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

Jens Axboe
2018-12-05T03:06:00Z

On 12/4/18 8:03 PM, Ming Lei wrote:
> On Wed, Dec 05, 2018 at 10:58:02AM +0800, Ming Lei wrote:
>> On Tue, Dec 04, 2018 at 07:30:24PM -0700, Jens Axboe wrote:
>>> On 12/4/18 7:27 PM, Ming Lei wrote:
>>>> On Tue, Dec 04, 2018 at 07:16:11PM -0700, 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 merge with this request, then the old tables are no longer valid.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.
>>>>
>>>> discard is kind of write, and it isn't treated very specially in make
>>>> request path, except for multi-range discard.
>>>
>>> The point of direct dispatch is to reduce latencies for requests,
>>> discards are so damn slow on ALL devices anyway that it doesn't make any
>>> sense to try direct dispatch to begin with, regardless of whether it
>>> possible or not.
>>
>> SCSI MQ device may benefit from direct dispatch from reduced lock contention.
>>
>>>
>>>>>> 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.
>>>>
>>>> But marking NOMERGE still may have a hole on re-insert discard request as
>>>> mentioned above.
>>>
>>> What I said was further limit the scope of direct dispatch, which means
>>> not allowing anything that isn't a read/write.
>>
>> IMO, the conservative approach is to take the one used in legacy io
>> path, in which it is never allowed to re-insert queued request to
>> scheduler queue except for requeue, however RQF_DONTPREP is cleared
>> before requeuing request to scheduler.
>>
>>>
>>>> Given we never allow to re-insert queued request to scheduler queue
>>>> except for 6ce3dd6eec1, I think it is the correct thing to do, and the
>>>> fix is simple too.
>>>
>>> As I said, it's not the time to experiment. This issue has been there
>>> since 4.19-rc1. The alternative is yanking both those patches, and then
>>> looking at it later when the direct issue path has been cleaned up
>>> first.
>>
>> The issue should have been there from v4.1, especially after commit
>> f984df1f0f7 ("blk-mq: do limited block plug for multiple queue case"),
>> which is the 1st one to re-insert the queued request into scheduler
>> queue.
>
> But at that time, there isn't io scheduler for MQ, so in theory the
> issue should be there since v4.11, especially 945ffb60c11d ("mq-deadline:
> add blk-mq adaptation of the deadline IO scheduler").

Ming, I'm getting really tired of this. As mentioned in the other email,
we're not having a theoretical or hypothetical debate here. The facts
are on the table, there's no point in trying to shift blame. We need
to deal with the current situation.

--
Jens Axboe