Re: [Report] blk-zoned/ZNS: non_power_of_2 of zone->len]

[Ming Lei <ming.lei@redhat.com>]

On Fri, Jan 12, 2024 at 12:05:45PM +0900, Damien Le Moal wrote:
> On 1/12/24 10:13, Ming Lei wrote:
> > Hello Damien and Guys,
> > 
> > Yi reported that the following failure:
> > 
> > Oct 18 15:24:15 localhost kernel: nvme nvme4: invalid zone size:196608 for namespace:1
> > Oct 18 15:24:33 localhost smartd[2303]: Device: /dev/nvme4, opened
> > Oct 18 15:24:33 localhost smartd[2303]: Device: /dev/nvme4, NETAPPX4022S173A4T0NTZ, S/N:S66NNE0T800169, FW:MVP40B7B, 4.09 TB
> > 
> > Looks current blk-zoned requires zone->len to be power_of_2() since
> > commit:
> > 
> > 6c6b35491422 ("block: set the zone size in blk_revalidate_disk_zones atomically")
> > 
> > And the original power_of_2() requirement is from the following commit
> > for ZBC and ZAC.
> > 
> > d9dd73087a8b ("block: Enhance blk_revalidate_disk_zones()")
> > 
> > Meantime block layer does support non-power_of_2 chunk sectors limit.
> 
> That is not true. It does. See blk_stack_limits which ahs:
> 
> 	/* Set non-power-of-2 compatible chunk_sectors boundary */
>         if (b->chunk_sectors)
>                 t->chunk_sectors = gcd(t->chunk_sectors, b->chunk_sectors);
> 
> and the absence of any check on the value of chunk_sectors in
> blk_queue_chunk_sectors().

I meant non-power_of_2 chunk sectors limit is supported, see

07d098e6bbad ("block: allow 'chunk_sectors' to be non-power-of-2")

And device mapper uses that.

> 
> > The question is if there is such hard requirement for ZNS, and I can't see
> > any such words in NVMe Zoned Namespace Command Set Specification.
> 
> No, there are no requirements in ZNS for the zone size to be a power of 2 number
> of sectors/LBAs. The same is also true for ZBC and ZAC (SCSI and ATA) SMR HDDs.
> The requirement for the zone size to be a power of 2 number of sectors is
> entirely in the kernel. The reason being that zoned block device support started
> with SMR HDDs which all had a zone size of 256 MB (and still do) and no user
> ever wanted anything else than that. So everything was coded with this
> requirement, as that allowed many nice things like bit-shift/mask arithmetic for
> conversions between zone number and sectors etc (and that of course is very
> efficient).

Thanks for the clarification.

> 
> > So is it one NVMe firmware issue? or blk-zoned problem with too strict(power_of_2)
> > requirement on zone->len?
> 
> It is the latter. There was a session at LSF/MM last year about this. I recall
> that the conclusion was that unless there is a strong user demand for non power
> of 2 zone size, we are not going to do anything about it. Because allowing
> non-power of 2 zone size has some serious consequences all over the place,
> including in FSes that natively support zoned devices. So relaxing that
> requirement is not trivial.

Just saw Bart's work on supporting non-power_of_2 zone len:

https://lore.kernel.org/linux-block/dc89c70e-4931-baaf-c450-6801c200c1d7@acm.org/

IMO FS support might be another topic, cause FS isn't the only user,
also without block layer support, the device isn't usable, not mention FS.

Since non-power2 zoned device does exists, I'd suggest Bart to restart the
work and let linux cover more zoned devices(include non-power 2 zone).


Thanks,
Ming