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 <[email protected]> --- 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
