linux-ext4 - Re: [PATCH 5/8] nowait aio: return on congested block device

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Date:   Mon, 24 Apr 2017 20:28:37 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Christoph Hellwig <hch@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, jack@...e.com,
        linux-block@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        sagi@...mberg.me, avi@...lladb.com, linux-api@...r.kernel.org,
        willy@...radead.org, tom.leiming@...il.com,
        Goldwyn Rodrigues <rgoldwyn@...e.com>
Subject: Re: [PATCH 5/8] nowait aio: return on congested block device

On 04/24/2017 03:10 PM, Goldwyn Rodrigues wrote:
> 
> 
> 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@...e.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?

You are misunderstanding. What Christoph (correctly) says is that request
based drivers do not block, so all of those are fine. What you need to
worry about is drivers that are NOT request based. In other words, drivers
that hook into make_request_fn and process bio's.

-- 
Jens Axboe