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    SubjectRe: [PATCH] tcp: do not promote SPLICE_F_NONBLOCK to socket O_NONBLOCK
    Date
    On Friday 18 July 2008, Evgeniy Polyakov wrote:
    > Hi.
    >
    > On Fri, Jul 18, 2008 at 08:04:44PM +0300, Octavian Purdila
    (opurdila@ixiacom.com) wrote:
    > > Suppose we have 20 packets in the socket queue and the pipe is empty and
    > > the application calls splice(sock, pipe, 17, flags=0).
    > >
    > > Then, tcp_splice_read will be called, which in turn calls tcp_read_sock.
    > >
    > > tcp_read_sock will loop until all the 17 bytes will be read from the
    > > socket. tcp_read_sock calls skb_splice_bits which calls splice_to_pipe.
    >
    > How come?
    > spd_fill_page() should fail when it will be called for the 17'th skb and
    > all reading from the socket will return, and thus can be sent to the
    > file.
    >

    spd_fill_page work with the splice_pipe_descriptor declared in
    skb_splice_bits, thus spd_fill_page does not have visibility across multiple
    skb_splice_bits calls.

    > > Now while skb_splice_bits is carefull to only put a maximum of
    > > PIPE_BUFFERS during its iteration, due to the looping in tcp_read_sock,
    > > we will end up with 17 calls to splice_to_pipe. Thus on the 17th call,
    > > splice_to_pipe will block.
    >
    > Where exactly?
    > Why
    > tcp_splice_data_recv()->skb_splice_bits()->__skb_splice_bits()->spd_fill_pa
    >ge() callchain does not return error and that pipe is full?


    Ok, let me try to move through the function calls:

    tcp_read_sock
    ... -> skb_splice_bits -> spd_fill_page;
    on return (spd->nr_page is 1 and pipe->nrbufs is 1)
    ... -> skb_splice_bits -> spd_fill_page;
    on return (spd->nr_page is 1 and pipe->nrbufs is 2)
    ... -> skb_splice_bits -> spd_fill_page;
    on return (spd->nr_page is 1 and pipe->nrbufs is 3)
    ...

    and so on until pipe->nrbufs is 16. At than point, we will block in pipe_wait,
    inside splice_to_pipe.

    Thanks,
    tavi










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