Re: [PATCH for-next] dm: split write BIOs on zone boundaries when zone append is not emulated

[Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>]

On Jul 17, 2025 / 17:18, Damien Le Moal wrote:
> On 2025/07/17 11:32, Shin'ichiro Kawasaki wrote:
> > Since commit f70291411ba2 ("block: Introduce
> > bio_needs_zone_write_plugging()"), blk_mq_submit_bio() no longer splits
> > write BIOs to comply with the requirements of zoned block devices. This
> > change applies to write BIOs issued by zoned DM targets. Following this
> > modification, it is expected that DM targets themselves split write BIOs
> > for zoned block devices.
> 
> This description is confusing: if the underlying device of a DM target is a
> blk-mq block device, then blk_mq_submit_bio() will be called and will split BIOs
> as needed, regardless of what the DM target did. For BIOs that are issued by the
> user/FS to the DM device, then that is when bio_needs_zone_write_plugging() is
> used by DM core to determine if a BIO must be split.
> 
> Anyway, I do not think you need to reference this commit. So I would drop this
> entire paragraph from the commit message, and start the explanation with the
> below paragraph. That is very clear I think.
> 
> > Commit 2df7168717b7 ("dm: Always split write BIOs to zoned device
> > limits") updates the device-mapper driver to perform splits for the
> > write BIOs. However, it did not address the cases where DM targets do
> > not emulate zone append, such as in the cases of dm-linear or dm-flakey.
> > For these targets, when the write BIOs span across zone boundaries, they
> > trigger WARN_ON_ONCE(bio_straddles_zones(bio)) in
> > blk_zone_wplug_handle_write(). This results in I/O errors. The errors
> > are reproduced by running blktests test case zbd/004 using zoned
> > dm-linear or dm-flakey devices.
> > 
> > To avoid the I/O errors, handle the write BIOs regardless whether DM
> > targets emulate zone append or not, so that all write BIOs are split at
> > zone boundaries. For that purpose, drop the check for zone append
> > emulation in dm_zone_bio_needs_split(). Its argument 'md' is no longer
> > used then drop it also.
> > 
> > Fixes: 2df7168717b7 ("dm: Always split write BIOs to zoned device limits")
> > Cc: stable@vger.kernel.org
> 
> The above patch is queued in block-next, so there is no need for this CC-stable.
> Remove it please.
> 
> > Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
> 
> One more nit below.
> 
> With these fixed, feel free to add:
> 
> Reviewed-by: Damien Le Moal <dlemoal@kernel.org>

Thanks. I will reflect your comments and send out v2 soon.