linux-kernel - Re: [patch] pipe: add support for shrinking and growing pipes

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Date:	Mon, 24 May 2010 10:51:38 -0400
From:	Brian Bloniarz <bmb@...enacr.com>
To:	mtk.manpages@...il.com
CC:	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] pipe: add support for shrinking and growing pipes

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.

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