Staging: add cowloop driver

Cowloop is a "copy-on-write" pseudo block driver. It can
be stacked on top of a "real" block driver, and catches
all write operations on their way from the file systems
layer above to the real driver below, effectively shielding
the lower driver from those write accesses. The requests are
then diverted to an ordinary file, located somewhere else
(configurable). Later read requests are checked to see whether
they can be serviced by the "real" block driver below, or
must be pulled in from the diverted location. More information
is on the project's website http://www.ATComputing.nl/cowloop/

From: "H.J. Thomassen" <hjt@ATComputing.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 2963 insertions, 0 deletions

diff --git a/drivers/staging/cowloop/cowloop.c b/drivers/staging/cowloop/cowloop.c
new file mode 100644
index 000000000000..23978012838a
--- /dev/null
+++ b/drivers/staging/cowloop/cowloop.c
@@ -0,0 +1,2897 @@
+/*
+**       COWLOOP block device driver (2.6 kernel compliant)
+** =======================================================================
+** Read-write loop-driver with copy-on-write functionality.
+**
+** Synopsis:
+**
+**     modprobe cowloop [maxcows=..] [rdofile=..... cowfile=.... [option=r]]
+**
+** Definition of number of configured cowdevices:
+**   maxcows=	number of configured cowdevices (default: 16)
+** (do not confuse this with MAXCOWS: absolute maximum as compiled)
+**
+** One pair of filenames can be supplied during insmod/modprobe to open
+** the first cowdevice:
+**   rdofile=	read-only file (or filesystem)
+**   cowfile=	storage-space for modified blocks of read-only file(system)
+**   option=r	repair cowfile automatically if it appears to be dirty
+**
+** Other cowdevices can be activated via the command "cowdev"
+** whenever the cowloop-driver is loaded.
+**
+** The read-only file may be of type 'regular' or 'block-device'.
+**
+** The cowfile must be of type 'regular'.
+** If an existing regular file is used as cowfile, its contents will be
+** used again for the current read-only file. When the cowfile has not been
+** closed properly during a previous session (i.e. rmmod cowloop), the
+** cowloop-driver refuses to open it unless the parameter "option=r" is
+** specified.
+**
+** Layout of cowfile:
+**
+** 	+-----------------------------+
+**	|       cow head block        |   MAPUNIT bytes
+**	|-----------------------------|
+**	|                             |   MAPUNIT bytes
+**	|---                       ---|
+**	|                             |   MAPUNIT bytes
+**	|---                       ---|
+**	|      used-block bitmap      |   MAPUNIT bytes
+**	|-----------------------------|
+**	|  gap to align start-offset  |
+**	|        to 4K multiple       |
+**	|-----------------------------|  <---- start-offset cow blocks
+**	|                             |
+**      |    written cow blocks       |   MAPUNIT bytes
+**      |          .....              |
+**
+** 	cowhead block:
+**   	  - contains general info about the rdofile which is related
+** 	    to this cowfile
+**
+** 	used-block bitmap:
+** 	  - contains one bit per block with a size of MAPUNIT bytes
+** 	  - bit-value '1' = block has been written on cow
+** 	              '0' = block unused on cow
+** 	  - total bitmap rounded to multiples of MAPUNIT
+**
+** ============================================================================
+** Author:             Gerlof Langeveld - AT Computing (March 2003)
+** Current maintainer: Hendrik-Jan Thomassen - AT Computing (Summer 2006)
+** Email:              hjt@ATComputing.nl
+** ----------------------------------------------------------------------------
+** Copyright (C) 2003-2009 AT Consultancy
+**
+** This program is free software; you can redistribute it and/or modify it
+** under the terms of the GNU General Public License as published by the
+** Free Software Foundation; either version 2, or (at your option) any
+** later version.
+**
+** This program is distributed in the hope that it will be useful, but
+** WITHOUT ANY WARRANTY; without even the implied warranty of
+** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+** See the GNU General Public License for more details.
+**
+** You should have received a copy of the GNU General Public License
+** along with this program; if not, write to the Free Software
+** Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+** ----------------------------------------------------------------------------
+**
+** Major modifications:
+**
+** 	200405	Ported to kernel-version 2.6		Hendrik-Jan Thomassen
+**	200405	Added cowhead to cowfile to garantee
+**		consistency with read-only file		Gerlof Langeveld
+**	200405	Postponed flushing of bitmaps to improve
+**		performance. 				Gerlof Langeveld
+**	200405	Inline recovery for dirty cowfiles.	Gerlof Langeveld
+**	200502	Redesign to support more cowdevices.	Gerlof Langeveld
+**	200502	Support devices/file > 2 Gbytes.	Gerlof Langeveld
+**	200507	Check for free space to expand cowfile.	Gerlof Langeveld
+**	200902  Upgrade for kernel 2.6.28               Hendrik-Jan Thomassen
+**
+** Inspired by
+**    loop.c  by Theodore Ts'o and
+**    cloop.c by Paul `Rusty' Russell & Klaus Knopper.
+**
+** Design-considerations:
+**
+**   For the first experiments with the cowloop-driver, the request-queue
+**   made use of the do_generic_file_read() which worked fine except
+**   in combination with the cloop-driver; that combination
+**   resulted in a non-interruptible hangup of the system during
+**   heavy load. Other experiments using the `make_request' interface also
+**   resulted in unpredictable system hangups (with proper use of spinlocks).
+**
+**   To overcome these problems, the cowloop-driver starts a kernel-thread
+**   for every active cowdevice.
+**   All read- and write-request on the read-only file and copy-on-write file
+**   are handled in the context of that thread.
+**   A scheme has been designed to wakeup the kernel-thread as
+**   soon as I/O-requests are available in the request-queue; this thread
+**   handles the requests one-by-one by calling the proper read- or
+**   write-function related to the open read-only file or copy-on-write file.
+**   When all pending requests have been handled, the kernel-thread goes
+**   back to sleep-state.
+**   This approach requires some additional context-switches; however the
+**   performance loss during heavy I/O is less than 3%.
+**
+** -------------------------------------------------------------------------*/
+/* The following is the cowloop package version number. It must be
+   identical to the content of the include-file "version.h" that is
+   used in all supporting utilities:                                  */
+char revision[] = "$Revision: 3.1 $"; /* cowlo_init_module() has
+			     assumptions about this string's format   */
+
+/* Note that the following numbers are *not* the cowloop package version
+   numbers, but separate revision history numbers to track the
+   modifications of this particular source file:                      */
+/* $Log: cowloop.c,v $
+**
+** Revision 1.30  2009/02/08 hjt
+** Integrated earlier fixes
+** Upgraded to kernel 2.6.28 (thanks Jerome Poulin)
+**
+** Revision 1.29  2006/12/03 22:12:00  hjt
+** changed 'cowdevlock' from spinlock to semaphore, to avoid
+** "scheduling while atomic". Contributed by Juergen Christ.
