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authorJeff Moyer <jmoyer@redhat.com>2014-08-08 11:03:41 -0400
committerMike Snitzer <snitzer@redhat.com>2014-08-10 20:54:49 -0400
commit200612ec33e555a356eebc717630b866ae2b694f (patch)
tree56f9ed8e6816988f64d375e9ba15f757641eed58 /Documentation
parent56b1ebf2d9798257c5932c8a0dd9da16796dbf36 (diff)
downloadop-kernel-dev-200612ec33e555a356eebc717630b866ae2b694f.zip
op-kernel-dev-200612ec33e555a356eebc717630b866ae2b694f.tar.gz
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); 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. 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. Signed-off-by: Jeff Moyer <jmoyer@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Cc: stable@vger.kernel.org # 3.16+
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