Messages in this thread | | | From | Octavian Purdila <> | Subject | Re: [PATCH] tcp: do not promote SPLICE_F_NONBLOCK to socket O_NONBLOCK | Date | Fri, 18 Jul 2008 21:16:43 +0300 |
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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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