Messages in this thread | | | From | Jeff Moyer <> | Subject | Re: [PATCH] blk-merge: fix blk_recount_segments | Date | Tue, 02 Sep 2014 13:01:07 -0400 |
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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
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