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

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Message-ID: <4abe98f4-8179-4422-aee5-ae47552e28b7@nvidia.com>
Date: Tue, 5 Mar 2024 16:46:33 +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>, 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 08:39, Chaitanya Kulkarni wrote:
> 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
>
>

For final version when DMA API is discussion is concluded, I've plan to use
the iod_mempool for allocation of nvme_iod->dma_link_address, however I'
not wait for that and send out a updated version with trimmed nvme_iod size.

If you guys have any other comments please let me know or we can 
continue the
discussion on once I post new version of this patch on this thread ...

Thanks a log Keith and Jens for looking into it ...

-ck