Re: [PATCH 5/8] nowait aio: return on congested block device

From: Goldwyn Rodrigues <rgoldwyn@suse.de>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-fsdevel@vger.kernel.org, jack@suse.com,
	linux-block@vger.kernel.org, linux-btrfs@vger.kernel.org,
	linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org,
	sagi@grimberg.me, avi@scylladb.com, axboe@kernel.dk,
	linux-api@vger.kernel.org, willy@infradead.org,
	tom.leiming@gmail.com, Goldwyn Rodrigues <rgoldwyn@suse.com>
Subject: Re: [PATCH 5/8] nowait aio: return on congested block device
Date: Mon, 24 Apr 2017 16:10:55 -0500	[thread overview]
Message-ID: <3485e23c-3a4d-23a5-50d5-69c824353d1a@suse.de> (raw)
In-Reply-To: <20170419064505.GD20053@infradead.org>

On 04/19/2017 01:45 AM, Christoph Hellwig wrote:
> On Fri, Apr 14, 2017 at 07:02:54AM -0500, Goldwyn Rodrigues wrote:
>> From: Goldwyn Rodrigues <rgoldwyn@suse.com>
>>
> 
>> +	/* Request queue supports BIO_NOWAIT */
>> +	queue_flag_set_unlocked(QUEUE_FLAG_NOWAIT, q);
> 
> BIO_NOWAIT is gone.  And the comment would not be needed if the
> flag had a more descriptive name, e.g. QUEUE_FLAG_NOWAIT_SUPPORT.
> 
> And I think all request based drivers should set the flag implicitly
> as ->queuecommand can't sleep, and ->queue_rq only when it's always
> offloaded to a workqueue when the BLK_MQ_F_BLOCKING flag is set.
> 

We introduced QUEUE_FLAG_NOWAIT for devices which would not wait for
request completions. The ones which wait are MD devices because of sync
or suspend operations.

The only user of BLK_MQ_F_NONBLOCKING seems to be nbd. As you mentioned,
it uses the flag to offload it to a workqueue.

The other way to do it implicitly is to change the flag to
BLK_MAY_BLOCK_REQS and use it for devices which do wait such as md/dm.
Is that what you are hinting at? Or do you have something else in mind?

-- 
Goldwyn