Re: file handle in statx

[Dave Chinner <david@fromorbit.com>]

On Wed, Dec 13, 2023 at 08:57:43AM +1100, NeilBrown wrote:
> On Wed, 13 Dec 2023, Dave Chinner wrote:
> > On Tue, Dec 12, 2023 at 09:15:29AM -0800, Frank Filz wrote:
> > > > On Tue, Dec 12, 2023 at 10:10:23AM +0100, Donald Buczek wrote:
> > > > > On 12/12/23 06:53, Dave Chinner wrote:
> > > > >
> > > > > > So can someone please explain to me why we need to try to re-invent
> > > > > > a generic filehandle concept in statx when we already have a have
> > > > > > working and widely supported user API that provides exactly this
> > > > > > functionality?
> > > > >
> > > > > name_to_handle_at() is fine, but userspace could profit from being
> > > > > able to retrieve the filehandle together with the other metadata in a
> > > > > single system call.
> > > > 
> > > > Can you say more?  What, specifically is the application that would want
> > > to do
> > > > that, and is it really in such a hot path that it would be a user-visible
> > > > improveable, let aloine something that can be actually be measured?
> > > 
> > > A user space NFS server like Ganesha could benefit from getting attributes
> > > and file handle in a single system call.
> > 
> > At the cost of every other application that doesn't need those
> > attributes.
> 
> Why do you think there would be a cost?

It's as much maintenance and testing cost as it is a runtime cost.
We have to test and check this functionality works as advertised,
and we have to maintain that in working order forever more. That's
not free, especially if it is decided that the implementation needs
to be hyper-optimised in each individual filesystem because of
performance cost reasons.

Indeed, even the runtime "do we need to fetch this information"
checks have a measurable cost, especially as statx() is a very hot
kernel path. We've been optimising branches out of things like
setting up kiocbs because when that path is taken millions of times
every second each logic branch that decides if something needs to be
done or not has a direct measurable cost. statx() is a hot path that
can be called millions of times a second.....

And then comes the cost of encoding dynamically sized information in
struct statx - filehandles are not fixed size - and statx is most
definitely not set up or intended for dynamically sized attribute
data. This adds more complexity to statx because it wasn't designed
or intended to handle dynamically sized attributes. Optional
attributes, yes, but not attributes that might vary in size from fs
to fs or even inode type to inode type within a fileystem (e.g. dir
filehandles can, optionally, encode the parent inode in them).

-Dave.
-- 
Dave Chinner
david@fromorbit.com