linux-kernel - Re: [LSF TOPIC] statx extensions for subvol/snapshot filesystems & more

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Date: Thu, 22 Feb 2024 10:14:20 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Josef Bacik <josef@...icpanda.com>
Cc: Kent Overstreet <kent.overstreet@...ux.dev>, linux-bcachefs@...r.kernel.org, 
	linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, lsf-pc@...ts.linux-foundation.org
Subject: Re: [LSF TOPIC] statx extensions for subvol/snapshot filesystems & more

On Wed, 21 Feb 2024 at 22:08, Josef Bacik <josef@...icpanda.com> wrote:
>
> On Wed, Feb 21, 2024 at 04:06:34PM +0100, Miklos Szeredi wrote:
> > On Wed, 21 Feb 2024 at 01:51, Kent Overstreet <kent.overstreet@...ux.dev> wrote:
> > >
> > > Recently we had a pretty long discussion on statx extensions, which
> > > eventually got a bit offtopic but nevertheless hashed out all the major
> > > issues.
> > >
> > > To summarize:
> > >  - guaranteeing inode number uniqueness is becoming increasingly
> > >    infeasible, we need a bit to tell userspace "inode number is not
> > >    unique, use filehandle instead"
> >
> > This is a tough one.   POSIX says "The st_ino and st_dev fields taken
> > together uniquely identify the file within the system."
> >
>
> Which is what btrfs has done forever, and we've gotten yelled at forever for
> doing it.  We have a compromise and a way forward, but it's not a widely held
> view that changing st_dev to give uniqueness is an acceptable solution.  It may
> have been for overlayfs because you guys are already doing something special,
> but it's not an option that is afforded the rest of us.

Overlayfs tries hard not to use st_dev to give uniqueness and instead
partitions the 64bit st_ino space within the same st_dev.  There are
various fallback cases, some involve switching st_dev and some using
non-persistent st_ino.

What overlayfs does may or may not be applicable to btrfs/bcachefs,
but that's not my point.  My point is that adding a flag to statx does
not solve anything.   You can't just say that from now on btrfs
doesn't have use unique st_ino/st_dev because we've just indicated
that in statx and everything is fine.   That will trigger the
no-regressions rule and then it's game over.  At least I would expect
that to happen.

What we can do instead is introduce a new API that is better, and
thankfully we already have one in the form of file handles.  The
problem I see is that you think you can get away with then reverting
back st_dev to be uniform across subvolumes.  But you can't.  I see
two options:

 a) do some hacks, like overlayfs does

 b) introduce a new "st_dev_v2" that will do the right thing and
applications can move over.

Thanks,
Miklos