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Date: Mon, 24 May 2010 10:51:38 -0400
From: Brian Bloniarz <bmb@athenacr.com>
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CC: Michael Kerrisk <mtk.manpages@googlemail.com>,
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       Andrew Morton <akpm@linux-foundation.org>,
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       Miklos Szeredi <miklos@szeredi.hu>, linux-fsdevel@vger.kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: [patch] pipe: add support for shrinking and growing pipes
References: <E1OEmP5-0006GU-KC@pomaz-ex.szeredi.hu> <alpine.LFD.2.00.1005190946540.23538@i5.linux-foundation.org> 	<AANLkTikkRywJSJ8HFsVhVU9dHz8cFGqlK9ZHXZuGQbb2@mail.gmail.com> 	<20100522223838.ebca396a.akpm@linux-foundation.org> <AANLkTinFOqDF0v_uMvkj4r8n4aX4xvpFvTi-9odUYa5F@mail.gmail.com> 	<20100523070917.GO23411@kernel.dk> <AANLkTimKFHQPi9kKhYYjRhMt0s5CDsBXmESQaH493hpR@mail.gmail.com> 	<20100523174706.GP23411@kernel.dk> <87632e846m.fsf@devron.myhome.or.jp> 	<20100524070416.GQ23411@kernel.dk> <AANLkTikwTa16lDh87zkI-L9SaI9jG1_eqZuo4z7dj2qP@mail.gmail.com>
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On 05/24/2010 03:28 AM, Michael Kerrisk wrote:
> Actually, SO_*BUF is pretty weird. It returns double what was
> supplied. It's not simply a matter of rounding up: it always doubles
> what was supplied.

Rationale in net/core/sock.c:

set_rcvbuf:
                sk->sk_userlocks |= SOCK_RCVBUF_LOCK;
                /*
                 * We double it on the way in to account for
                 * "struct sk_buff" etc. overhead.   Applications
                 * assume that the SO_RCVBUF setting they make will
                 * allow that much actual data to be received on that
                 * socket.
                 *
                 * Applications are unaware that "struct sk_buff" and
                 * other overheads allocate from the receive buffer
                 * during socket buffer allocation.
                 *
                 * And after considering the possible alternatives,
                 * returning the value we actually used in getsockopt
                 * is the most desirable behavior.
                 */
                if ((val * 2) < SOCK_MIN_RCVBUF)
                        sk->sk_rcvbuf = SOCK_MIN_RCVBUF;
                else
                        sk->sk_rcvbuf = val * 2;
                break;

I'm guessing pipes don't have this kind of wrinkle.