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[94.196.228.157]) by smtp.gmail.com with ESMTPSA id a5-20020a05600c224500b003b4fac020c8sm14022447wmm.16.2022.09.27.07.30.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 27 Sep 2022 07:30:11 -0700 (PDT) Message-ID: <021d8ea4-891c-237d-686e-64cecc2cc842@gmail.com> Date: Tue, 27 Sep 2022 15:28:34 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.12.0 Subject: Re: [PATCH net-next 0/4] shrink struct ubuf_info Content-Language: en-US To: Paolo Abeni , netdev@vger.kernel.org Cc: "David S . Miller" , Jakub Kicinski , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org, Wei Liu , Paul Durrant , kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, "Michael S . Tsirkin" , Jason Wang References: <7fef56880d40b9d83cc99317df9060c4e7cdf919.camel@redhat.com> From: Pavel Begunkov In-Reply-To: <7fef56880d40b9d83cc99317df9060c4e7cdf919.camel@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Paolo, On 9/27/22 14:49, Paolo Abeni wrote: > Hello, > > On Fri, 2022-09-23 at 17:39 +0100, Pavel Begunkov wrote: >> struct ubuf_info is large but not all fields are needed for all >> cases. We have limited space in io_uring for it and large ubuf_info >> prevents some struct embedding, even though we use only a subset >> of the fields. It's also not very clean trying to use this typeless >> extra space. >> >> Shrink struct ubuf_info to only necessary fields used in generic paths, >> namely ->callback, ->refcnt and ->flags, which take only 16 bytes. And >> make MSG_ZEROCOPY and some other users to embed it into a larger struct >> ubuf_info_msgzc mimicking the former ubuf_info. >> >> Note, xen/vhost may also have some cleaning on top by creating >> new structs containing ubuf_info but with proper types. > > That sounds a bit scaring to me. If I read correctly, every uarg user > should check 'uarg->callback == msg_zerocopy_callback' before accessing > any 'extend' fields. Providers of ubuf_info access those fields via callbacks and so already know the actual structure used. The net core, on the opposite, should keep it encapsulated and not touch them at all. The series lists all places where we use extended fields just on the merit of stripping the structure of those fields and successfully building it. The only user in net/ipv{4,6}/* is MSG_ZEROCOPY, which again uses callbacks. Sounds like the right direction for me. There is a couple of places where it might get type safer, i.e. adding types instead of void* in for struct tun_msg_ctl and getting rid of one macro hiding types in xen. But seems more like TODO for later. > AFAICS the current code sometimes don't do the > explicit test because the condition is somewhat implied, which in turn > is quite hard to track. > > clearing uarg->zerocopy for the 'wrong' uarg was armless and undetected > before this series, and after will trigger an oops.. And now we don't have this field at all to access, considering that nobody blindly casts it. > There is some noise due to uarg -> uarg_zc renaming which make the > series harder to review. Have you considered instead keeping the old > name and introducing a smaller 'struct ubuf_info_common'? the overall > code should be mostly the same, but it will avoid the above mentioned > noise. I don't think there will be less noise this way, but let me try and see if I can get rid of some churn. -- Pavel Begunkov