Re: [RFC RESEND 16/16] nvme-pci: use blk_rq_dma_map() for NVMe SGL

[Chaitanya Kulkarni]

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