linux-kernel - Re: [PATCH RFC V2 6/6] RDMA/rxe: Support PAGE

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Date:   Mon, 6 Nov 2023 03:07:01 +0000
From:   "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>
To:     Bart Van Assche <bvanassche@....org>,
        "zyjzyj2000@...il.com" <zyjzyj2000@...il.com>,
        "jgg@...pe.ca" <jgg@...pe.ca>, "leon@...nel.org" <leon@...nel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rpearsonhpe@...il.com" <rpearsonhpe@...il.com>,
        "Daisuke Matsuda (Fujitsu)" <matsuda-daisuke@...itsu.com>,
        "yi.zhang@...hat.com" <yi.zhang@...hat.com>
Subject: Re: [PATCH RFC V2 6/6] RDMA/rxe: Support PAGE_SIZE aligned MR



On 03/11/2023 23:04, Bart Van Assche wrote:
> 
> On 11/3/23 02:55, Li Zhijian wrote:
>> -    return ib_sg_to_pages(ibmr, sgl, sg_nents, sg_offset, rxe_set_page);
>> +    for_each_sg(sgl, sg, sg_nents, i) {
>> +        u64 dma_addr = sg_dma_address(sg) + sg_offset;
>> +        unsigned int dma_len = sg_dma_len(sg) - sg_offset;
>> +        u64 end_dma_addr = dma_addr + dma_len;
>> +        u64 page_addr = dma_addr & PAGE_MASK;
>> +
>> +        if (sg_dma_len(sg) == 0) {
>> +            rxe_dbg_mr(mr, "empty SGE\n");
>> +            return -EINVAL;
>> +        }
>> +        do {
>> +            int ret = rxe_store_page(mr, page_addr);
>> +            if (ret)
>> +                return ret;
>> +
>> +            page_addr += PAGE_SIZE;
>> +        } while (page_addr < end_dma_addr);
>> +        sg_offset = 0;
>> +    }
>> +
>> +    return ib_sg_to_pages(ibmr, sgl, sg_nents, sg_offset_p, rxe_set_page);
>>   }
> 
> Is this change necessary? 

There is already a loop in ib_sg_to_pages()
> that splits SG entries that are larger than mr->page_size into entries
> with size mr->page_size.

I see.

My thought was that we are only able to safely access PAGE_SIZE memory scope [page_va, page_va + PAGE_SIZE)
from the return of kmap_local_page(page).
However when mr->page_size is larger than PAGE_SIZE, we may access the next pages without mapping it.

Thanks
Zhijian