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Date: Wed, 19 Aug 2026 10:51:37 +0200
From: Christoph Hellwig <hch@lst.de>
To: John Garry <john.g.garry@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>, Joerg Roedel <joro@8bytes.org>,
	Will Deacon <will@kernel.org>, iommu@lists.linux.dev,
	linux-nvme@lists.infradead.org
Subject: Re: dma_opt_mapping_size returns way too low sizes when using IOMMU
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References: <20260817083654.GA23414@lst.de> <86f7fbdc-3c7f-4d84-871e-52ed3cfe8d58@oracle.com> <20260817091840.GA26843@lst.de> <8b7e86a7-6cb1-4007-bd99-1c4be8e6828e@oracle.com>
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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.