+** Added version.h again
+**
+** Revision 1.28  2006/08/16 16:00:00  hjt
+** malloc each individual cowloopdevice struct separately
+**
+** Revision 1.27  2006/03/14 14:57:03  root
+** Removed include version.h
+**
+** Revision 1.26  2005/08/08 11:22:48  root
+** Implement possibility to close a cow file or reopen a cowfile read-only.
+**
+** Revision 1.25  2005/08/03 14:00:39  root
+** Added modinfo info to driver.
+**
+** Revision 1.24  2005/07/21 06:14:53  root
+** Cosmetic changes source code.
+**
+** Revision 1.23  2005/07/20 13:07:32  root
+** Supply ioctl to write watchdog program to react on lack of cowfile space.
+**
+** Revision 1.22  2005/07/20 07:53:34  root
+** Regular verification of free space in filesystem holding the cowfile
+** (give warnings whenever space is almost exhausted).
+** Terminology change: checksum renamed to fingerprint.
+**
+** Revision 1.21  2005/07/19 09:21:52  root
+** Removing maximum limit of 16 Gb per cowdevice.
+**
+** Revision 1.20  2005/07/19 07:50:33  root
+** Minor bugfixes and cosmetic changes.
+**
+** Revision 1.19  2005/06/10 12:29:55  root
+** Removed lock/unlock operation from cowlo_open().
+**
+** Revision 1.18  2005/05/09 12:56:26  root
+** Allow a cowdevice to be open more than once
+** (needed for support of ReiserFS and XFS).
+**
+** Revision 1.17  2005/03/17 14:36:16  root
+** Fixed some license issues.
+**
+** Revision 1.16  2005/03/07 14:42:05  root
+** Only allow one parallel open per cowdevice.
+**
+** Revision 1.15  2005/02/18 11:52:04  gerlof
+** Redesign to support more than one cowdevice > 2 Gb space.
+**
+** Revision 1.14  2004/08/17 14:19:16  gerlof
+** Modified output of /proc/cowloop.
+**
+** Revision 1.13  2004/08/16 07:21:10  gerlof
+** Separate statistical counter for read on rdofile and cowfile.
+**
+** Revision 1.12  2004/08/11 06:52:11  gerlof
+** Modified messages.
+**
+** Revision 1.11  2004/08/11 06:44:11  gerlof
+** Modified log messages.
+**
+** Revision 1.10  2004/08/10 12:27:27  gerlof
+** Cosmetic changes.
+**
+** Revision 1.9  2004/08/09 11:43:37  gerlof
+** Removed double definition of major number (COWMAJOR).
+**
+** Revision 1.8  2004/08/09 08:03:39  gerlof
+** Cleanup of messages.
+**
+** Revision 1.7  2004/05/27 06:37:33  gerlof
+** Modified /proc message.
+**
+** Revision 1.6  2004/05/26 21:23:28  gerlof
+** Modified /proc output.
+**
+** Revision 1.5  2004/05/26 13:23:34  gerlof
+** Support cowsync to force flushing the bitmaps and cowhead.
+**
+** Revision 1.4  2004/05/26 11:11:10  gerlof
+** Updated the comment to the actual situation.
+**
+** Revision 1.3  2004/05/26 10:50:00  gerlof
+** Implemented recovery-option.
+**
+** Revision 1.2  2004/05/25 15:14:41  gerlof
+** Modified bitmap flushing strategy.
+**
+*/
+
+#define COWMAJOR	241
+
+// #define COWDEBUG
+
+#ifdef 	COWDEBUG
+#define DEBUGP		printk
+#define DCOW		KERN_ALERT
+#else
+#define DEBUGP(format, x...)
+#endif
+
+#include <linux/types.h>
+#include <linux/autoconf.h>
+#ifndef AUTOCONF_INCLUDED
+#include <linux/config.h>
+#endif
+#include <linux/module.h>
+#include <linux/version.h>
+#include <linux/moduleparam.h>
+#include <linux/init.h>
+#include <linux/errno.h>
+#include <linux/kernel.h>
+#include <linux/major.h>
+#include <linux/sched.h>
+#include <linux/fs.h>
+#include <linux/file.h>
+#include <linux/stat.h>
+#include <linux/vmalloc.h>
+#include <linux/slab.h>
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,27))
+#include <linux/semaphore.h>
+#else
+#include <asm/semaphore.h>
+#endif
+#include <asm/uaccess.h>
+#include <linux/proc_fs.h>
+#include <linux/blkdev.h>
+#include <linux/buffer_head.h>
+#include <linux/hdreg.h>
+#include <linux/genhd.h>
+#include <linux/statfs.h>
+
+#include "cowloop.h"
+
+MODULE_LICENSE("GPL");
+/* MODULE_AUTHOR("Gerlof Langeveld <gerlof@ATComputing.nl>");     obsolete address */
+MODULE_AUTHOR("Hendrik-Jan Thomassen <hjt@ATComputing.nl>"); /* current maintainer */
+MODULE_DESCRIPTION("Copy-on-write loop driver");
+MODULE_PARM_DESC(maxcows, " Number of configured cowdevices (default 16)");
+MODULE_PARM_DESC(rdofile, " Read-only file for /dev/cow/0");
+MODULE_PARM_DESC(cowfile, " Cowfile for /dev/cow/0");
+MODULE_PARM_DESC(option, "  Repair cowfile if inconsistent: option=r");
+
+#define DEVICE_NAME	"cow"
+
+#define	DFLCOWS		16		/* default cowloop devices	*/
+
+static int maxcows = DFLCOWS;
+module_param(maxcows, int, 0);
+static char *rdofile = "";
+module_param(rdofile, charp, 0);
+static char *cowfile = "";
+module_param(cowfile, charp, 0);
+static char *option = "";
+module_param(option, charp, 0);
+
+/*
+** per cowdevice several bitmap chunks are allowed of MAPCHUNKSZ each
+**
+** each bitmap chunk can describe MAPCHUNKSZ * 8 * MAPUNIT bytes of data
+** suppose:
+**	MAPCHUNKSZ 4096 and MAPUNIT 1024 --> 4096 * 8 * 1024 = 32 Mb per chunk
+*/
+#define	MAPCHUNKSZ	4096	/* #bytes per bitmap chunk  (do not change)  */
+
+#define SPCMINBLK	100	/* space threshold to give warning messages  */
+#define SPCDFLINTVL	16	/* once every SPCDFLINTVL writes to cowfile, */
+				/* available space in filesystem is checked  */
+
+#define	CALCMAP(x)	((x)/(MAPCHUNKSZ*8))
+#define	CALCBYTE(x)	(((x)%(MAPCHUNKSZ*8))>>3)
