linux-kernel - Re: [PATCH] nvme-pci: 512 byte dma pool segment quirk

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Message-ID: <Zyz2oiQ2pco15HHT@kbusch-mbp>
Date: Thu, 7 Nov 2024 10:19:30 -0700
From: Keith Busch <kbusch@...nel.org>
To: Bob Beckett <bob.beckett@...labora.com>
Cc: Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Sagi Grimberg <sagi@...mberg.me>, kernel@...labora.com,
	linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme-pci: 512 byte dma pool segment quirk

On Thu, Nov 07, 2024 at 04:50:46PM +0000, Bob Beckett wrote:
> @@ -611,7 +612,7 @@ static blk_status_t nvme_pci_setup_prps(struct nvme_dev *dev,
>  	}
>  
>  	nprps = DIV_ROUND_UP(length, NVME_CTRL_PAGE_SIZE);
> -	if (nprps <= (256 / 8)) {
> +	if (nprps <= (dev->small_dmapool_seg_size / 8)) {
>  		pool = dev->prp_small_pool;
>  		iod->nr_allocations = 0;
>  	} else {

We have a constant expression currently, and this is changing it a full
division in the IO path. :(

Could we leave the pool selection check size as-is and just say the cost
of the quirk is additional memory overhead?

> @@ -2700,8 +2701,9 @@ static int nvme_setup_prp_pools(struct nvme_dev *dev)
>  		return -ENOMEM;
>  
>  	/* Optimisation for I/Os between 4k and 128k */
> -	dev->prp_small_pool = dma_pool_create("prp list 256", dev->dev,
> -						256, 256, 0);
> +	dev->prp_small_pool = dma_pool_create("prp list small", dev->dev,
> +						dev->small_dmapool_seg_size,
> +						dev->small_dmapool_seg_size, 0);

I think it should work if we only change the alignment property of the
pool. Something like this:

	if (dev->ctrl.quirks & NVME_QUIRK_SMALL_DMAPOOL_512)
		dev->prp_small_pool = dma_pool_create("prp list 256", dev->dev,
						      256, 512, 0);
	else
		dev->prp_small_pool = dma_pool_create("prp list 256", dev->dev,
						      256, 256, 0);