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Thu, 22 Jul 2021 05:18:30 -0700 (PDT) Subject: Re: [PATCH v5 net-next 01/36] net: Introduce direct data placement tcp offload To: Eric Dumazet , Boris Pismenny Cc: David Ahern , Jakub Kicinski , David Miller , Saeed Mahameed , Christoph Hellwig , sagi@grimberg.me, axboe@fb.com, kbusch@kernel.org, Al Viro , smalin@marvell.com, boris.pismenny@gmail.com, linux-nvme@lists.infradead.org, netdev , benishay@nvidia.com, ogerlitz@nvidia.com, yorayz@nvidia.com, Boris Pismenny , Ben Ben-Ishay , Or Gerlitz , Yoray Zack References: <20210722110325.371-1-borisp@nvidia.com> <20210722110325.371-2-borisp@nvidia.com> From: Boris Pismenny Message-ID: <7fdb948a-6411-fce5-370f-90567d2fe985@gmail.com> Date: Thu, 22 Jul 2021 15:18:27 +0300 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 22/07/2021 14:26, Eric Dumazet wrote: > On Thu, Jul 22, 2021 at 1:04 PM Boris Pismenny wrote: >> >> From: Boris Pismenny >> >> > ... > >> }; >> >> const char >> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c >> index e6ca5a1f3b59..4a7160bba09b 100644 >> --- a/net/ipv4/tcp_input.c >> +++ b/net/ipv4/tcp_input.c >> @@ -5149,6 +5149,9 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root, >> memcpy(nskb->cb, skb->cb, sizeof(skb->cb)); >> #ifdef CONFIG_TLS_DEVICE >> nskb->decrypted = skb->decrypted; >> +#endif >> +#ifdef CONFIG_ULP_DDP >> + nskb->ddp_crc = skb->ddp_crc; > > Probably you do not want to attempt any collapse if skb->ddp_crc is > set right there. > Right. >> #endif >> TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start; >> if (list) >> @@ -5182,6 +5185,11 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root, >> #ifdef CONFIG_TLS_DEVICE >> if (skb->decrypted != nskb->decrypted) >> goto end; >> +#endif >> +#ifdef CONFIG_ULP_DDP >> + >> + if (skb->ddp_crc != nskb->ddp_crc) > > This checks only the second, third, and remaining skbs. > Right, as we handle the head skb above. Could you clarify? >> + goto end; >> #endif >> } >> } > > > tcp_collapse() is copying data from small skbs to pack it to bigger > skb (one page of payload), in case > of memory emergency/pressure (socket queues are full) > > If your changes are trying to avoid 'needless' copies, maybe you > should reconsider and let the emergency packing be done. > > If the copy is not _possible_, you should rephrase your changelog to > clearly state the kernel _cannot_ access this memory in any way. > The issue is that skb_condense also gets called on many skbs in tcp_add_backlog and it will identify skbs that went through DDP as ideal for packing, even though they are not small and packing is counter-productive as data already resides in its destination. As mentioned above, it is possible to copy, but it is counter-productive in this case. If there was a real need to access this memory, then it is allowed.