On Fri, Aug 08, 2014 at 11:03:41AM -0400, Jeff Moyer wrote: > Hi, > > Commit 05f1dd5 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); > > The queue's max is set here: > blk_queue_max_segments(q, vblk->sg_elems-2); > > Basically, what happens is that a bio is built up for the dm device > (which does not have the QUEUE_FLAG_NO_SG_MERGE flag set) using > bio_add_page. That path will call into __blk_recalc_rq_segments, so > what you end up with is bi_phys_segments being much smaller than bi_vcnt > (and bi_vcnt grows beyond the maximum sg elements). Then, when the bio > is submitted, it gets cloned. When the cloned bio is submitted, it will > end up in blk_recount_segments, here: > > if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags)) > bio->bi_phys_segments = bio->bi_vcnt; > > and now we've set bio->bi_phys_segments to a number that is beyond what > was registered as queue_max_segments by the driver. > > The right way to fix this is to propagate the queue flag up the stack. > Attached is a patch that does this, tested and confirmed to fix the > problem in my environment. > > The rules for propagating the flag are simple: > - if the flag is set for any underlying device, it must be set for the > upper device > - consequently, if the flag is not set for any underlying device, it > should not be set for the upper device. > > stable notes: this patch should be applied to 3.16. > > Signed-off-by: Jeff Moyer <jmoyer@xxxxxxxxxx> > <formletter> This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read Documentation/stable_kernel_rules.txt for how to do this properly. </formletter> -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
