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    SubjectRE: zero copy issue while receiving the data (counter part of sen dfile)
    Date

    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;
    -----------------------------------------------------------------------

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