Re: [PATCH] blk-merge: fix blk_recount_segments

Jeff Moyer
2014-09-02T17:01:15Z

Jens Axboe <axboe@kernel.dk> writes:

> On 09/02/2014 09:02 AM, Ming Lei wrote:
>> QUEUE_FLAG_NO_SG_MERGE is set at default for blk-mq devices,
>> so bio->bi_phys_segment computed may be bigger than
>> queue_max_segments(q) for blk-mq devices, then drivers will
>> fail to handle the case, for example, BUG_ON() in
>> virtio_queue_rq() can be triggerd for virtio-blk:
>>
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1359146
>>
>> This patch fixes the issue by ignoring the QUEUE_FLAG_NO_SG_MERGE
>> flag if the computed bio->bi_phys_segment is bigger than
>> queue_max_segments(q), and the regression is caused by commit
>> 05f1dd53152173(block: add queue flag for disabling SG merging).
>>
>> Reported-by: Kick In <pierre-andre.morey@canonical.com>
>> Tested-by: Chris J Arges <chris.j.arges@canonical.com>
>> Signed-off-by: Ming Lei <ming.lei@canonical.com>
>
> Thanks Ming, this looks nice and clean. Will apply for 3.17.

This sounds an awful lot like the bug I fixed in dm:

commit 200612ec33e555a356eebc717630b866ae2b694f
Author: Jeff Moyer <jmoyer@redhat.com>
Date: Fri Aug 8 11:03:41 2014 -0400

dm table: propagate QUEUE_FLAG_NO_SG_MERGE

Commit 05f1dd5 ("block: add queue flag for disabling SG merging")
introduced a new queue flag: QUEUE_FLAG_NO_SG_MERGE. This gets set by
default in blk_mq_init_queue for mq-enabled devices. The effect of
the flag is to bypass the SG segment merging. Instead, the
bio->bi_vcnt is used as the number of hardware segments.

With a device mapper target on top of a device with
QUEUE_FLAG_NO_SG_MERGE set, we can end up sending down more segments
than a driver is prepared to handle. I ran into this when backporting
the virtio_blk mq support. It triggerred this BUG_ON, in
virtio_queue_rq:

BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);

...

Is bcache a stacking driver in this case? Should it not inherit this
flag, just as DM now does?

-Jeff