linux-kernel - Re: [PATCH v9 22/24] nvme-pci: use a better encoding for small prp pool allocations

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Message-ID: <20250423090552.GA381@lst.de>
Date: Wed, 23 Apr 2025 11:05:52 +0200
From: Christoph Hellwig <hch@....de>
To: Leon Romanovsky <leon@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
	Keith Busch <kbusch@...nel.org>, Jake Edge <jake@....net>,
	Jonathan Corbet <corbet@....net>, Jason Gunthorpe <jgg@...pe.ca>,
	Zhu Yanjun <zyjzyj2000@...il.com>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>,
	Yishai Hadas <yishaih@...dia.com>,
	Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Jérôme Glisse <jglisse@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.org, linux-rdma@...r.kernel.org,
	iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org,
	linux-pci@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
	Niklas Schnelle <schnelle@...ux.ibm.com>,
	Chuck Lever <chuck.lever@...cle.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Kanchan Joshi <joshi.k@...sung.com>,
	Chaitanya Kulkarni <kch@...dia.com>,
	Leon Romanovsky <leonro@...dia.com>
Subject: Re: [PATCH v9 22/24] nvme-pci: use a better encoding for small prp
 pool allocations

On Wed, Apr 23, 2025 at 11:13:13AM +0300, Leon Romanovsky wrote:
> From: Christoph Hellwig <hch@....de>
> 
> There is plenty of unused space in the iod next to nr_descriptors.
> Add a separate flag to encode that the transfer is using the full
> page sized pool, and use a normal 0..n count for the number of
> descriptors.
> 
> Signed-off-by: Christoph Hellwig <hch@....de>
> Tested-by: Jens Axboe <axboe@...nel.dk>
> [ Leon: changed original bool variable to be flag as was proposed by Kanchan ]
> Signed-off-by: Kanchan Joshi <joshi.k@...sung.com>
> Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> ---
>  drivers/nvme/host/pci.c | 93 ++++++++++++++++++++---------------------
>  1 file changed, 46 insertions(+), 47 deletions(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index 638e759b29ad..7e93536d01cb 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -44,6 +44,7 @@
>  #define NVME_MAX_SEGS	128
>  #define NVME_MAX_META_SEGS 15
>  #define NVME_MAX_NR_DESCRIPTORS	5
> +#define NVME_SMALL_DESCRIPTOR_SIZE 256
>  
>  static int use_threaded_interrupts;
>  module_param(use_threaded_interrupts, int, 0444);
> @@ -219,6 +220,10 @@ struct nvme_queue {
>  	struct completion delete_done;
>  };
>  
> +enum {
> +	IOD_LARGE_DESCRIPTORS = 1, /* uses the full page sized descriptor pool */

This is used as a ORable flag, I'd make that explicit:

	/* uses the full page sized descriptor pool */
	IOD_LARGE_DESCRIPTORS		= 1U << 0,

and similar for the next flag added in the next patch.

>  	struct nvme_request req;
>  	struct nvme_command cmd;
>  	bool aborted;
> -	/* # of PRP/SGL descriptors: (0 for small pool) */
> -	s8 nr_descriptors;
> +	u8 nr_descriptors;	/* # of PRP/SGL descriptors */
> +	unsigned int flags;

And this should be limited to a u16 to not bloat the structure.