linux-kernel - Re: [RFC RESEND 16/16] nvme-pci: use blk_rq_dma

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Date: Tue, 5 Mar 2024 16:39:39 +0000
From: Chaitanya Kulkarni <chaitanyak@...dia.com>
To: Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>, Leon
 Romanovsky <leon@...nel.org>
CC: Christoph Hellwig <hch@....de>, Robin Murphy <robin.murphy@....com>, Marek
 Szyprowski <m.szyprowski@...sung.com>, Joerg Roedel <joro@...tes.org>, Will
 Deacon <will@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>, Chaitanya Kulkarni
	<chaitanyak@...dia.com>, Jonathan Corbet <corbet@....net>, Sagi Grimberg
	<sagi@...mberg.me>, 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-doc@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-block@...r.kernel.org"
	<linux-block@...r.kernel.org>, "linux-rdma@...r.kernel.org"
	<linux-rdma@...r.kernel.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "linux-nvme@...ts.infradead.org"
	<linux-nvme@...ts.infradead.org>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, Bart Van
 Assche <bvanassche@....org>, Damien Le Moal
	<damien.lemoal@...nsource.wdc.com>, Amir Goldstein <amir73il@...il.com>,
	"josef@...icpanda.com" <josef@...icpanda.com>, "Martin K. Petersen"
	<martin.petersen@...cle.com>, "daniel@...earbox.net" <daniel@...earbox.net>,
	Dan Williams <dan.j.williams@...el.com>, "jack@...e.com" <jack@...e.com>,
	Leon Romanovsky <leonro@...dia.com>, Zhu Yanjun <zyjzyj2000@...il.com>
Subject: Re: [RFC RESEND 16/16] nvme-pci: use blk_rq_dma_map() for NVMe SGL

On 3/5/24 08:08, Jens Axboe wrote:
> On 3/5/24 8:51 AM, Keith Busch wrote:
>> On Tue, Mar 05, 2024 at 01:18:47PM +0200, Leon Romanovsky wrote:
>>> @@ -236,7 +236,9 @@ struct nvme_iod {
>>>   	unsigned int dma_len;	/* length of single DMA segment mapping */
>>>   	dma_addr_t first_dma;
>>>   	dma_addr_t meta_dma;
>>> -	struct sg_table sgt;
>>> +	struct dma_iova_attrs iova;
>>> +	dma_addr_t dma_link_address[128];
>>> +	u16 nr_dma_link_address;
>>>   	union nvme_descriptor list[NVME_MAX_NR_ALLOCATIONS];
>>>   };
>> That's quite a lot of space to add to the iod. We preallocate one for
>> every request, and there could be millions of them.
> Yeah, that's just a complete non-starter. As far as I can tell, this
> ends up adding 1052 bytes per request. Doing the quick math on my test
> box (24 drives), that's just a smidge over 3GB of extra memory. That's
> not going to work, not even close.
>

I don't have any intent to use more space for the nvme_iod than what
it is now. I'll trim down the iod structure and send out a patch soon with
this fixed to continue the discussion here on this thread ...

-ck