Okay, As per my understanding .... a) pre-fill and use "struct iovec" with sock_recvmsg() Using this option, data will first be copied from the NIC's buffer to sk_buff (which are allocated in the NIC's device driver via the dev_alloc_skb(). And then during tcp_recvmsg(), the SAME data will be copied from sk_buff to the iovecs that I pass to sock_recvmsg(). But actually it is this very copying that I'm trying to avoid. b) intercept socket's receive callback with tcp_read_sock() and use skb_copy_bits() to copy data from skb to your destination buffer. Again in this option as well, data will first be copied from the NIC's buffer to sk_buff. And this is some thing that cannot be avoided. However, if I use skb_copy_bits(), the data (as you said already) will be AGAIN copied from the sk_buff to my destination buffer. My question is that if I'm developing a module (i.e. if I'm executing in the kernel space), can't I directly use the buffers from sk_buff ... Instead of copying them to a destination buffer. This way, we can implement a functionality similar to send page. Any experience / ideas are welcome. Thanks & Regards, Rajat -----Original Message----- From: Dmitry Yusupov [mailto:dima@xxxxxxxx] Sent: Friday, December 17, 2004 11:01 PM To: Rajat Jain, Noida Cc: linux-newbie@xxxxxxxxxxxxxxx; linux-net@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; kernelnewbies@xxxxxxxxxxxx; Sanjay Kumar, Noida; Deepak Kumar Gupta, Noida Subject: RE: zero copy issue while receiving the data (counter part of sen dfil e) On Fri, 2004-12-17 at 21:54 +0530, Rajat Jain, Noida wrote: > > Hi, > > Thanks for the reply. > > Actually I am developing a loadable kernel module. I agree that at the > bare minimum, I need to copy from the NIC's device buffer to kernel's > allocated sk_buff (socket buffer). What I want is to avoid FURTHER > coying of data from the sk_buffs to the buffers allocated by the module. Looks like you have two options: a) pre-fill and use "struct iovec" with sock_recvmsg() b) intercept socket's receive callback with tcp_read_sock() and use skb_copy_bits() to copy data from skb to your destination buffer. Regards, Dima > > And hence I expected to pass the address of a buffer pointer to > tcp_read_sock(). And I expected this function to set it to socket buffer. > Any pointers on the functionality of tcp_read_sock()?? > > Rajat > > > -----Original Message----- > From: Dmitry Yusupov [mailto:dima@xxxxxxxx] > Sent: Friday, December 17, 2004 7:07 AM > To: Rajat Jain, Noida > Cc: linux-net@xxxxxxxxxxxxxxx; Sanjay Kumar, Noida; Deepak Kumar > Gupta, Noida > Subject: Re: zero copy issue while receiving the data (counter part of > sendfil e) > > Hi Rajat, > > I was using this function some times back... It's been working for me > just fine. Also kernel's RPC (see xprt* files) uses it. So you might > want to take a look. > > In general, it is not possible to fully avoid copying. You need at > least copy data from NIC's skb to the destination. It might be user > buffer or kernel buffer(depends on application). > > Regards, > Dmitry > > > On Thu, 2004-12-16 at 19:38 +0530, Rajat Jain, Noida wrote: > > > > Hi, > > > > I'm experimenting on stock kernel 2.6.8 > > > > I was looking for an interface that could directly receive data from > > a network socket, WITHOUT coying from kernel space to user space. > > (Like for sending data, "sendfile" provides to send data to network > > socket without copying it to kernel space). I came across > > tcp_read_sock() interface in net/ipv4/tcp.c. > > > > Has anybody tried tcp_read_sock()?? Is there any known issue with it > > ?? If somebody has some idea, I would appreciate if you can share. > > > > I might be wrong, but what I perceive is that I will pass a pointer > > to this function. And when the function returns, I expect it to be > > set to the kernel buffer (corresponding to socket). > > > > 1) To fulfill this objective, I expect to pass a pointer to pointer > > & only then it can be done. (If we have to modify a pointer's value, > > we have to pass its address ... Right??). However, this function > > expects a char * buf (in read_descriptor_t argument). Any ideas ????????? > > > > 2) This code also frees the space allocated to sk_buffs etc using > > sk_eat_skb(sk, skb) and cleanup_rbuf(sk, copied) etc. But this > > function is supposed to return these locations to the calling code ... > Right??? > > > > Any pointers are more than welcome. I have provided the code for > reference. > > Please cc the reply to me as I'm not on the list. > > > > Thanks & regards, > > > > Rajat Jain > > > > -------------------------------------------------------------------- > > -- > > - > > /* net/ipv4/tcp.c > > * This routine provides an alternative to tcp_recvmsg() for > > routines > > * that would like to handle copying from skbuffs directly in 'sendfile' > > * fashion. > > * Note: > > * - It is assumed that the socket was locked by the caller. > > * - The routine does not block. > > * - At present, there is no support for reading OOB data > > * or for 'peeking' the socket using this routine > > * (although both would be easy to implement). > > */ > > int tcp_read_sock(struct sock *sk, read_descriptor_t *desc, > > sk_read_actor_t recv_actor) { > > struct sk_buff *skb; > > struct tcp_opt *tp = tcp_sk(sk); > > u32 seq = tp->copied_seq; > > u32 offset; > > int copied = 0; > > > > if (sk->sk_state == TCP_LISTEN) > > return -ENOTCONN; > > while ((skb = tcp_recv_skb(sk, seq, &offset)) != NULL) { > > if (offset < skb->len) { > > size_t used, len; > > > > len = skb->len - offset; > > /* Stop reading if we hit a patch of urgent > > data > */ > > if (tp->urg_data) { > > u32 urg_offset = tp->urg_seq - seq; > > if (urg_offset < len) > > len = urg_offset; > > if (!len) > > break; > > } > > used = recv_actor(desc, skb, offset, len); > > if (used <= len) { > > seq += used; > > copied += used; > > offset += used; > > } > > if (offset != skb->len) > > break; > > } > > if (skb->h.th->fin) { > > sk_eat_skb(sk, skb); > > ++seq; > > break; > > } > > sk_eat_skb(sk, skb); > > if (!desc->count) > > break; > > } > > tp->copied_seq = seq; > > > > tcp_rcv_space_adjust(sk); > > > > /* Clean up data we have read: This will do ACK frames. */ > > if (copied) > > cleanup_rbuf(sk, copied); > > return copied; > > }------------------------------------------------------------------- > > -- > > -- > > > > read_descriptor_t is defined as: > > > > /* > > * include/linux/fs.h > > */ > > typedef struct { > > size_t written; > > size_t count; > > union { > > char __user * buf; > > void *data; > > } arg; > > int error; > > } read_descriptor_t; > > -------------------------------------------------------------------- > > -- > > - > > > > - > > To unsubscribe from this list: send the line "unsubscribe linux-net" > > in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo > > info at http://vger.kernel.org/majordomo-info.html -- Kernelnewbies: Help each other learn about the Linux kernel. 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