+#define	CALCBIT(x)	((x)&7)
+
+#define ALLCOW		1
+#define ALLRDO		2
+#define MIXEDUP		3
+
+static char	allzeroes[MAPUNIT];
+
+/*
+** administration per cowdevice (pair of cowfile/rdofile)
+*/
+
+/* bit-values for state */
+#define	COWDEVOPEN	0x01	/* cowdevice opened                          */
+#define	COWRWCOWOPEN	0x02	/* cowfile opened read-write                 */
+#define	COWRDCOWOPEN	0x04	/* cowfile opened read-only                  */
+#define	COWWATCHDOG	0x08 	/* ioctl for watchdog cowfile space active   */
+
+#define	COWCOWOPEN	(COWRWCOWOPEN|COWRDCOWOPEN)
+
+struct cowloop_device
+{
+	/*
+	** current status
+	*/
+	int		state;			/* bit-values (see above)    */
+	int		opencnt;		/* # opens for cowdevice     */
+
+        /*
+	** open file pointers
+	*/
+        struct file  	*rdofp,   *cowfp;	/* open file pointers        */
+	char		*rdoname, *cowname;	/* file names                */
+
+	/*
+	** request queue administration
+	*/
+	struct request_queue	*rqueue;
+	spinlock_t		rqlock;
+	struct gendisk		*gd;
+
+	/*
+	** administration about read-only file
+	*/
+	unsigned int	     numblocks;	/* # blocks input file in MAPUNIT    */
+	unsigned int	     blocksz;   /* minimum unit to access this dev   */
+	unsigned long	     fingerprint; /* fingerprint of current rdofile  */
+	struct block_device  *belowdev;	/* block device below us             */
+	struct gendisk       *belowgd;  /* gendisk for blk dev below us      */
+	struct request_queue *belowq;	/* req. queue of blk dev below us    */
+
+	/*
+	** bitmap administration to register which blocks are modified
+	*/
+	long int	mapsize;	/* total size of bitmap (bytes)      */
+	long int	mapremain;	/* remaining bytes in last bitmap    */
+	int		mapcount;       /* number of bitmaps in use          */
+	char 		**mapcache;	/* area with pointers to bitmaps     */
+
+	char		*iobuf;		/* databuffer of MAPUNIT bytes       */
+	struct cowhead	*cowhead;	/* buffer containing cowhead         */
+
+	/*
+	** administration for interface with the kernel-thread
+	*/
+	int		pid;		/* pid==0: no thread available       */
+	struct request	*req;		/* request to be handled now         */
+	wait_queue_head_t waitq;	/* wait-Q: thread waits for work     */
+	char		closedown;	/* boolean: thread exit required     */
+	char		qfilled;	/* boolean: I/O request pending      */
+	char		iobusy;		/* boolean: req under treatment      */
+
+	/*
+	** administration to keep track of free space in cowfile filesystem
+	*/
+	unsigned long	blksize;	/* block size of fs (bytes)          */
+	unsigned long	blktotal;	/* recent total space in fs (blocks) */
+	unsigned long	blkavail;	/* recent free  space in fs (blocks) */
+
+	wait_queue_head_t watchq;	/* wait-Q: watcher awaits threshold  */
+	unsigned long	watchthresh;	/* threshold of watcher (blocks)     */
+
+	/*
+	** statistical counters
+	*/
+	unsigned long	rdoreads;	/* number of  read-actions rdo       */
+	unsigned long	cowreads;	/* number of  read-actions cow       */
+	unsigned long	cowwrites;	/* number of write-actions           */
+	unsigned long	nrcowblocks;	/* number of blocks in use on cow    */
+};
+
+static struct cowloop_device	**cowdevall;	/* ptr to ptrs to all cowdevices */
+static struct semaphore 	cowdevlock;	/* generic lock for cowdevs      */
+
+static struct gendisk		*cowctlgd;	/* gendisk control channel       */
+static spinlock_t		cowctlrqlock;   /* for req.q. of ctrl. channel   */
+
+/*
+** private directory /proc/cow
+*/
+struct proc_dir_entry	*cowlo_procdir;
+
+/*
+** function prototypes
+*/
+static long int cowlo_do_request (struct request *req);
+static void	cowlo_sync       (void);
+static int	cowlo_checkio    (struct cowloop_device *,         int, loff_t);
+static int	cowlo_readmix    (struct cowloop_device *, void *, int, loff_t);
+static int	cowlo_writemix   (struct cowloop_device *, void *, int, loff_t);
+static long int cowlo_readrdo    (struct cowloop_device *, void *, int, loff_t);
+static long int cowlo_readcow    (struct cowloop_device *, void *, int, loff_t);
+static long int cowlo_readcowraw (struct cowloop_device *, void *, int, loff_t);
+static long int cowlo_writecow   (struct cowloop_device *, void *, int, loff_t);
+static long int cowlo_writecowraw(struct cowloop_device *, void *, int, loff_t);
+
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28))
+static int      cowlo_ioctl      (struct block_device *, fmode_t,
+           			 		unsigned int, unsigned long);
+#else
+static int      cowlo_ioctl      (struct inode *, struct file *,
+           			 		unsigned int, unsigned long);
+#endif
+
+static int	cowlo_makepair    (struct cowpair __user *);
+static int	cowlo_removepair  (unsigned long  __user *);
+static int	cowlo_watch       (struct cowpair __user *);
+static int	cowlo_cowctl      (unsigned long  __user *, int);
+static int	cowlo_openpair    (char *, char *, int, int);
+static int 	cowlo_closepair   (struct cowloop_device *);
+static int	cowlo_openrdo     (struct cowloop_device *, char *);
+static int	cowlo_opencow     (struct cowloop_device *, char *, int);
+static void	cowlo_undo_openrdo(struct cowloop_device *);
+static void	cowlo_undo_opencow(struct cowloop_device *);
