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+
1 file changed
