On Mon, Aug 17, 2026 at 03:42:02PM +0200, Sergii Ushakov wrote:
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 <redacted>
---
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(+)
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>