[PATCH v2] 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).

Automatically clamp sg_elems to (ring_size - 2) when indirect
descriptors are disabled.

Signed-off-by: Sergii Ushakov <sergiiushakov@google.com>
---
v1 -> v2:
- Drop max_segments module parameter and rely solely on automatic clamping
  when indirect descriptors are disabled (suggested by Christoph Hellwig).
- Guard (ring_size - 2) calculation with ring_size > 2 to prevent underflow.
- Update commit description accordingly.

 drivers/block/virtio_blk.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 32bf3ba07a9d..8f5a2d5323a6 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -1267,6 +1267,13 @@ 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 (!virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC)) {
+		u32 ring_size = virtqueue_get_vring_size(vblk->vqs[0].vq);
+
+		if (ring_size > 2)
+			sg_elems = min(sg_elems, ring_size - 2);
+	}
+
 	/* We can handle whatever the host told us to handle. */
 	lim->max_segments = sg_elems;
 
-- 
2.55.0.691.gc56d675ccc-goog