Re: [PATCH] xfs: validate that zoned RT devices are zone aligned

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Dec 10, 2025 at 05:54:38PM +0100, Christoph Hellwig wrote:
> On Wed, Dec 10, 2025 at 08:48:59AM -0800, Darrick J. Wong wrote:
> > mkfs doesn't enforce that when you're creating a zoned filesystem on
> > non-zoned storage:
> ...
> > (The mkfs enforcement does work if you have an actual zoned storage
> > device since mkfs complains about changes in the zone sizes.)
> 
> Ugg, and I thought only my horrible hacks caused that..
> 
> > 
> > > to avoid getting into trouble due to fuzzers or mkfs bugs.
> > >
> > > Fixes: 2167eaabe2fa ("xfs: define the zoned on-disk format")
> > > Signed-off-by: Christoph Hellwig <hch@lst.de>
> > 
> > How many filesystems are there in the wild with rump rtgroups?
> 
> I suspect very, very few as zoned mode on non-zoned devices is not a
> widely advertised feature, and then you'd also need wiredly sized device
> or manual override to get it.  And then scrub would complain about it.

<nod>

> > Given that runt zoned rtgroups can exist in the wild, how hard would it
> > be to fix zonegc?
> 
> Very nasty.  We can't ever GC into one.

How nasty is it, exactly?  AFAICT,

 * The zone targetting code (aka the zone we copy into) then has to
   know to avoid a runt endzone?

 * Thresholding gets weird because they don't apply right to the runt
   zone, which means the victim selection is also off.

 * The code that reserves zones for gc or other ENOSPC handling then has
   to ensure it never picks a runt zone to avoid corner case problems

Any other reasons?  Given that zoned is still experimental I think I'm
ok with adding this restriction, but only after some more thorough
understanding. :)

Also does growfs need patching so that it doesn't create a runt zone?

--D