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

[Ming Lei <ming.lei@redhat.com>]

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 merge with this request, then the old tables are no longer valid.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

> 
> > 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.

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.

Thanks,
Ming