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

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Message-ID: <20080718175341.GA21496@2ka.mipt.ru>
Date:	Fri, 18 Jul 2008 21:53:41 +0400
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Octavian Purdila <opurdila@...acom.com>
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

Hi.

On Fri, Jul 18, 2008 at 08:04:44PM +0300, Octavian Purdila (opurdila@...acom.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.

> 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_page()
callchain does not return error and that pipe is full?

-- 
	Evgeniy Polyakov
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