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

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Message-ID: <AANLkTilJqTEZwQdZOQiiwGzQRu_-d4voY5GlwpuW_epl@mail.gmail.com>
Date:	Mon, 24 May 2010 17:43:32 +0200
From:	Michael Kerrisk <mtk.manpages@...glemail.com>
To:	Brian Bloniarz <bmb@...enacr.com>
Cc:	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 Mon, May 24, 2010 at 4:51 PM, Brian Bloniarz <bmb@...enacr.com> wrote:
> 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.

Yes, all of the above is understood. It's exposing these details to
userspace that's weird...

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface" http://blog.man7.org/
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