Re: next-20260917 and next-20260918: merge of the block tree breaks all zoned devices

[Stephen Rothwell <sfr@canb.auug.org.au>]

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Hi Tao,

On Sun, 20 Sep 2026 20:58:34 +0800 Tao Cui <cui.tao@linux.dev> wrote:
>
> Hi Stephen,

I no longer run linux-next, please email Mark Brown (cc'd).

[The rest kept for Mark's benefit]

> I ran into this while testing a blk-iocost series of mine (charging
> zone appends as writes) against a zoned null_blk: the device simply
> does not show up, whether configured through module parameters or
> configfs.  null_blk prints "using native zone append" and then
> add_disk() fails silently with -ENODEV.
> 
> My series only touches blk-iocost.c, and the failure reproduces on
> plain linux-next, so it is not mine.  Bisecting over the tags (qemu,
> "null_blk.zoned=1 null_blk.gb=1 null_blk.zone_size=64" on the command
> line) narrows it down to one day:
> 
>   next-20260916: nullb0 created, /sys/block/nullb0/queue/zoned = host-managed
>   next-20260917: no device, add_disk() fails
>   next-20260918: same failure
> 
> The only blk-zoned.c changes between next-20260916 and next-20260917
> come in through the merge of the block tree, and next-20260918 repeats
> the same resolution in the same place:
> 
>   66c8b6b56c09 ("Merge branch 'for-next' of .../axboe/linux.git", 2026-09-17)
>   a9ce2250a714 ("Merge branch 'for-next' of .../axboe/linux.git", 2026-09-18)
> 
> As far as I can tell the resolved disk_revalidate_capacity() matches
> neither parent: it gains a
> 
>   if (args->capacity >= args->nr_zones)
> 
> check that compares a sector count against a zone count, with
> args->nr_zones still 0 on the first call, so it always trips and
> blk_revalidate_disk_zones() always returns -ENODEV.  The zone_sectors
> power-of-two check that both parents have is gone as well.  This hits
> every zoned device, not just null_blk.
> 
> Taking the axboe side of the function makes nullb0 come back on
> next-20260918 here (host-managed, chunk_sectors 131072), so it looks
> like the merge should just have taken that side.
> 
> Same conflict, same bad outcome twice in a row now, so tomorrow's
> merge may hit it again unless the block tree has moved on in this
> area.
> 
> Regards,
> Tao
> 

-- 
Cheers,
Stephen Rothwell

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