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

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Message-ID: <4C1D08B4.50806@fusionio.com>
Date:	Sat, 19 Jun 2010 20:13:08 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	"mtk.manpages@...il.com" <mtk.manpages@...il.com>
CC:	Miklos Szeredi <miklos@...redi.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch] pipe: add support for shrinking and growing pipes

On 19/06/10 07.45, Michael Kerrisk wrote:
> Hello Jens,
> 
> I've tested this F_SETPIPE_SZ code as it appears in 2.6.35-rc3, and
> things seem to work as advertised, if the text below is the correct
> advertisement. Could you please review the following man-pages text,
> and let me know if it's okay.
> 
> fcntl.2:
>        F_SETPIPE_SZ (long; since Linux 2.6.35)
>               Change  the  capacity of the pipe referred to by
>               fd to be at least arg  bytes.   An  unprivileged
>               process  can  adjust  the  pipe  capacity to any
>               value between the system page size and the limit
>               defined   in   /proc/sys/fs/pipe-size-max   (see
>               proc(5)).  Attempts to  set  the  pipe  capacity
>               below  the  page size are silently rounded up to
>               the page size.  Attempts by an unprivileged pro-
>               cess to set the pipe capacity above the limit in
>               /proc/sys/fs/pipe-size-max   yield   the   error
>               EPERM;  a  privileged process (CAP_SYS_RESOURCE)
>               can override the  limit.   When  allocating  the
>               buffer for the pipe, the kernel may use a capac-
>               ity larger than arg, if that is  convenient  for
>               the  implementation.  The F_GETPIPE_SZ operation
>               returns the actual size used.  Attempting to set
>               the  pipe  capacity  smaller  than the amount of
>               buffer space currently used to store  data  pro-
>               duces the error EBUSY.
> 
>        F_GETPIPE_SZ (void; since Linux 2.6.35)
>               Return  (as the function result) the capacity of
>               the pipe referred to by fd.
> 
> And in proc.5:
>        /proc/sys/fs/pipe-max-size (since Linux 2.6.35)
>               The value in this file defines  an  upper  limit
>               for  raising  the  capacity  of a pipe using the
>               fcntl(2)  F_SETPIPE_SZ  operation.   This  limit
>               applies  only  to  unprivileged  processes.  The
>               default value for this file is  1,048,576.   The
>               value  assigned  to  this  file  may  be rounded
>               upward, to reflect the value  actually  employed
>               for  a  convenient implementation.  To determine
>               the rounded-up value, display  the  contents  of
>               this  file  after  assigning a value to it.  The
>               minimum value that can be assigned to this  file
>               is the system page size.

Looks good to me.

-- 
Jens Axboe

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