linux-kernel - Re: [PATCH v4 12/14] blk-mq: remove set of bd->last when get driver tag for next request fails

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Date:   Thu, 19 Jan 2023 09:45:04 +0800
From:   Kemeng Shi <shikemeng@...weicloud.com>
To:     Christoph Hellwig <hch@....de>
Cc:     axboe@...nel.dk, dwagner@...e.de, hare@...e.de,
        ming.lei@...hat.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, john.garry@...wei.com, jack@...e.cz
Subject: Re: [PATCH v4 12/14] blk-mq: remove set of bd->last when get driver
 tag for next request fails



on 1/19/2023 1:42 AM, Christoph Hellwig wrote:
> On Wed, Jan 18, 2023 at 05:37:24PM +0800, Kemeng Shi wrote:
>> Commit 113285b473824 ("blk-mq: ensure that bd->last is always set
>> correctly") will set last if we failed to get driver tag for next
>> request to avoid flush miss as we break the list walk and will not
>> send the last request in the list which will be sent with last set
>> normally.
>> This code seems stale now becase the flush introduced is always
>> redundant as:
>> For case tag is really out, we will send a extra flush if we find
>> list is not empty after list walk.
>> For case some tag is freed before retry in blk_mq_prep_dispatch_rq for
>> next, then we can get a tag for next request in retry and flush notified
>> already is not necessary.
> 
> I think Ming will know this code better than me, but aren't we
> losing the blk_mq_get_driver_tag call entirely here now.  Where
> is getting the driver tag covered now?
> 
We will get driver tag in blk_mq_prep_dispatch_rq at beginning of dispatch
loop, so it's fine to remove blk_mq_get_driver_tag here. Thanks.
-- 
Best wishes
Kemeng Shi