[patch] dm: propagate QUEUE_FLAG_NO_SG_MERGE

From: Jeff Moyer
Date: Fri Aug 08 2014 - 11:03:50 EST


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

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>

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 5f59f1e..bf756d1 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -1386,6 +1386,14 @@ static int device_is_not_random(struct dm_target *ti, struct dm_dev *dev,
return q && !blk_queue_add_random(q);

+static int queue_supports_sg_merge(struct dm_target *ti, struct dm_dev *dev,
+ sector_t start, sector_t len, void *data)
+ struct request_queue *q = bdev_get_queue(dev->bdev);
+ return q && !test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags);
static bool dm_table_all_devices_attribute(struct dm_table *t,
iterate_devices_callout_fn func)
@@ -1464,6 +1472,9 @@ void dm_table_set_restrictions(struct dm_table *t, struct request_queue *q,
if (!dm_table_supports_write_same(t))
q->limits.max_write_same_sectors = 0;

+ if (!dm_table_all_devices_attribute(t, queue_supports_sg_merge))
+ queue_flag_set_unlocked(QUEUE_FLAG_NO_SG_MERGE, q);

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