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

[Christoph Hellwig]

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?