+
+/*****************************************************************************/
+/* System call handling                                                      */
+/*****************************************************************************/
+
+/*
+** handle system call open()/mount()
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28))
+cowlo_open(struct block_device *bdev, fmode_t mode)
+#else
+cowlo_open(struct inode *inode, struct file *file)
+#endif
+{
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28))
+	struct inode *inode = bdev->bd_inode;
+#endif
+	if (!inode)
+		return -EINVAL;
+
+	if (imajor(inode) != COWMAJOR) {
+		printk(KERN_WARNING
+		       "cowloop - unexpected major %d\n", imajor(inode));
+		return -ENODEV;
+	}
+
+	switch (iminor(inode)) {
+	   case COWCTL:
+		DEBUGP(DCOW"cowloop - open %d control\n", COWCTL);
+		break;
+
+	   default:
+		DEBUGP(DCOW"cowloop - open minor %d\n", iminor(inode));
+
+		if ( iminor(inode) >= maxcows )
+			return -ENODEV;
+
+		if ( !((cowdevall[iminor(inode)])->state & COWDEVOPEN) )
+			return -ENODEV;
+
+		(cowdevall[iminor(inode)])->opencnt++;
+	}
+
+	return 0;
+}
+
+/*
+** handle system call close()/umount()
+**
+** returns:
+** 	0   - okay
+*/
+static int
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28))
+cowlo_release(struct gendisk *gd, fmode_t mode)
+#else
+cowlo_release(struct inode *inode, struct file *file)
+#endif
+{
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28))
+	struct block_device *bdev;
+	struct inode *inode;
+
+	bdev = bdget_disk(gd, 0);
+	inode = bdev->bd_inode;
+#endif
+	if (!inode)
+		return 0;
+
+	DEBUGP(DCOW"cowloop - release (close) minor %d\n", iminor(inode));
+
+	if ( iminor(inode) != COWCTL)
+		(cowdevall[iminor(inode)])->opencnt--;
+
+	return 0;
+}
+
+/*
+** handle system call ioctl()
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28))
+cowlo_ioctl(struct block_device *bdev, fmode_t mode,
+            unsigned int cmd, unsigned long arg)
+#else
+cowlo_ioctl(struct inode *inode, struct file *filp,
+            unsigned int cmd, unsigned long arg)
+#endif
+{
+	struct hd_geometry	geo;
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,28))
+	struct inode *inode = bdev->bd_inode;
+#endif
+
+	DEBUGP(DCOW "cowloop - ioctl cmd %x\n", cmd);
+
+	switch ( iminor(inode) ) {
+
+	   /*
+	   ** allowed via control device only
+	   */
+	   case COWCTL:
+		switch (cmd) {
+		   /*
+		   ** write all bitmap chunks and cowheaders to cowfiles
+		   */
+		   case COWSYNC:
+			down(&cowdevlock);
+			cowlo_sync();
+			up(&cowdevlock);
+			return 0;
+
+		   /*
+		   ** open a new cowdevice (pair of rdofile/cowfile)
+		   */
+		   case COWMKPAIR:
+			return cowlo_makepair((void __user *)arg);
+
+		   /*
+		   ** close a cowdevice (pair of rdofile/cowfile)
+		   */
+		   case COWRMPAIR:
+			return cowlo_removepair((void __user *)arg);
+
+		   /*
+		   ** watch free space of filesystem containing cowfile
+		   */
+		   case COWWATCH:
+			return cowlo_watch((void __user *)arg);
+
+		   /*
+		   ** close cowfile for active device
+		   */
+		   case COWCLOSE:
+			return cowlo_cowctl((void __user *)arg, COWCLOSE);
+
+		   /*
+		   ** reopen cowfile read-only for active device
+		   */
+		   case COWRDOPEN:
+			return cowlo_cowctl((void __user *)arg, COWRDOPEN);
+
+		   default:
+			return -EINVAL;
+		} /* end of switch on command */
+
+	   /*
+	   ** allowed for any other cowdevice
+	   */
+	   default:
+		switch (cmd) {
+		   /*
+		   ** HDIO_GETGEO must be supported for fdisk, etc
+		   */
+		   case HDIO_GETGEO:
+			geo.cylinders = 0;
+			geo.heads     = 0;
+			geo.sectors   = 0;
+
+			if (copy_to_user((void __user *)arg, &geo, sizeof geo))
+				return -EFAULT;
+			return 0;
+
+		   default:
+			return -EINVAL;
+		} /* end of switch on ioctl-cmd code parameter */
+	} /* end of switch on minor number */
+}
+
+static struct block_device_operations cowlo_fops =
+{
+	.owner	 =     THIS_MODULE,
+        .open    =     cowlo_open,	/* called upon open  */
+        .release =     cowlo_release,	/* called upon close */
+        .ioctl   =     cowlo_ioctl,     /* called upon ioctl */
+};
+
+/*
+** handle ioctl-command COWMKPAIR:
+**	open a new cowdevice (pair of rdofile/cowfile) on-the-fly
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_makepair(struct cowpair __user *arg)
+{
+	int		i, rv=0;
+	struct cowpair	cowpair;
+	unsigned char	*cowpath;
+	unsigned char	*rdopath;
+
+	/*
+	** retrieve info about pathnames
+	*/
+	if ( copy_from_user(&cowpair, arg, sizeof cowpair) )
+		return -EFAULT;
+
+	if ( (MAJOR(cowpair.device) != COWMAJOR) && (cowpair.device != ANYDEV) )
+		return -EINVAL;
+
+	if ( (MINOR(cowpair.device) >= maxcows)  && (cowpair.device != ANYDEV) )
+		return -EINVAL;
+
+	/*
+	** retrieve pathname strings
+	*/
+	if ( (cowpair.cowflen > PATH_MAX) || (cowpair.rdoflen > PATH_MAX) )
+		return -ENAMETOOLONG;
+
+	if ( !(cowpath = kmalloc(cowpair.cowflen+1, GFP_KERNEL)) )
+		return -ENOMEM;
+
+	if ( copy_from_user(cowpath, (void __user *)cowpair.cowfile,
+	                                            cowpair.cowflen) ) {
+		kfree(cowpath);
+		return -EFAULT;
+	}
