Re: [PATCH] blk-mq: fix corruption with direct issue

[Jens Axboe <axboe@kernel.dk>]

On 12/4/18 7:16 PM, Jens Axboe wrote:
> On 12/4/18 6:37 PM, Ming Lei wrote:
>> On Tue, Dec 04, 2018 at 03:47:46PM -0700, Jens Axboe wrote:
>>> If we attempt a direct issue to a SCSI device, and it returns BUSY, then
>>> we queue the request up normally. However, the SCSI layer may have
>>> already setup SG tables etc for this particular command. If we later
>>> merge with this request, then the old tables are no longer valid. Once
>>> we issue the IO, we only read/write the original part of the request,
>>> not the new state of it.
>>>
>>> This causes data corruption, and is most often noticed with the file
>>> system complaining about the just read data being invalid:
>>>
>>> [  235.934465] EXT4-fs error (device sda1): ext4_iget:4831: inode #7142: comm dpkg-query: bad extra_isize 24937 (inode size 256)
>>>
>>> because most of it is garbage...
>>>
>>> This doesn't happen from the normal issue path, as we will simply defer
>>> the request to the hardware queue dispatch list if we fail. Once it's on
>>> the dispatch list, we never merge with it.
>>>
>>> Fix this from the direct issue path by flagging the request as
>>> REQ_NOMERGE so we don't change the size of it before issue.
>>>
>>> See also:
>>>   https://bugzilla.kernel.org/show_bug.cgi?id=201685
>>>
>>> Fixes: 6ce3dd6eec1 ("blk-mq: issue directly if hw queue isn't busy in case of 'none'")
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>>
>>> ---
>>>
>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>> index 3f91c6e5b17a..d8f518c6ea38 100644
>>> --- a/block/blk-mq.c
>>> +++ b/block/blk-mq.c
>>> @@ -1715,6 +1715,15 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
>>>  		break;
>>>  	case BLK_STS_RESOURCE:
>>>  	case BLK_STS_DEV_RESOURCE:
>>> +		/*
>>> +		 * If direct dispatch fails, we cannot allow any merging on
>>> +		 * this IO. Drivers (like SCSI) may have set up permanent state
>>> +		 * for this request, like SG tables and mappings, and if we
>>> +		 * merge to it later on then we'll still only do IO to the
>>> +		 * original part.
>>> +		 */
>>> +		rq->cmd_flags |= REQ_NOMERGE;
>>> +
>>>  		blk_mq_update_dispatch_busy(hctx, true);
>>>  		__blk_mq_requeue_request(rq);
>>>  		break;
>>>
>>
>> Not sure it is enough to just mark it as NOMERGE, for example, driver
>> may have setup the .special_vec for discard, and NOMERGE may not prevent
>> request from entering elevator queue completely. Cause 'rq.rb_node' and
>> 'rq.special_vec' share same space.
> 
> We should rather limit the scope of the direct dispatch instead. It
> doesn't make sense to do for anything but read/write anyway.
> 
>> So how about inserting this request via blk_mq_request_bypass_insert()
>> in case that direct issue returns BUSY? Then it is invariant that
>> any request queued via .queue_rq() won't enter scheduler queue.
> 
> I did consider this, but I didn't want to experiment with exercising
> a new path for an important bug fix. You do realize that your original
> patch has been corrupting data for months? I think a little caution
> is in order here.

Here's a further limiting version. And we seriously need to clean up the
direct issue paths, it's ridiculous.


diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3f91c6e5b17a..3262d83b9e07 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1715,6 +1715,15 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
 		break;
 	case BLK_STS_RESOURCE:
 	case BLK_STS_DEV_RESOURCE:
+		/*
+		 * If direct dispatch fails, we cannot allow any merging on
+		 * this IO. Drivers (like SCSI) may have set up permanent state
+		 * for this request, like SG tables and mappings, and if we
+		 * merge to it later on then we'll still only do IO to the
+		 * original part.
+		 */
+		rq->cmd_flags |= REQ_NOMERGE;
+
 		blk_mq_update_dispatch_busy(hctx, true);
 		__blk_mq_requeue_request(rq);
 		break;
@@ -1727,6 +1736,18 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
 	return ret;
 }
 
+/*
+ * Don't allow direct dispatch of anything but regular reads/writes,
+ * as some of the other commands can potentially share request space
+ * with data we need for the IO scheduler. If we attempt a direct dispatch
+ * on those and fail, we can't safely add it to the scheduler afterwards
+ * without potentially overwriting data that the driver has already written.
+ */
+static bool blk_rq_can_direct_dispatch(struct request *rq)
+{
+	return req_op(rq) == REQ_OP_READ || req_op(rq) == REQ_OP_WRITE;
+}
+
 static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 						struct request *rq,
 						blk_qc_t *cookie,
@@ -1748,7 +1769,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 		goto insert;
 	}
 
-	if (q->elevator && !bypass_insert)
+	if (!blk_rq_can_direct_dispatch(rq) || (q->elevator && !bypass_insert))
 		goto insert;
 
 	if (!blk_mq_get_dispatch_budget(hctx))
@@ -1810,6 +1831,9 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
 		struct request *rq = list_first_entry(list, struct request,
 				queuelist);
 
+		if (!blk_rq_can_direct_dispatch(rq))
+			break;
+
 		list_del_init(&rq->queuelist);
 		ret = blk_mq_request_issue_directly(rq);
 		if (ret != BLK_STS_OK) {

-- 
Jens Axboe