Re: [dm-devel] dm: add support for DM_TARGET_NOWAIT for various targets

[JeffleXu <jefflexu@linux.alibaba.com>]

On 11/12/20 2:05 PM, JeffleXu wrote:
>>
>> dm-table.c:dm_table_set_restrictions() has:
>>
>>          if (dm_table_supports_nowait(t))
>>                  blk_queue_flag_set(QUEUE_FLAG_NOWAIT, q);
>>          else
>>                  blk_queue_flag_clear(QUEUE_FLAG_NOWAIT, q);
>>
>>> This patch adds support for DM_TARGET_NOWAIT for those dm targets, the
>>> .map() algorithm of which just involves sector recalculation.
>> So you're looking to constrain which targets will properly support
>> REQ_NOWAIT, based on whether they do a simple remapping?
>
> To be honest, I'm a little confused about the semantics of REQ_NOWAIT. 
> Jens may had ever
>
> explained it in block or io_uring mailing list, but I can't find the 
> specific mail.
>
I find it here 
https://lore.kernel.org/linux-block/f1a6ae88-1436-e947-1124-41e10b3ea9bc@kernel.dk/


So if the IO is offloaded to workqueue and the current process context 
will not get blocked,

then is this device capable of handling REQ_NOWAIT or not?

-- 
Thanks,
Jeffle