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

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Message-ID: <20240222114417.wpcdkgsed7wklv3h@quack3>
Date: Thu, 22 Feb 2024 12:44:17 +0100
From: Jan Kara <jack@...e.cz>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Jan Kara <jack@...e.cz>, Miklos Szeredi <miklos@...redi.hu>,
	Josef Bacik <josef@...icpanda.com>, linux-kernel@...r.kernel.org,
	linux-bcachefs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	lsf-pc@...ts.linux-foundation.org, linux-btrfs@...r.kernel.org
Subject: Re: [Lsf-pc] [LSF TOPIC] statx extensions for subvol/snapshot
 filesystems & more

On Thu 22-02-24 06:27:14, Kent Overstreet wrote:
> On Thu, Feb 22, 2024 at 12:01:38PM +0100, Jan Kara wrote:
> > On Thu 22-02-24 04:42:07, Kent Overstreet wrote:
> > > On Thu, Feb 22, 2024 at 10:14:20AM +0100, Miklos Szeredi wrote:
> > > > 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.
> > > 
> > > Yeah no, you can't crap multiple 64 bit inode number spaces into 64
> > > bits: pigeonhole principle.
> > > 
> > > We need something better than "hacks".
> > 
> > I agree we should have a better long-term plan than finding ways how to
> > cram things into 64-bits inos. However I don't see a realistic short-term
> > solution other than that.
> > 
> > To explicit: Currently, tar and patch and very likely other less well-known
> > tools are broken on bcachefs due to non-unique inode numbers. If you want
> > ot fix them, either you find ways how bcachefs can cram things into 64-bit
> > ino_t or you go and modify these tools (or prod maintainers or whatever) to
> > not depend on ino_t for uniqueness. The application side of things isn't
> > going to magically fix itself by us telling "bad luck, ino_t isn't unique
> > anymore".
> 
> My intent is to make a real effort towards getting better interfaces
> going, prod those maintainers, _then_ look at adding those hacks (that
> will necessarily be short term solutions since 64 bits is already
> looking cramped).

OK, fine by me :) So one thing is still not quite clear to me - how do you
expect the INO_NOT_UNIQUE flag to be used by these apps? Do you expect them
to use st_dev + st_ino by default and fall back to fsid + fhandle only when
INO_NOT_UNIQUE is set?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR