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Subject[PATCH] virtio_blk: fix defaults for max_hw_sectors and max_segment_size
virtio_blk incorrectly established -1U as the default for these
queue_limits. Set these limits to sane default values to avoid crashing
the kernel. But the virtio-blk protocol should probably be extended to
allow proper stacking of the disk's limits from the host.

This change fixes a crash that was reported when virtio-blk was used to
test linux-dm.git commit 604ea90641b4 ("dm thin: adjust max_sectors_kb
based on thinp blocksize") that will initially set max_sectors to
max_hw_sectors and then rounddown to the first power-of-2 factor of the
DM thin-pool's blocksize. Basically that commit assumes drivers don't
suck when establishing max_hw_sectors so it acted like a canary in the
coal mine.

In the case of a DM thin-pool built ontop of virtio-blk data device
these are the insane limits that were established for the DM thin-pool:

# cat /sys/block/dm-6/queue/max_sectors_kb
# cat /sys/block/dm-6/queue/max_hw_sectors_kb

by stacking the virtio-blk device's limits:

# cat /sys/block/vdb/queue/max_sectors_kb
# cat /sys/block/vdb/queue/max_hw_sectors_kb

Attempting to mkfs.xfs against a thin device from this thin-pool quickly
resulted in fs/direct-io.c:dio_send_cur_page()'s BUG_ON.

Signed-off-by: Mike Snitzer <>
drivers/block/virtio_blk.c | 9 ++++++---
1 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index c6a27d5..68efbdc 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -674,8 +674,11 @@ static int virtblk_probe(struct virtio_device *vdev)
/* No need to bounce any requests */
blk_queue_bounce_limit(q, BLK_BOUNCE_ANY);

- /* No real sector limit. */
- blk_queue_max_hw_sectors(q, -1U);
+ /*
+ * Limited by disk's max_hw_sectors in host, but
+ * without that info establish a sane default.
+ */
+ blk_queue_max_hw_sectors(q, BLK_DEF_MAX_SECTORS);

/* Host can optionally specify maximum segment size and number of
* segments. */
@@ -684,7 +687,7 @@ static int virtblk_probe(struct virtio_device *vdev)
if (!err)
blk_queue_max_segment_size(q, v);
- blk_queue_max_segment_size(q, -1U);
+ blk_queue_max_segment_size(q, BLK_MAX_SEGMENT_SIZE);

/* Host can optionally specify the block size of the device */
err = virtio_cread_feature(vdev, VIRTIO_BLK_F_BLK_SIZE,

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