nvme-5.3 ssd performance regression

[ming.lei@redhat.com (Ming Lei)]

On Tue, Jul 16, 2019@11:36:17AM +0200, Christoph Hellwig wrote:
> On Tue, Jul 16, 2019@03:12:06PM +0800, Ming Lei wrote:
> > It should be caused by the following commit, and that patch uses
> > single mapping size to limit max hw sectors, and looks that way
> > is wrong. For example, on qemu, max_hw_sectors is decreased to
> > 512. You can try to revert the patch and see if it makes a difference.
> > 
> > I feel we might need the max segment size limit too.
> 
> No, with swiotlb it really is the whole request size that ?s limited
> by the swiotlb buffer size.  Similar for potential iommus where
> again it is the whole thing.

Documentation/DMA-API.txt:

		size_t
		dma_max_mapping_size(struct device *dev);
	
	Returns the maximum size of a mapping for the device. The size parameter
	of the mapping functions like dma_map_single(), dma_map_page() and
	others should not be larger than the returned value.

And dma_map_single() & dma_map_page() is usually for mapping single
element of SGL, instead of whole SGL.

Thanks,
Ming