+	*(cowpath+cowpair.cowflen) = 0;
+
+	if ( !(rdopath = kmalloc(cowpair.rdoflen+1, GFP_KERNEL)) ) {
+		kfree(cowpath);
+		return -ENOMEM;
+	}
+
+	if ( copy_from_user(rdopath, (void __user *)cowpair.rdofile,
+	                                            cowpair.rdoflen) ) {
+		kfree(rdopath);
+		kfree(cowpath);
+		return -EFAULT;
+	}
+	*(rdopath+cowpair.rdoflen) = 0;
+
+	/*
+	** open new cowdevice
+	*/
+	if ( cowpair.device == ANYDEV) {
+		/*
+		** search first unused minor
+		*/
+		for (i=0, rv=-EBUSY; i < maxcows; i++) {
+			if ( !((cowdevall[i])->state & COWDEVOPEN) ) {
+				rv = cowlo_openpair(rdopath, cowpath, 0, i);
+				break;
+			}
+		}
+
+		if (rv) { 		/* open failed? */
+			kfree(rdopath);
+			kfree(cowpath);
+			return rv;
+		}
+
+		/*
+		** return newly allocated cowdevice to user space
+		*/
+		cowpair.device = MKDEV(COWMAJOR, i);
+
+		if ( copy_to_user(arg, &cowpair, sizeof cowpair)) {
+			kfree(rdopath);
+			kfree(cowpath);
+			return -EFAULT;
+		}
+	} else { 		/* specific minor requested */
+		if ( (rv = cowlo_openpair(rdopath, cowpath, 0,
+						MINOR(cowpair.device)))) {
+			kfree(rdopath);
+			kfree(cowpath);
+			return rv;
+		}
+	}
+
+	return 0;
+}
+
+/*
+** handle ioctl-command COWRMPAIR:
+** 	deactivate an existing cowdevice (pair of rdofile/cowfile) on-the-fly
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_removepair(unsigned long __user *arg)
+{
+	unsigned long		cowdevice;
+	struct cowloop_device	*cowdev;
+
+	/*
+	** retrieve info about device to be removed
+	*/
+	if ( copy_from_user(&cowdevice, arg, sizeof cowdevice))
+		return -EFAULT;
+
+	/*
+	** verify major-minor number
+	*/
+	if ( MAJOR(cowdevice) != COWMAJOR)
+		return -EINVAL;
+
+	if ( MINOR(cowdevice) >= maxcows)
+		return -EINVAL;
+
+	cowdev = cowdevall[MINOR(cowdevice)];
+
+	if ( !(cowdev->state & COWDEVOPEN) )
+		return -ENODEV;
+
+	/*
+	** synchronize bitmaps and close cowdevice
+	*/
+	if (cowdev->state & COWRWCOWOPEN) {
+		down(&cowdevlock);
+		cowlo_sync();
+		up(&cowdevlock);
+	}
+
+	return cowlo_closepair(cowdev);
+}
+
+/*
+** handle ioctl-command COWWATCH:
+**	watch the free space of the filesystem containing a cowfile
+**      of an open cowdevice
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_watch(struct cowpair __user *arg)
+{
+	struct cowloop_device	*cowdev;
+	struct cowwatch		cowwatch;
+
+	/*
+	** retrieve structure holding info
+	*/
+	if ( copy_from_user(&cowwatch, arg, sizeof cowwatch))
+		return -EFAULT;
+
+	/*
+	** verify if cowdevice exists and is currently open
+	*/
+	if ( MINOR(cowwatch.device) >= maxcows)
+		return -EINVAL;
+
+	cowdev = cowdevall[MINOR(cowwatch.device)];
+
+	if ( !(cowdev->state & COWDEVOPEN) )
+		return -ENODEV;
+
+	/*
+	** if the WATCHWAIT-option is set, wait until the indicated
+	** threshold is reached (only one waiter allowed)
+	*/
+	if (cowwatch.flags & WATCHWAIT) {
+		/*
+		** check if already another waiter active
+		** for this cowdevice
+		*/
+		if (cowdev->state & COWWATCHDOG)
+			return -EAGAIN;
+
+		cowdev->state |= COWWATCHDOG;
+
+		cowdev->watchthresh = (unsigned long long)
+		                      cowwatch.threshold /
+				      (cowdev->blksize / 1024);
+
+		if (wait_event_interruptible(cowdev->watchq,
+		                    cowdev->watchthresh >= cowdev->blkavail)) {
+			cowdev->state &= ~COWWATCHDOG;
+			return EINTR;
+		}
+
+		cowdev->state &= ~COWWATCHDOG;
+	}
+
+	cowwatch.totalkb = (unsigned long long)cowdev->blktotal *
+	                                       cowdev->blksize / 1024;
+	cowwatch.availkb = (unsigned long long)cowdev->blkavail *
+	                                       cowdev->blksize / 1024;
+
+	if ( copy_to_user(arg, &cowwatch, sizeof cowwatch))
+		return -EFAULT;
+
+	return 0;
+}
+
+/*
+** handle ioctl-commands COWCLOSE and COWRDOPEN:
+**	COWCLOSE  - close the cowfile while the cowdevice remains open;
+**                  this allows an unmount of the filesystem on which
+**                  the cowfile resides
+**	COWRDOPEN - close the cowfile and reopen it for read-only;
+**                  this allows a remount read-ony of the filesystem
+**                  on which the cowfile resides
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_cowctl(unsigned long __user *arg, int cmd)
+{
+	struct cowloop_device	*cowdev;
+	unsigned long		cowdevice;
+
+	/*
+	** retrieve info about device to be removed
+	*/
+	if ( copy_from_user(&cowdevice, arg, sizeof cowdevice))
+		return -EFAULT;
+
+	/*
+	** verify major-minor number
+	*/
+	if ( MAJOR(cowdevice) != COWMAJOR)
+		return -EINVAL;
+
+	if ( MINOR(cowdevice) >= maxcows)
+		return -EINVAL;
+
+	cowdev = cowdevall[MINOR(cowdevice)];
+
+	if ( !(cowdev->state & COWDEVOPEN) )
+		return -ENODEV;
+
+	/*
+	** synchronize bitmaps and close cowfile
+	*/
+	if (cowdev->state & COWRWCOWOPEN) {
+		down(&cowdevlock);
+		cowlo_sync();
+		up(&cowdevlock);
+	}
+
+	/*
+	** handle specific ioctl-command
+	*/
+	switch (cmd) {
+	   case COWRDOPEN:
+		/*
+		** if the cowfile is still opened read-write
+		*/
+		if (cowdev->state & COWRWCOWOPEN) {
+			/*
+			** close the cowfile
+			*/
+  			if (cowdev->cowfp)
+		  		filp_close(cowdev->cowfp, 0);
+
+			cowdev->state &= ~COWRWCOWOPEN;
