Re: [PATCH v1 2/2] nvme-pci: handle dma_opt_mapping_size() returning 0

[John Garry <john.g.garry@oracle.com>]

On 16/03/2026 20:39, Ionut Nechita (Wind River) wrote:
> From: Ionut Nechita <ionut.nechita@windriver.com>
> 
> After the previous commit, dma_opt_mapping_size() returns 0 when no DMA
> backend provides an optimal mapping size hint (e.g. IOMMU in passthrough
> mode with no ops->opt_mapping_size callback).
> 
> The NVMe PCI driver used min_t(u32, NVME_MAX_BYTES >> SECTOR_SHIFT,
> dma_opt_mapping_size() >> 9) to cap max_hw_sectors.  With a 0 return
> value this would set max_hw_sectors to 0, which is invalid.

With the first patch you have introduced a temporary breakage.

> 
> Guard the min_t so that max_hw_sectors is only capped when
> dma_opt_mapping_size() provides a real hint.  When it returns 0, fall
> back to the existing NVME_MAX_BYTES >> SECTOR_SHIFT default.
> 
> Fixes: 3710e2b056cb ("nvme-pci: clamp max_hw_sectors based on DMA optimized limitation")
> Cc: stable@vger.kernel.org
> Signed-off-by: Ionut Nechita <ionut.nechita@windriver.com>
> ---
>   drivers/nvme/host/pci.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index b78ba239c8ea8..dc148fb6eff28 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -3640,6 +3640,7 @@ static struct nvme_dev *nvme_pci_alloc_dev(struct pci_dev *pdev,
>   {
>   	unsigned long quirks = id->driver_data;
>   	int node = dev_to_node(&pdev->dev);
> +	size_t dma_opt;
>   	struct nvme_dev *dev;
>   	struct quirk_entry *qentry;
>   	int ret = -ENOMEM;
> @@ -3691,12 +3692,16 @@ static struct nvme_dev *nvme_pci_alloc_dev(struct pci_dev *pdev,
>   	dma_set_max_seg_size(&pdev->dev, 0xffffffff);
>   
>   	/*
> -	 * Limit the max command size to prevent iod->sg allocations going
> -	 * over a single page.
> +	 * Limit the max command size to prevent iod->sg allocations
> +	 * going over a single page.  Only apply the DMA optimal mapping
> +	 * size limit when the DMA layer actually provides one (non-zero
> +	 * return from dma_opt_mapping_size()).
>   	 */
> -	dev->ctrl.max_hw_sectors = min_t(u32,
> -			NVME_MAX_BYTES >> SECTOR_SHIFT,
> -			dma_opt_mapping_size(&pdev->dev) >> 9);
> +	dev->ctrl.max_hw_sectors = NVME_MAX_BYTES >> SECTOR_SHIFT;
> +	dma_opt = dma_opt_mapping_size(&pdev->dev);
> +	if (dma_opt)
> +		dev->ctrl.max_hw_sectors =
> +			min_t(u32, dev->ctrl.max_hw_sectors, dma_opt >> 9); 

SECTOR_SHIFT can be used instead of hard-coded '9'

>   	dev->ctrl.max_segments = NVME_MAX_SEGS;
>   	dev->ctrl.max_integrity_segments = 1;
>   	return dev;