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

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Message-ID: <yaq7zpqe3x67igm6kvzbs4jczgd7pqhx733jha4fmzihr6e67c@pm5nfkpabtk7>
Date: Thu, 22 Feb 2024 06:19:19 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: Josef Bacik <josef@...icpanda.com>, 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 Thu, Feb 22, 2024 at 11:25:12AM +0100, Miklos Szeredi wrote:
> On Thu, 22 Feb 2024 at 10:42, Kent Overstreet <kent.overstreet@...ux.dev> wrote:
> 
> > Yeah no, you can't crap multiple 64 bit inode number spaces into 64
> > bits: pigeonhole principle.
> 
> Obviously not.  And I have no idea about the inode number allocation
> strategy of bcachefs and how many bits would be needed for subvolumes,
> etc..   I was just telling what overlayfs does and why.  It's a
> pragmatic solution that works.  I'd very much like to move to better
> interfaces, but creating good interfaces is never easy.

You say "creating good interfaces is never easy" - but we've got a
proposal, that's bounced around a fair bit, and you aren't saying
anything concrete.

> > We need something better than "hacks".
> 
> That's the end goal, obviously.   But we also need to take care of
> legacy.  Always have.

So what are you proposing?

> > This isn't a serious proposal.
> 
> If not, then what is?
> 
> BTW to expand on the st_dev_v2 idea, it can be done by adding a
> STATX_DEV_V2 query mask.

Didn't you see Josef just say they're trying to get away from st_dev?

> The other issue is adding subvolume ID.  You seem to think that it's
> okay to add that to statx and let userspace use (st_ino, st_subvoid)
> to identify the inode.  I'm saying this is wrong, because it doesn't
> work in the general case.

No, I explicitly said that when INO_NOT_UNIQUE is set the _filehandle_
would be the means to identify the file.