+
+			/*
+			** open again for read-only
+			*/
+			cowdev->cowfp = filp_open(cowdev->cowname,
+		                          O_RDONLY|O_LARGEFILE, 0600);
+
+			if ( (cowdev->cowfp == NULL) || IS_ERR(cowdev->cowfp) ) {
+				printk(KERN_ERR
+				     "cowloop - failed to reopen cowfile %s\n",
+				     cowdev->cowname);
+				return -EINVAL;
+			}
+
+			/*
+			** mark cowfile open for read-only
+			*/
+			cowdev->state |= COWRDCOWOPEN;
+		} else {
+			return -EINVAL;
+		}
+		break;
+
+	   case COWCLOSE:
+		/*
+		** if the cowfile is still open
+		*/
+		if (cowdev->state & COWCOWOPEN) {
+			/*
+			** close the cowfile
+			*/
+  			if (cowdev->cowfp)
+		  		filp_close(cowdev->cowfp, 0);
+
+			cowdev->state &= ~COWCOWOPEN;
+		}
+	}
+
+	return 0;
+}
+
+
+/*****************************************************************************/
+/* Handling of I/O-requests for a cowdevice                                  */
+/*****************************************************************************/
+
+/*
+** function to be called by core-kernel to handle the I/O-requests
+** in the queue
+*/
+static void
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,25))
+cowlo_request(struct request_queue *q)
+#else
+cowlo_request(request_queue_t *q)
+#endif
+{
+	struct request		*req;
+	struct cowloop_device	*cowdev;
+
+	DEBUGP(DCOW "cowloop - request function called....\n");
+
+	while((req = elv_next_request(q)) != NULL) {
+		DEBUGP(DCOW "cowloop - got next request\n");
+
+		if (! blk_fs_request(req)) {
+               		 /* this is not a normal file system request */
+                	end_request(req, 0);
+                	continue;
+        	}
+		cowdev = req->rq_disk->private_data;
+
+		if (cowdev->iobusy)
+			return;
+		else
+			cowdev->iobusy = 1;
+
+		/*
+		** when no kernel-thread is available, the request will
+		** produce an I/O-error
+		*/
+		if (!cowdev->pid) {
+			printk(KERN_ERR"cowloop - no thread available\n");
+			end_request(req, 0);	/* request failed */
+			cowdev->iobusy	= 0;
+			continue;
+		}
+
+		/*
+		** handle I/O-request in the context of the kernel-thread
+		*/
+		cowdev->req 	= req;
+		cowdev->qfilled	= 1;
+
+		wake_up_interruptible_sync(&cowdev->waitq);
+
+		/*
+		** get out of this function now while the I/O-request is
+		** under treatment of the kernel-thread; this function
+		** will be called again after the current I/O-request has
+		** been finished by the thread
+		*/
+		return;
+	}
+}
+
+/*
+** daemon-process (kernel-thread) executes this function
+*/
+static int
+cowlo_daemon(struct cowloop_device *cowdev)
+{
+	int	rv;
+	int     minor;
+	char	myname[16];
+
+	for (minor = 0; minor < maxcows; minor++) {
+		if (cowdev == cowdevall[minor]) break;
+	}
+	sprintf(myname, "cowloopd%d", minor);
+
+        daemonize(myname);
+
+	while (!cowdev->closedown) {
+		/*
+		** sleep while waiting for an I/O request;
+		** note that no non-interruptible wait has been used
+		** because the non-interruptible version of
+		** a *synchronous* wake_up does not exist (any more)
+		*/
+		if (wait_event_interruptible(cowdev->waitq, cowdev->qfilled)){
+			flush_signals(current); /* ignore signal-based wakeup */
+			continue;
+		}
+
+		if (cowdev->closedown)		/* module will be unloaded ? */{
+			cowdev->pid = 0;
+			return 0;
+		}
+
+		/*
+		** woken up by the I/O-request handler:	treat requested I/O
+		*/
+		cowdev->qfilled = 0;
+
+		rv = cowlo_do_request(cowdev->req);
+
+		/*
+		** reacquire the queue-spinlock for manipulating
+		** the request-queue and dequeue the request
+		*/
+		spin_lock_irq(&cowdev->rqlock);
+
+		end_request(cowdev->req, rv);
+		cowdev->iobusy = 0;
+
+		/*
+		** initiate the next request from the queue
+		*/
+		cowlo_request(cowdev->rqueue);
+
+		spin_unlock_irq(&cowdev->rqlock);
+	}
+	return 0;
+}
+
+/*
+** function to be called in the context of the kernel thread
+** to handle the queued I/O-requests
+**
+** returns:
+** 	0   - fail
+**      1   - success
+*/
+static long int
+cowlo_do_request(struct request *req)
+{
+	unsigned long		len;
+	long int		rv;
+	loff_t 			offset;
+	struct cowloop_device	*cowdev = req->rq_disk->private_data;
+
+	/*
+	** calculate some variables which are needed later on
+	*/
+	len     =          req->current_nr_sectors << 9;
+	offset  = (loff_t) req->sector             << 9;
+
+	DEBUGP(DCOW"cowloop - req cmd=%d offset=%lld len=%lu addr=%p\n",
+				*(req->cmd), offset, len, req->buffer);
+
+	/*
+	** handle READ- or WRITE-request
+	*/
+	switch (rq_data_dir(req)) {
+	   /**********************************************************/
+	   case READ:
+		switch ( cowlo_checkio(cowdev, len, offset) ) {
+		   case ALLCOW:
+			rv = cowlo_readcow(cowdev, req->buffer, len, offset);
+			break;
+
+		   case ALLRDO:
+			rv = cowlo_readrdo(cowdev, req->buffer, len, offset);
+			break;
+
+	   	   case MIXEDUP:
+			rv = cowlo_readmix(cowdev, req->buffer, len, offset);
+			break;
+
+		   default:
+			rv = 0;	/* never happens */
+		}
+		break;
+
+	   /**********************************************************/
+	   case WRITE:
+		switch ( cowlo_checkio(cowdev, len, offset) ) {
+		   case ALLCOW:
+			/*
+			** straight-forward write will do...
