On Fri, Aug 14, 2026 at 12:59:54PM +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). > > 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.
What is the reason for the override?
