an error message. I've found this in
admin-guide/sysfs-rules.rst that's perhaps closest to a recommendation
we could follow:

172 - When reading and writing sysfs device attribute files, avoid dependency
173     on specific error codes wherever possible. This minimizes coupling to
174     the error handling implementation within the kernel.

So I take it as that error codes belong to the sysfs layer and the
validity of the contents is up to the sysfs user, ie. btrfs in this
case.