[PATCH 3/4] btrfs: zoned: fail mount if the device does not support zone append

[David Sterba]

On Mon, Apr 19, 2021 at 09:46:36AM +0000, Damien Le Moal wrote:
> On 2021/04/19 18:41, hch at infradead.org wrote:
> > On Mon, Apr 19, 2021 at 09:35:37AM +0000, Damien Le Moal wrote:
> >> This is only to avoid someone from running zoned-btrfs on top of dm-crypt.
> >> Without this patch, mount will be OK and file data writes will also actually be
> >> OK. But all reads will miserably fail... I would rather have this patch in than
> >> deal with the "bug reports" about btrfs failing to read files. No ?
> >>
> >> Note that like you, I dislike having to add such code. But it was my oversight
> >> when I worked on getting dm-crypt to work on zoned drives. Zone append was
> >> overlooked at that time... My bad, really.
> > 
> > dm-crypt needs to stop pretending it supports zoned devices if it
> > doesn't.  Note that dm-crypt could fairly trivially support zone append
> > by doing the same kind of emulation that the sd driver does.
> 
> I am not so sure about the "trivial" but yes, it is feasible. Let me think about
> something then. Whatever we do, performance with ZNS will no be great, for
> sure... But for SMR HDDs, we likely will not notice any difference in performance.

So this needs to be fixed outside of btrfs. The fix in btrfs would make
sense in case we can't sync the dm-crypt and btrfs in a released kernel.
Having a mount check sounds like a better option to me than to fail
reads, we can revert it in a release once everything woks as expected.