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[75.142.250.213]) by smtp.gmail.com with ESMTPSA id c27sm315917qko.71.2021.04.14.12.33.19 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Apr 2021 12:33:20 -0700 (PDT) Subject: Re: Proposal of improvement for DMA - direct passing of hugepages to the SG list To: wzab@ise.pw.edu.pl, LKML Cc: linux-fpga@vger.kernel.org References: <6c7a1cff-3ad6-0a75-c45b-ce437ea156ef@elektron.elka.pw.edu.pl> From: Tom Rix Message-ID: Date: Wed, 14 Apr 2021 12:33:18 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <6c7a1cff-3ad6-0a75-c45b-ce437ea156ef@elektron.elka.pw.edu.pl> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 4/14/21 4:58 AM, wzab@ise.pw.edu.pl wrote: > Hi, > > I'm working both on DMA engines implementations in FPGA and their > Linux drivers. > Now I need to create an engine that takes the hugepages-backed buffer > allocated > by the user-space application and passes it to the device. > My current solution: > > https://forums.xilinx.com/t5/Embedded-Linux/How-to-pass-efficiently-the-hugepages-backed-buffer-to-the-BM/m-p/1229340/highlight/true#M49777 > > or https://stackoverflow.com/a/67065962/1735409 > > uses the get_user_pages_fast function, to create the kernel mapping, > and then uses sg_alloc_table_from_pages to build sg_table for it. > > I have verified that the created sg_table has one entry for each > individual > hugepage (so I can efficiently map it for my SG-capable DMA device). > > The disadvantage of that solution is that I need to create and keep a > huge set > of standard-size pages. Because the "struct page" occupies between 56 > and 80 > bytes, I get the overhead up to 80/4096 which is approx. 2%. > > The open question is if I can free those pages as soon as the sg_table > is created? (As far as I know, the hugepages are locked automatically). > Of course it is unclear what happens if the application crashes and > its mmaped > hugepage-based buffer gets freed. Will the driver be notified about > closing the > file so that it can disable DMA before that memory can be taken for other > purposes? > > To be sure that it doesn't happen maybe it is good to keep those pages > locked > in the kernel as well. > The big improvement would be if we could have the get_user_hugepages_fast > function. The user should be allowed to create a smaller number of > page structs. > The function should check if the requested region really consists of > hugepages > and return an error if it doesn't. > > Another important fact is that we don't need a kernel mapping for > those pages > at all. So it would be good to have yet another function: > sg_alloc_table_from_user_pages > which should take an additional "flag" argument enabling the user to > decide > if the area used consists of normal pages or of hugepages. > > The function should return an error in  case if the flag does not > match the > properties of the region. Of course the function should also lock the > pages, > and sg_free_table should unlock them (protecting against the danger > of application crash, that I described above). > > As a temporary workaround, is it possible to "manually" walk the pages > creating the application-delivered buffer, verify that they are > hugepages, > lock them and create the sg_table? > What functions/macros should be used for that to ensure that the > implemented > solution be portable and keeps working in a reasonable number of > future versions > of the Linux kernel? > > I'll appreciate comments, suggestions and considering of the above > proposal. > With best regards, > Wojtek > Do you have trial patch you could RFC ? Are you aware of the xilinx qdma patchset ? https://www.xilinx.com/support/answers/71453.html https://github.com/Xilinx/dma_ip_drivers Maybe its kernel or dpdk driver has what you need. Tom