Re: [PATCH] block: Disable write plugging for zoned block devices

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jul 09, 2019 at 06:02:19PM +0900, Damien Le Moal wrote:
> Simultaneously writing to a sequential zone of a zoned block device
> from multiple contexts requires mutual exclusion for BIO issuing to
> ensure that writes happen sequentially. However, even for a well
> behaved user correctly implementing such synchronization, BIO plugging
> may interfere and result in BIOs from the different contextx to be
> reordered if plugging is done outside of the mutual exclusion section,
> e.g. the plug was started by a function higher in the call chain than
> the function issuing BIOs.
> 
>       Context A                           Context B
> 
>    | blk_start_plug()
>    | ...
>    | seq_write_zone()
>      | mutex_lock(zone)
>      | submit_bio(bio-0)
>      | submit_bio(bio-1)
>      | mutex_unlock(zone)
>      | return
>    | ------------------------------> | seq_write_zone()
>   				       | mutex_lock(zone)
> 				       | submit_bio(bio-2)
> 				       | mutex_unlock(zone)
>    | <------------------------------ |
>    | blk_finish_plug()
> 
> In the above example, despite the mutex synchronization resulting in the
> correct BIO issuing order 0, 1, 2, context A BIOs 0 and 1 end up being
> issued after BIO 2 when the plug is released with blk_finish_plug().

I am wondering how you guarantee that context B is always run after
context A.

> 
> To fix this problem, introduce the internal helper function
> blk_mq_plug() to access the current context plug, return the current
> plug only if the target device is not a zoned block device or if the
> BIO to be plugged not a write operation. Otherwise, ignore the plug and
> return NULL, resulting is all writes to zoned block device to never be
> plugged.

Another candidate approach is to run the following code before
releasing 'zone' lock:

	if (current->plug)
		blk_finish_plug(context->plug)

Then we can fix zone specific issue in zone code only, and avoid generic
blk-core change for zone issue.

Thanks,
Ming