[Report] requests are submitted to hardware in reverse order from nvme/virtio-blk queue_rqs()

[Keith Busch]

On Wed, Jan 24, 2024 at 07:59:54PM +0800, Ming Lei wrote:
> Requests are added to plug list in reverse order, and both virtio-blk
> and nvme retrieves request from plug list in order, so finally requests
> are submitted to hardware in reverse order via nvme_queue_rqs() or
> virtio_queue_rqs, see:
> 
> 	io_uring       submit_bio  vdb      6302096     4096
> 	io_uring       submit_bio  vdb     12235072     4096
> 	io_uring       submit_bio  vdb      7682280     4096
> 	io_uring       submit_bio  vdb     11912464     4096
> 	io_uring virtio_queue_rqs  vdb     11912464     4096
> 	io_uring virtio_queue_rqs  vdb      7682280     4096
> 	io_uring virtio_queue_rqs  vdb     12235072     4096
> 	io_uring virtio_queue_rqs  vdb      6302096     4096
> 
> 
> May this reorder be one problem for virtio-blk and nvme-pci?

For nvme, it depends. Usually it's probably not a problem, though some
pci ssd's have optimizations for sequential IO that might not work if
these get reordered.