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

Ming Lei
2018-12-05T02:58:14Z

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 merge with this request, then the old tables are no longer valid.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

Thanks,
Ming