linux-kernel - Re: [PATCH v2] blk-mq: fix io hung due to missing commit

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Message-ID: <c720cb4f-2e74-b319-c7e2-2672f2890ed7@huaweicloud.com>
Date:   Tue, 26 Jul 2022 14:30:07 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Ming Lei <ming.lei@...hat.com>, Yu Kuai <yukuai1@...weicloud.com>
Cc:     axboe@...nel.dk, osandov@...com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, yi.zhang@...wei.com
Subject: Re: [PATCH v2] blk-mq: fix io hung due to missing commit_rqs

Hi, Ming!

在 2022/07/26 12:21, Ming Lei 写道:
> On Tue, Jul 26, 2022 at 11:35:19AM +0800, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> Currently, in virtio_scsi, if 'bd->last' is not set to true while
>> dispatching request, such io will stay in driver's queue, and driver
>> will wait for block layer to dispatch more rqs. However, if block
>> layer failed to dispatch more rq, it should trigger commit_rqs to
>> inform driver.
>>
>> There is a problem in blk_mq_try_issue_list_directly() that commit_rqs
>> won't be called:
>>
>> // assume that queue_depth is set to 1, list contains two rq
>> blk_mq_try_issue_list_directly
>>   blk_mq_request_issue_directly
>>   // dispatch first rq
>>   // last is false
>>    __blk_mq_try_issue_directly
>>     blk_mq_get_dispatch_budget
>>     // succeed to get first budget
>>     __blk_mq_issue_directly
>>      scsi_queue_rq
>>       cmd->flags |= SCMD_LAST
>>        virtscsi_queuecommand
>>         kick = (sc->flags & SCMD_LAST) != 0
>>         // kick is false, first rq won't issue to disk
>>   queued++
>>
>>   blk_mq_request_issue_directly
>>   // dispatch second rq
>>    __blk_mq_try_issue_directly
>>     blk_mq_get_dispatch_budget
>>     // failed to get second budget
>>   ret == BLK_STS_RESOURCE
>>    blk_mq_request_bypass_insert
>>   // errors is still 0
>>
>>   if (!list_empty(list) || errors && ...)
>>    // won't pass, commit_rqs won't be called
>>
>> In this situation, first rq relied on second rq to dispatch, while
>> second rq relied on first rq to complete, thus they will both hung.
>> And same problem exists in blk_mq_dispatch_rq_list()
>>
>> Fix the problem by also treat 'BLK_STS_*RESOURCE' as 'errors' since
>> it means that request is not queued successfully.
>>
>> Fixes: d666ba98f849 ("blk-mq: add mq_ops->commit_rqs()")
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>> Changes in v2:
>>   - suggested by Ming, handle blk_mq_dispatch_rq_list() as well.
>>   - change title and modify commit message.
>>
>>   block/blk-mq.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 70177ee74295..ee1e065fe63f 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -1909,6 +1909,7 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
>>   			fallthrough;
>>   		case BLK_STS_DEV_RESOURCE:
>>   			blk_mq_handle_dev_resource(rq, list);
>> +			errors++;
>>   			goto out;
>>   		case BLK_STS_ZONE_RESOURCE:
>>   			/*
>> @@ -1918,6 +1919,7 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
>>   			 */
>>   			blk_mq_handle_zone_resource(rq, &zone_list);
>>   			needs_resource = true;
>> +			errors++;
> 
> But accounting error here may break return value of
> blk_mq_dispatch_rq_list(), see:
> 
> /* Returns true if we did some work AND can potentially do more. */

Yes, you're right. I'll try to fix that in v3.

Thanks,
Kuai
>