From 200612ec33e555a356eebc717630b866ae2b694f Mon Sep 17 00:00:00 2001 From: Jeff Moyer Date: Fri, 8 Aug 2014 11:03:41 -0400 Subject: 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 Signed-off-by: Mike Snitzer Cc: stable@vger.kernel.org # 3.16+ --- drivers/md/dm-table.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c index 3c72bf1..f9c6cb8 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) { @@ -1501,6 +1509,11 @@ 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_clear_unlocked(QUEUE_FLAG_NO_SG_MERGE, q); + else + queue_flag_set_unlocked(QUEUE_FLAG_NO_SG_MERGE, q); + dm_table_set_integrity(t); /* -- cgit v1.1