[PATCH 7/9] nvme-pci: convert the data mapping blk_rq_dma_map

[Daniel Gomez]

On 10/06/2025 07.06, Christoph Hellwig wrote:
> Use the blk_rq_dma_map API to DMA map requests instead of scatterlists.
> This removes the need to allocate a scatterlist covering every segment,
> and thus the overall transfer length limit based on the scatterlist
> allocation.
> 
> Instead the DMA mapping is done by iterating the bio_vec chain in the
> request directly.  The unmap is handled differently depending on how
> we mapped:
> 
>  - when using an IOMMU only a single IOVA is used, and it is stored in
>    iova_state
>  - for direct mappings that don't use swiotlb and are cache coherent no
>    unmap is needed at all
>  - for direct mappings that are not cache coherent or use swiotlb, the
>    physical addresses are rebuild from the PRPs or SGL segments
> 
> The latter unfortunately adds a fair amount of code to the driver, but
> it is code not used in the fast path.
> 
> The conversion only covers the data mapping path, and still uses a
> scatterlist for the multi-segment metadata case.  I plan to convert that
> as soon as we have good test coverage for the multi-segment metadata
> path.
> 
> Thanks to Chaitanya Kulkarni for an initial attempt at a new DMA API
> conversion for nvme-pci, Kanchan Joshi for bringing back the single
> segment optimization, Leon Romanovsky for shepherding this through a
> gazillion rebases and Nitesh Shetty for various improvements.
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  drivers/nvme/host/pci.c | 388 +++++++++++++++++++++++++---------------
>  1 file changed, 242 insertions(+), 146 deletions(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index 04461efb6d27..2d3573293d0c 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
...
> @@ -2908,26 +3018,14 @@ static int nvme_disable_prepare_reset(struct nvme_dev *dev, bool shutdown)
>  static int nvme_pci_alloc_iod_mempool(struct nvme_dev *dev)

Since this pool is now used exclusively for metadata, it makes sense to update
the function name accordingly:

static int nvme_pci_alloc_iod_meta_mempool(struct nvme_dev *dev)

>  {
>  	size_t meta_size = sizeof(struct scatterlist) * (NVME_MAX_META_SEGS + 1);
> -	size_t alloc_size = sizeof(struct scatterlist) * NVME_MAX_SEGS;
> -
> -	dev->iod_mempool = mempool_create_node(1,
> -			mempool_kmalloc, mempool_kfree,
> -			(void *)alloc_size, GFP_KERNEL,
> -			dev_to_node(dev->dev));
> -	if (!dev->iod_mempool)
> -		return -ENOMEM;
>  
>  	dev->iod_meta_mempool = mempool_create_node(1,
>  			mempool_kmalloc, mempool_kfree,
>  			(void *)meta_size, GFP_KERNEL,
>  			dev_to_node(dev->dev));
>  	if (!dev->iod_meta_mempool)
> -		goto free;
> -
> +		return -ENOMEM;
>  	return 0;
> -free:
> -	mempool_destroy(dev->iod_mempool);
> -	return -ENOMEM;
>  }
>  
>  static void nvme_free_tagset(struct nvme_dev *dev)