Re: [PATCH] md: remove REQ_NOWAIT support

[Jens Axboe]

On 7/30/26 4:50 AM, yu kuai wrote:
> Hi,
> 
> ? 2026/6/28 22:27, Abd-Alrhman Masalkhi ??:
>> REQ_NOWAIT support in md is fundamentally incomplete. While reads can
>> avoid some blocking paths, write requests can still encounter cases
>> where one mirror succeeds while another returns -EAGAIN. At that point
>> md cannot distinguish queue pressure from a real device failure, so it
>> can neither record a bad block nor safely retry the write without
>> REQ_NOWAIT, leaving mirrors with divergent data.
>>
>> Rather than continue advertising REQ_NOWAIT support that cannot be
>> implemented correctly, remove it from md and its raid personalities.
>> This simplifies the waiting paths and avoids exposing incorrect
>> non-blocking semantics.
>>
>> Fixes: bf2c411bb1cf ("md: raid456 add nowait support")
>> Fixes: c9aa889b035f ("md: raid10 add nowait support")
>> Fixes: 5aa705039c4f ("md: raid1 add nowait support")
>> Fixes: f51d46d0e7cb ("md: add support for REQ_NOWAIT")
>> Suggested-by: Yu Kuai<yukuai@xxxxxxx>
>> Signed-off-by: Abd-Alrhman Masalkhi<abd.masalkhi@xxxxxxxxx>
>> ---
>>   drivers/md/md-bitmap.c   |  9 +---
>>   drivers/md/md-bitmap.h   |  2 +-
>>   drivers/md/md-llbitmap.c | 10 +----
>>   drivers/md/md.c          | 15 ++-----
>>   drivers/md/raid1-10.c    |  8 ++--
>>   drivers/md/raid1.c       | 91 +++++++++-------------------------------
>>   drivers/md/raid10.c      | 80 +++++++++--------------------------
>>   drivers/md/raid5.c       | 13 ------
>>   8 files changed, 51 insertions(+), 177 deletions(-)
> 
> Applied to md-7.3 with a few changes, please check:
> 
> md: remove REQ_NOWAIT support from raid1/10/456 - 
> kernel/git/mdraid/linux.git - song's fork of linux.git <https://git.kernel.org/pub/scm/linux/kernel/git/mdraid/linux.git/commit/?h=md-7.3&id=3fe5b7c9fb72ccc29bfd0f955b124892af7e3674>
> 
> - rebase raid1/raid10 with latest code;
> - keep nowait for raid0/linear;
> - keep nowait for md_handle_request, as raid0/linear will need it;

What's the plan for making REQ_NOWAIT _actually_ work with md? Just
yanking it wholesale is a massive performance problem, and it will
regress workloads. Has any time been spent on making this more solid?

-- 
Jens Axboe