+			*/
+			DEBUGP(DCOW"cowloop - write straight ");
+
+			rv = cowlo_writecow(cowdev, req->buffer, len, offset);
+			break;	/* from switch */
+
+		   case ALLRDO:
+			if ( (len & MUMASK) == 0) {
+				DEBUGP(DCOW"cowloop - write straight ");
+
+				rv = cowlo_writecow(cowdev, req->buffer,
+								len, offset);
+				break;
+			}
+
+	   	   case MIXEDUP:
+			rv = cowlo_writemix(cowdev, req->buffer, len, offset);
+			break;
+
+		   default:
+			rv = 0;	/* never happens */
+		}
+		break;
+
+	   default:
+		printk(KERN_ERR
+		       "cowloop - unrecognized command %d\n", *(req->cmd));
+		rv = 0;
+	}
+
+	return (rv <= 0 ? 0 : 1);
+}
+
+/*
+** check for a given I/O-request if all underlying blocks
+** (with size MAPUNIT) are either in the read-only file or in
+** the cowfile (or a combination of the two)
+**
+** returns:
+** 	ALLRDO  - all underlying blocks in rdofile
+**      ALLCOW  - all underlying blocks in cowfile
+**      MIXEDUP - underlying blocks partly in rdofile and partly in cowfile
+*/
+static int
+cowlo_checkio(struct cowloop_device *cowdev, int len, loff_t offset)
+{
+	unsigned long	mapnum, bytenum, bitnum, blocknr, partlen;
+	long int	totcnt, cowcnt;
+	char		*mc;
+
+	/*
+	** notice that the requested block might cross
+	** a blocksize boundary while one of the concerned
+	** blocks resides in the read-only file and another
+	** one in the copy-on-write file; in that case the
+        ** request will be broken up into pieces
+	*/
+	if ( (len <= MAPUNIT) &&
+	     (MAPUNIT - (offset & MUMASK) <= len) ) {
+		/*
+		** easy situation:
+		** requested data-block entirely fits within
+		** the mapunit used for the bitmap
+		** check if that block is located in rdofile or
+		** cowfile
+		*/
+		blocknr = offset >> MUSHIFT;
+
+		mapnum  = CALCMAP (blocknr);
+		bytenum = CALCBYTE(blocknr);
+		bitnum  = CALCBIT (blocknr);
+
+		if (*(*(cowdev->mapcache+mapnum)+bytenum)&(1<<bitnum))
+			return ALLCOW;
+		else
+			return ALLRDO;
+	}
+
+	/*
+	** less easy situation:
+	** the requested data-block does not fit within the mapunit
+	** used for the bitmap
+	** check if *all* underlying blocks involved reside on the rdofile
+       	** or the cowfile (so still no breakup required)
+	*/
+	for (cowcnt=totcnt=0; len > 0; len-=partlen, offset+=partlen, totcnt++){
+		/*
+		** calculate blocknr of involved block
+		*/
+		blocknr = offset >> MUSHIFT;
+
+		/*
+		** calculate partial length for this transfer
+		*/
+		partlen = MAPUNIT - (offset & MUMASK);
+		if (partlen > len)
+			partlen = len;
+
+		/*
+		** is this block located in the cowfile
+		*/
+		mapnum  = CALCMAP (blocknr);
+		bytenum = CALCBYTE(blocknr);
+		bitnum  = CALCBIT (blocknr);
+
+		mc	= *(cowdev->mapcache+mapnum);
+
+		if (*(mc+bytenum)&(1<<bitnum))
+			cowcnt++;;
+
+		DEBUGP(DCOW
+		       "cowloop - check %lu - map %lu, byte %lu, bit %lu, "
+		       "cowcnt %ld, totcnt %ld %02x %p\n",
+			blocknr, mapnum, bytenum, bitnum, cowcnt, totcnt,
+			*(mc+bytenum), mc);
+	}
+
+	if (cowcnt == 0)	/* all involved blocks on rdofile? */
+		return ALLRDO;
+
+	if (cowcnt == totcnt)	/* all involved blocks on cowfile? */
+		return ALLCOW;
+
+	/*
+	** situation somewhat more complicated:
+	** involved underlying blocks spread over both files
+	*/
+	return MIXEDUP;
+}
+
+/*
+** read requested chunk partly from rdofile and partly from cowfile
+**
+** returns:
+** 	0   - fail
+**      1   - success
+*/
+static int
+cowlo_readmix(struct cowloop_device *cowdev, void *buf, int len, loff_t offset)
+{
+	unsigned long	mapnum, bytenum, bitnum, blocknr, partlen;
+	long int	rv;
+	char		*mc;
+
+	/*
+	** complicated approach: breakup required of read-request
+	*/
+	for (rv=1; len > 0; len-=partlen, buf+=partlen, offset+=partlen) {
+		/*
+		** calculate blocknr of entire block
+		*/
+		blocknr = offset >> MUSHIFT;
+
+		/*
+		** calculate partial length for this transfer
+		*/
+		partlen = MAPUNIT - (offset & MUMASK);
+		if (partlen > len)
+			partlen = len;
+
+		/*
+		** is this block located in the cowfile
+		*/
+		mapnum  = CALCMAP (blocknr);
+		bytenum = CALCBYTE(blocknr);
+		bitnum  = CALCBIT (blocknr);
+		mc	= *(cowdev->mapcache+mapnum);
+
+		if (*(mc+bytenum)&(1<<bitnum)) {
+			/*
+			** read (partial) block from cowfile
+			*/
+			DEBUGP(DCOW"cowloop - split read "
+				"cow partlen=%ld off=%lld\n", partlen, offset);
+
+			if (cowlo_readcow(cowdev, buf, partlen, offset) <= 0)
+				rv = 0;
+		} else {
+			/*
+			** read (partial) block from rdofile
+			*/
+			DEBUGP(DCOW"cowloop - split read "
+				"rdo partlen=%ld off=%lld\n", partlen, offset);
+
+			if (cowlo_readrdo(cowdev, buf, partlen, offset) <= 0)
+				rv = 0;
+		}
+	}
+
+	return rv;
+}
+
+/*
+** chunk to be written to the cowfile needs pieces to be
+** read from the rdofile
+**
+** returns:
+** 	0   - fail
+**      1   - success
+*/
+static int
+cowlo_writemix(struct cowloop_device *cowdev, void *buf, int len, loff_t offset)
+{
+	unsigned long	mapnum, bytenum, bitnum, blocknr, partlen;
+	long int	rv;
+	char		*mc;
+
+	/*
+	** somewhat more complicated stuff is required:
+	** if the request is larger than one underlying
+	** block or is spread over two underlying blocks,
+	** split the request into pieces; if a block does not
+	** start at a block boundary, take care that
+	** surrounding data is read first (if needed),
+	** fit the new data in and write it as a full block
+	*/
+	for (rv=1; len > 0; len-=partlen, buf+=partlen, offset+=partlen) {
+		/*
+		** calculate partial length for this transfer
+		*/
+		partlen = MAPUNIT - (offset & MUMASK);
+		if (partlen > len)
+			partlen = len;
+
+		/*
+		** calculate blocknr of entire block
+		*/
+		blocknr = offset >> MUSHIFT;
+
+		/*
+		** has this block been written before?
