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

Jens Axboe
2018-12-05T02:16:17Z

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 merge with this request, then the old tables are no longer valid.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.

--
Jens Axboe