netdev - Re: [PATCH] tcp: do not promote SPLICE_F_NONBLOCK to socket O

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Date:	Fri, 18 Jul 2008 20:04:44 +0300
From:	Octavian Purdila <opurdila@...acom.com>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	axboe@...nel.dk
Subject: Re: [PATCH] tcp: do not promote SPLICE_F_NONBLOCK to socket O_NONBLOCK

On Friday 18 July 2008, Evgeniy Polyakov wrote:

> > It will take 17 because this is what the user requested. And when trying
> > to push the 17th on the pipe, it will block. I base this both on
> > experiments and on my understanding of the tcp splice receive
> > implementation.
>
> Seems like we do not understand each other. How it can take 17 if there
> are only 16 pages? By 'push' you mean splice-into-pipe or 
> splice-out-of-pipe-into-other-fd? Where exactly will it block?
>

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.

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.

Does this make sense?

Thanks,
tavi
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