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Date: Mon, 17 Aug 2026 10:36:54 +0200
From: Christoph Hellwig <hch@lst.de>
To: Marek Szyprowski <m.szyprowski@samsung.com>,
	Robin Murphy <robin.murphy@arm.com>, Joerg Roedel <joro@8bytes.org>,
	Will Deacon <will@kernel.org>, John Garry <john.g.garry@oracle.com>,
	iommu@lists.linux.dev, linux-nvme@lists.infradead.org
Subject: dma_opt_mapping_size returns way too low sizes when using IOMMU
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User-Agent: Mutt/1.5.17 (2007-11-01)

Hi all,

I got reports that NVMe devices were arbitrarily limited to 128kiB
transfers in recent kernel.  It turns out that this only happens when
using an IOMMU and is caused by iommu_dma_opt_mapping_size returning
iova_rcache_range(), which is defined as follows:

unsigned long iova_rcache_range(void)
{
	return PAGE_SIZE << (IOVA_RANGE_CACHE_MAX_SIZE - 1);
}

and thus indeed hardcodes a 128kiB return on 4k page size architectures.

Both the NVMe performance numbers and common sense suggest that this
is NOT the optimal DMA mapping granularity.  Can we pick a saner value
for iommu_dma_opt_mapping_size that does not restrict common I/O sizes?