Re: [PATCH v2] dm-delay: Prevent zoned write reordering on suspend

[Mikulas Patocka <mpatocka@redhat.com>]

On Fri, 11 Apr 2025, Damien Le Moal wrote:

> When a dm-delay device is being suspended, the .presuspend() operation
> is first executed (delay_presuspend()) to immediately issue all the BIOs
> present in the delayed list of the device and also sets the device
> may_delay boolean to false. At the same time, if any new BIO is issued
> to the device will not be delayed and immediately issued with
> delay_bio() returning DM_MAPIO_REMAPPED. This creates a situation where
> potentially 2 different contexts may be issuing write BIOs to the same
> zone of a zone device without respecting the issuing order from the
> user, that is, a newly issued write BIO may be issued before other write
> BIOs for the same target zone that are in the device delayed list. If
> such situation occurs, write BIOs may be failed by the underlying zoned
> device due to an unaligned write error.
> 
> Prevent this situation from happening by always handling newly issued
> write BIOs using the delayed list of BIOs, even when the device is being
> suspended. This is done by forcing the use of the worker kthread for
> zoned devices, and by modifying flush_worker_fn() to always flush all
> delayed BIOs if the device may_delay boolean is false.
> 
> Reported-by: Benjamin Marzinski <bmarzins@redhat.com>
> Fixes: d43929ef65a6 ("dm-delay: support zoned devices")
> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
> ---
> Changes from v1:
>  - Fixed typo in commit message
>  - Added reported-by tag
> 
>  drivers/md/dm-delay.c | 29 +++++++++++++++++++++--------
>  1 file changed, 21 insertions(+), 8 deletions(-)

Hi

I looked at the generic device mapper code and it seems that ordering of 
write bios is not guaranteed with any target in case of suspend/resume.

* we suspend the device:
* received bios are added to md->deferred in queue_io

* we resume the device:
* __dm_resume calls dm_queue_flush
* dm_queue_flush clears DMF_BLOCK_IO_FOR_SUSPEND and submits work item 
  &md->work (dm_wq_work)
* dm_resume clears DMF_SUSPENDED
* the device starts accepting new bios in dm_submit_bio
* dm_wq_work runs concurrently with new bios that are received, so 
  ordering of bios is not preserved

So it doesn't make much sense to try to fix it in dm-delay, if it isn't 
supposed to work at all.

Mikulas