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

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Date:	Sat, 19 Jun 2010 07:45:45 +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

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.

Cheers,

Michael
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