[PATCH] virtio-blk: clamp max_segments when indirect descriptors are disabled

[Sergii Ushakov <sergiiushakov@google.com>]

When VIRTIO_RING_F_INDIRECT_DESC is not negotiated by the host, every
scatter-gather segment in a request must consume a physical slot in
the virtqueue ring.

If the host does not advertise VIRTIO_BLK_F_SEG_MAX and provides a small
virtqueue (e.g. 128 descriptors on QNX Hypervisor), the block layer
defaults max_segments to BLK_MAX_SEGMENTS (1024). When a multi-page
compound bio arrives from the page cache, virtqueue_add_split() rejects
the request with -ENOSPC and triggers:

  WARNING: at drivers/virtio/virtio_ring.c:1493 virtqueue_add+...
  WARN_ON_ONCE(total_sg > vq->split.vring.num && !vq->indirect);

This permanently wedges the blk-mq queue and blocks all subsequent disk
I/O in uninterruptible sleep (D state).

Add a virtio_blk.max_segments module parameter to allow runtime cmdline
overrides, and automatically clamp sg_elems to
(virtqueue_get_vring_size - 2) when indirect descriptors are disabled.

Signed-off-by: Sergii Ushakov <sergiiushakov@google.com>
---
 drivers/block/virtio_blk.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 32bf3ba07a9d..082acd90a02d 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -41,6 +41,10 @@ static unsigned int poll_queues;
 module_param(poll_queues, uint, 0644);
 MODULE_PARM_DESC(poll_queues, "The number of dedicated virtqueues for polling I/O");
 
+static unsigned int max_segments;
+module_param(max_segments, uint, 0644);
+MODULE_PARM_DESC(max_segments, "Override maximum number of segments per request");
+
 static int major;
 static DEFINE_IDA(vd_index_ida);
 
@@ -1267,6 +1271,12 @@ static int virtblk_read_limits(struct virtio_blk *vblk,
 	/* Prevent integer overflows and honor max vq size */
 	sg_elems = min_t(u32, sg_elems, VIRTIO_BLK_MAX_SG_ELEMS - 2);
 
+	if (max_segments)
+		sg_elems = min_t(u32, sg_elems, max_segments);
+	else if (!virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC))
+		sg_elems = min_t(u32, sg_elems,
+				 virtqueue_get_vring_size(vblk->vqs[0].vq) - 2);
+
 	/* We can handle whatever the host told us to handle. */
 	lim->max_segments = sg_elems;
 
-- 
2.55.0.691.gc56d675ccc-goog