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

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Message-ID: <AANLkTik6gxq_mR8fgz_rCu5libi7Ntp23dnPLUOHfCkD@mail.gmail.com>
Date:	Sun, 20 Jun 2010 07:37:13 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Jens Axboe <jaxboe@...ionio.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 Sat, Jun 19, 2010 at 8:13 PM, Jens Axboe <jaxboe@...ionio.com> wrote:
> 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.

Thanks!

Cheers,

Michael


-- 
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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