Re: [PATCH v8 24/24] nvme-pci: optimize single-segment handling

[Damien Le Moal <dlemoal@kernel.org>]

On 4/18/25 15:47, Leon Romanovsky wrote:
> From: Kanchan Joshi <joshi.k@samsung.com>
> 
> blk_rq_dma_map API is costly for single-segment requests.
> Avoid using it and map the bio_vec directly.
> 
> Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
> Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
> ---
>  drivers/nvme/host/pci.c | 65 +++++++++++++++++++++++++++++++++++++----
>  1 file changed, 60 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index 8d99a8f871ea..cf020de82962 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -216,6 +216,11 @@ struct nvme_queue {
>  	struct completion delete_done;
>  };
>  
> +enum {
> +	IOD_LARGE_DESCRIPTORS = 1, /* uses the full page sized descriptor pool */
> +	IOD_SINGLE_SEGMENT = 2, /* single segment dma mapping */
> +};
> +
>  /*
>   * The nvme_iod describes the data in an I/O.
>   */
> @@ -224,7 +229,7 @@ struct nvme_iod {
>  	struct nvme_command cmd;
>  	bool aborted;
>  	u8 nr_descriptors;	/* # of PRP/SGL descriptors */
> -	bool large_descriptors;	/* uses the full page sized descriptor pool */
> +	unsigned int flags;
>  	unsigned int total_len; /* length of the entire transfer */
>  	unsigned int total_meta_len; /* length of the entire metadata transfer */
>  	dma_addr_t meta_dma;
> @@ -529,7 +534,7 @@ static inline bool nvme_pci_use_sgls(struct nvme_dev *dev, struct request *req,
>  static inline struct dma_pool *nvme_dma_pool(struct nvme_dev *dev,
>  		struct nvme_iod *iod)
>  {
> -	if (iod->large_descriptors)
> +	if (iod->flags & IOD_LARGE_DESCRIPTORS)
>  		return dev->prp_page_pool;
>  	return dev->prp_small_pool;
>  }
> @@ -630,6 +635,15 @@ static void nvme_free_sgls(struct nvme_dev *dev, struct request *req)
>  static void nvme_unmap_data(struct nvme_dev *dev, struct request *req)
>  {
>  	struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
> +	unsigned int nr_segments = blk_rq_nr_phys_segments(req);
> +	dma_addr_t dma_addr;
> +
> +	if (nr_segments == 1 && (iod->flags & IOD_SINGLE_SEGMENT)) {

nvme_pci_setup_prps() calls nvme_try_setup_prp_simple() which sets
IOD_SINGLE_SEGMENT if and only if the req has a single phys segment. So why do
you need to count the segments again here ? Looking at the flag only should be
enough, no ?

> +		dma_addr = le64_to_cpu(iod->cmd.common.dptr.prp1);
> +		dma_unmap_page(dev->dev, dma_addr, iod->total_len,
> +				rq_dma_dir(req));
> +		return;
> +	}
>  
>  	if (!blk_rq_dma_unmap(req, dev->dev, &iod->dma_state, iod->total_len)) {
>  		if (iod->cmd.common.flags & NVME_CMD_SGL_METABUF)
> @@ -642,6 +656,41 @@ static void nvme_unmap_data(struct nvme_dev *dev, struct request *req)
>  		nvme_free_descriptors(dev, req);
>  }
>  
> +static bool nvme_try_setup_prp_simple(struct nvme_dev *dev, struct request *req,
> +				      struct nvme_rw_command *cmnd,
> +				      struct blk_dma_iter *iter)
> +{
> +	struct nvme_iod *iod = blk_mq_rq_to_pdu(req);
> +	struct bio_vec bv = req_bvec(req);
> +	unsigned int first_prp_len;
> +
> +	if (is_pci_p2pdma_page(bv.bv_page))
> +		return false;
> +	if ((bv.bv_offset & (NVME_CTRL_PAGE_SIZE - 1)) + bv.bv_len >
> +	    NVME_CTRL_PAGE_SIZE * 2)
> +		return false;
> +
> +	iter->addr = dma_map_bvec(dev->dev, &bv, rq_dma_dir(req), 0);
> +	if (dma_mapping_error(dev->dev, iter->addr)) {
> +		iter->status = BLK_STS_RESOURCE;
> +		goto out;
> +	}
> +	iod->total_len = bv.bv_len;
> +	cmnd->dptr.prp1 = cpu_to_le64(iter->addr);
> +
> +	first_prp_len = NVME_CTRL_PAGE_SIZE -
> +			(bv.bv_offset & (NVME_CTRL_PAGE_SIZE - 1));
> +	if (bv.bv_len > first_prp_len)
> +		cmnd->dptr.prp2 = cpu_to_le64(iter->addr + first_prp_len);
> +	else
> +		cmnd->dptr.prp2 = 0;
> +
> +	iter->status = BLK_STS_OK;
> +	iod->flags |= IOD_SINGLE_SEGMENT;
> +out:
> +	return true;
> +}
> +
>  static blk_status_t nvme_pci_setup_prps(struct nvme_dev *dev,
>  					struct request *req)
>  {
> @@ -652,6 +701,12 @@ static blk_status_t nvme_pci_setup_prps(struct nvme_dev *dev,
>  	dma_addr_t prp1_dma, prp2_dma = 0;
>  	unsigned int prp_len, i;
>  	__le64 *prp_list;
> +	unsigned int nr_segments = blk_rq_nr_phys_segments(req);
> +
> +	if (nr_segments == 1) {
> +		if (nvme_try_setup_prp_simple(dev, req, cmnd, &iter))
> +			return iter.status;
> +	}
>  
>  	if (!blk_rq_dma_map_iter_start(req, dev->dev, &iod->dma_state, &iter))
>  		return iter.status;
> @@ -693,7 +748,7 @@ static blk_status_t nvme_pci_setup_prps(struct nvme_dev *dev,
>  
>  	if (DIV_ROUND_UP(length, NVME_CTRL_PAGE_SIZE) >
>  	    NVME_SMALL_DESCRIPTOR_SIZE / sizeof(__le64))
> -		iod->large_descriptors = true;
> +		iod->flags |= IOD_LARGE_DESCRIPTORS;
>  
>  	prp_list = dma_pool_alloc(nvme_dma_pool(dev, iod), GFP_ATOMIC,
>  			&prp2_dma);
> @@ -808,7 +863,7 @@ static blk_status_t nvme_pci_setup_sgls(struct nvme_dev *dev,
>  	}
>  
>  	if (entries > NVME_SMALL_DESCRIPTOR_SIZE / sizeof(*sg_list))
> -		iod->large_descriptors = true;
> +		iod->flags |= IOD_LARGE_DESCRIPTORS;
>  
>  	sg_list = dma_pool_alloc(nvme_dma_pool(dev, iod), GFP_ATOMIC, &sgl_dma);
>  	if (!sg_list)
> @@ -932,7 +987,7 @@ static blk_status_t nvme_prep_rq(struct nvme_dev *dev, struct request *req)
>  
>  	iod->aborted = false;
>  	iod->nr_descriptors = 0;
> -	iod->large_descriptors = false;
> +	iod->flags = 0;
>  	iod->total_len = 0;
>  	iod->total_meta_len = 0;
>  


-- 
Damien Le Moal
Western Digital Research