dma_opt_mapping_size returns way too low sizes when using IOMMU

[Christoph Hellwig]

On Mon, Aug 17, 2026 at 11:11:43AM +0100, John Garry wrote:
>> At least on the four different nvme devices I tested, the larger I/O
>> sizes made up for this.  But maybe the details depend on other
>> factors as well.
> Are you saying that you tried increasing IOVA_RANGE_CACHE_MAX_SIZE and got 
> better performance?

No, just disabling this hint.

> As I remember, I was told that the value of 6 for IOVA_RANGE_CACHE_MAX_SIZE 
> was originally chosen from the value then in max page order, i.e. the idea 
> was that we should not be getting streaming IOs larger than that value. But 
> in looking at lore, 8 was very originally proposed, but I can't see any 
> discussion on why that changed or any relation to page max order.

But that's a bit weird.  A bio already does up to 256 entries, and a
request can have many more.