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Subject: Re: [PATCH] blk-mq: insert rq with DONTPREP to hctx dispatch list
 when requeue
From:   Jens Axboe <axboe@kernel.dk>
To:     Jianchao Wang <jianchao.w.wang@oracle.com>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <1549863665-1691-1-git-send-email-jianchao.w.wang@oracle.com>
 <937e9207-dcf7-dab5-1917-8a10303edc67@kernel.dk>
Message-ID: <82deee13-a4ff-9bf5-5e01-0e4df01b7ff7@kernel.dk>
Date:   Mon, 11 Feb 2019 16:15:44 -0700
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On 2/11/19 8:59 AM, Jens Axboe wrote:
> On 2/10/19 10:41 PM, Jianchao Wang wrote:
>> When requeue, if RQF_DONTPREP, rq has contained some driver
>> specific data, so insert it to hctx dispatch list to avoid any
>> merge. Take scsi as example, here is the trace event log (no
>> io scheduler, because RQF_STARTED would prevent merging),
>>
>>    kworker/0:1H-339   [000] ...1  2037.209289: block_rq_insert: 8,0 R 4096 () 32768 + 8 [kworker/0:1H]
>> scsi_inert_test-1987  [000] ....  2037.220465: block_bio_queue: 8,0 R 32776 + 8 [scsi_inert_test]
>> scsi_inert_test-1987  [000] ...2  2037.220466: block_bio_backmerge: 8,0 R 32776 + 8 [scsi_inert_test]
>>    kworker/0:1H-339   [000] ....  2047.220913: block_rq_issue: 8,0 R 8192 () 32768 + 16 [kworker/0:1H]
>> scsi_inert_test-1996  [000] ..s1  2047.221007: block_rq_complete: 8,0 R () 32768 + 8 [0]
>> scsi_inert_test-1996  [000] .Ns1  2047.221045: block_rq_requeue: 8,0 R () 32776 + 8 [0]
>>    kworker/0:1H-339   [000] ...1  2047.221054: block_rq_insert: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
>>    kworker/0:1H-339   [000] ...1  2047.221056: block_rq_issue: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
>> scsi_inert_test-1986  [000] ..s1  2047.221119: block_rq_complete: 8,0 R () 32776 + 8 [0]
>>
>> (32768 + 8) was requeued by scsi_queue_insert and had RQF_DONTPREP.
>> Then it was merged with (32776 + 8) and issued. Due to RQF_DONTPREP,
>> the sdb only contained the part of (32768 + 8), then only that part
>> was completed. The lucky thing was that scsi_io_completion detected
>> it and requeued the remaining part. So we didn't get corrupted data.
>> However, the requeue of (32776 + 8) is not expected.
> 
> Good catch, but how about something like this? Makes it more integrated,
> I think that's cleaner.

This is probably better (and safer):


diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8f5b533764ca..b3908eb3881c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -737,12 +737,21 @@ static void blk_mq_requeue_work(struct work_struct *work)
 	spin_unlock_irq(&q->requeue_lock);
 
 	list_for_each_entry_safe(rq, next, &rq_list, queuelist) {
-		if (!(rq->rq_flags & RQF_SOFTBARRIER))
+		if (!(rq->rq_flags & (RQF_SOFTBARRIER | RQF_DONTPREP)))
 			continue;
 
 		rq->rq_flags &= ~RQF_SOFTBARRIER;
 		list_del_init(&rq->queuelist);
-		blk_mq_sched_insert_request(rq, true, false, false);
+
+		/*
+		 * If RQF_DONTPREP is set, rq may contain some driver
+		 * specific data. Insert it to hctx dispatch list to avoid
+		 * any merge.
+		 */
+		if (rq->rq_flags & RQF_DONTPREP)
+			blk_mq_sched_insert_request(rq, true, false, false);
+		else
+			blk_mq_request_bypass_insert(rq, false);
 	}
 
 	while (!list_empty(&rq_list)) {

-- 
Jens Axboe