Re: [PATCH] block: introduce QUEUE_FLAG_POLL_CAP flag

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

On 4/16/21 5:07 PM, Ming Lei wrote:
> On Fri, Apr 16, 2021 at 04:00:37PM +0800, Jeffle Xu wrote:
>> Hi,
>> How about this patch to remove the extra poll_capable() method?
>>
>> And the following 'dm: support IO polling for bio-based dm device' needs
>> following change.
>>
>> ```
>> +       /*
>> +        * Check for request-based device is remained to
>> +        * dm_mq_init_request_queue()->blk_mq_init_allocated_queue().
>> +        * For bio-based device, only set QUEUE_FLAG_POLL when all underlying
>> +        * devices supporting polling.
>> +        */
>> +       if (__table_type_bio_based(t->type)) {
>> +               if (dm_table_supports_poll(t)) {
>> +                       blk_queue_flag_set(QUEUE_FLAG_POLL_CAP, q);
>> +                       blk_queue_flag_set(QUEUE_FLAG_POLL, q);
>> +               }
>> +               else {
>> +                       blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
>> +                       blk_queue_flag_clear(QUEUE_FLAG_POLL_CAP, q);
>> +               }
>> +       }
>> ```
> 
> Frankly speaking, I don't see any value of using QUEUE_FLAG_POLL_CAP for
> DM, and the result is basically subset of treating DM as always being capable
> of polling.
> 
> Also underlying queue change(either limits or flag) won't be propagated
> to DM/MD automatically. Strictly speaking it doesn't matter if all underlying
> queues are capable of supporting polling at the exact time of 'write sysfs/poll',
> cause any of them may change in future.

Yes it is.

> 
> So why not start with the simplest approach(always capable of polling)
> which does meet normal bio based polling requirement?
> 

I agree if we have no better way. Though the handling of "sysfs/io_poll"
is somehow inconsistent between blk-mq and bio-based device then.

-- 
Thanks,
Jeffle