Subject: Re: [PATCH v4 12/14] blk-mq: remove set of bd->last when get driver
 tag for next request fails
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Cc:     ming.lei@redhat.com, Christoph Hellwig <hch@lst.de>,
        axboe@kernel.dk, dwagner@suse.de, hare@suse.de,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        john.garry@huawei.com, jack@suse.cz
References: <20230118093726.3939160-1-shikemeng@huaweicloud.com>
 <20230118093726.3939160-12-shikemeng@huaweicloud.com>
 <20230118174208.GH12399@lst.de>
 <1f116b9b-8194-54c8-eedb-7f2c9cd493c6@huaweicloud.com>
 <0526b518-0894-10f6-e428-c03644d39c02@huaweicloud.com>
Message-ID: <119aeade-151b-e375-f89b-20956bd9f3fb@huaweicloud.com>
Date:   Mon, 6 Feb 2023 09:05:25 +0800
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on 1/28/2023 10:03 AM, Kemeng Shi wrote:
> 
> 
> on 1/19/2023 9:45 AM, Kemeng Shi wrote:
>>
>>
>> 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.
>>
> 
> Hi Ming and everyone familiar with code invovled, could you help with
> reviewing this patch and patch "[PATCH v4 04/14] blk-mq: Fix potential
> io hung for shared sbitmap per tagset" in the same patchset.
> Thanks.
> 
Could anyone please help review the last two patches without reviewed-by.
Thanks.

-- 
Best wishes
Kemeng Shi