+		*/
+		mapnum  = CALCMAP (blocknr);
+		bytenum = CALCBYTE(blocknr);
+		bitnum  = CALCBIT (blocknr);
+		mc	= *(cowdev->mapcache+mapnum);
+
+		if (*(mc+bytenum)&(1<<bitnum)) {
+			/*
+			** block has been written before;
+			** write transparantly to cowfile
+			*/
+			DEBUGP(DCOW
+			       "cowloop - splitwr transp\n");
+
+			if (cowlo_writecow(cowdev, buf, partlen, offset) <= 0)
+				rv = 0;
+		} else {
+			/*
+			** block has never been written before,
+			** so read entire block from
+			** read-only file first, unless
+			** a full block is requested to
+			** be written
+			*/
+			if (partlen < MAPUNIT) {
+				if (cowlo_readrdo(cowdev, cowdev->iobuf,
+				      MAPUNIT, (loff_t)blocknr << MUSHIFT) <= 0)
+					rv = 0;
+			}
+
+			/*
+			** transfer modified part into
+			** the block just read
+			*/
+			memcpy(cowdev->iobuf + (offset & MUMASK), buf, partlen);
+
+			/*
+			** write entire block to cowfile
+			*/
+			DEBUGP(DCOW"cowloop - split "
+				"partlen=%ld off=%lld\n",
+				partlen, (loff_t)blocknr << MUSHIFT);
+
+			if (cowlo_writecow(cowdev, cowdev->iobuf, MAPUNIT,
+					     (loff_t)blocknr << MUSHIFT) <= 0)
+				rv = 0;
+		}
+	}
+
+	return rv;
+}
+
+/*****************************************************************************/
+/* I/O-support for read-only file and copy-on-write file                     */
+/*****************************************************************************/
+
+/*
+** read data from the read-only file
+**
+** return-value: similar to user-mode read
+*/
+static long int
+cowlo_readrdo(struct cowloop_device *cowdev, void *buf, int len, loff_t offset)
+{
+	long int	rv;
+	mm_segment_t	old_fs;
+	loff_t		saveoffset = offset;
+
+	DEBUGP(DCOW"cowloop - readrdo called\n");
+
+        old_fs = get_fs();
+	set_fs( get_ds() );
+	rv = cowdev->rdofp->f_op->read(cowdev->rdofp, buf, len, &offset);
+        set_fs(old_fs);
+
+	if (rv < len) {
+		printk(KERN_WARNING "cowloop - read-failure %ld on rdofile"
+		                    "- offset=%lld len=%d\n",
+					rv, saveoffset, len);
+	}
+
+	cowdev->rdoreads++;
+	return rv;
+}
+
+/*
+** read cowfile from a modified offset, i.e. skipping the bitmap and cowhead
+**
+** return-value: similar to user-mode read
+*/
+static long int
+cowlo_readcow(struct cowloop_device *cowdev, void *buf, int len, loff_t offset)
+{
+	DEBUGP(DCOW"cowloop - readcow called\n");
+
+	offset += cowdev->cowhead->doffset;
+
+	return cowlo_readcowraw(cowdev, buf, len, offset);
+}
+
+/*
+** read cowfile from an absolute offset
+**
+** return-value: similar to user-mode read
+*/
+static long int
+cowlo_readcowraw(struct cowloop_device *cowdev,
+					void *buf, int len, loff_t offset)
+{
+	long int	rv;
+	mm_segment_t	old_fs;
+	loff_t		saveoffset = offset;
+
+	DEBUGP(DCOW"cowloop - readcowraw called\n");
+
+	/*
+	** be sure that cowfile is opened for read-write
+	*/
+	if ( !(cowdev->state & COWCOWOPEN) ) {
+		 printk(KERN_WARNING
+		        "cowloop - read request from cowfile refused\n");
+
+		return -EBADF;
+	}
+
+	/*
+	** issue low level read
+	*/
+        old_fs = get_fs();
+	set_fs( get_ds() );
+	rv = cowdev->cowfp->f_op->read(cowdev->cowfp, buf, len, &offset);
+        set_fs(old_fs);
+
+	if (rv < len) {
+		printk(KERN_WARNING
+		       "cowloop - read-failure %ld on cowfile"
+		       "- offset=%lld len=%d\n", rv, saveoffset, len);
+	}
+
+	cowdev->cowreads++;
+	return rv;
+}
+
+/*
+** write cowfile from a modified offset, i.e. skipping the bitmap and cowhead
+**
+** if a block is written for the first time while its contents consists
+** of binary zeroes only, the concerning bitmap is flushed to the cowfile
+**
+** return-value: similar to user-mode write
+*/
+static long int
+cowlo_writecow(struct cowloop_device *cowdev, void *buf, int len, loff_t offset)
+{
+	long int	rv;
+	unsigned long	mapnum=0, mapbyte=0, mapbit=0, cowblock=0, partlen;
+	char		*tmpptr,  *mapptr = NULL;
+	loff_t		tmpoffset, mapoffset = 0;
+
+	DEBUGP(DCOW"cowloop - writecow called\n");
+
+	/*
+	** be sure that cowfile is opened for read-write
+	*/
+	if ( !(cowdev->state & COWRWCOWOPEN) ) {
+		 printk(KERN_WARNING
+		        "cowloop - Write request to cowfile refused\n");
+
+		return -EBADF;
+	}
+
+	/*
+	** write the entire block to the cowfile
+	*/
+	tmpoffset = offset + cowdev->cowhead->doffset;
+
+	rv = cowlo_writecowraw(cowdev, buf, len, tmpoffset);
+
+	/*
+	** verify if enough space available on filesystem holding
+	** the cowfile
+	**   - when the last write failed (might be caused by lack of space)
+	**   - when a watcher is active (to react adequatly)
+	**   - when the previous check indicated fs was almost full
+	**   - with regular intervals
+	*/
+	if ( (rv <= 0)				       ||
+	     (cowdev->state        & COWWATCHDOG)      ||
+	     (cowdev->blkavail / 2 < SPCDFLINTVL)      ||
+	     (cowdev->cowwrites    % SPCDFLINTVL == 0) ) {
+		struct kstatfs		ks;
+
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,18))
+		if (vfs_statfs(cowdev->cowfp->f_dentry, &ks)==0){
+#else
+		if (vfs_statfs(cowdev->cowfp->f_dentry->d_inode->i_sb, &ks)==0){
+#endif
+			if (ks.f_bavail <= SPCMINBLK) {
+				switch (ks.f_bavail) {
+				   case 0:
+				   case 1:
+				   case 2:
+				   case 3:
+					printk(KERN_ALERT
+					       "cowloop - "
+					       "ALERT: cowfile full!\n");
+					break;
+
+				   default:
+					printk(KERN_WARNING
+					       "cowloop - cowfile almost "
+					       "full (only %llu Kb free)\n",
+						(unsigned long long)
+                                                ks.f_bsize * ks.f_bavail /1024);
+				}
+			}
+
+			cowdev->blktotal = ks.f_blocks;
+			cowdev->blkavail = ks.f_bavail;
+
+			/*
+			** wakeup watcher if threshold has been reached
+			*/
+			if ( (cowdev->state & COWWATCHDOG) &&
+			    (cowdev->watchthresh >= cowdev->blkavail) ) {
+				wake_up_interruptible(&cowdev->watchq);
+			}
+		}
+	}
+
+	if (rv <= 0)
+		return rv;
+
+	DEBUGP(DCOW"cowloop - block written\n");
+
+	/*
+	** check if block(s) is/are written to the cowfile
+	** for the first time; if so, adapt the bitmap
+	*/
+	for (; len > 0; len-=partlen, offset+=partlen, buf+=partlen) {
+		/*
+		** calculate partial length for this transfer
+		*/
+		partlen = MAPUNIT - (offset & MUMASK);
+		if (partlen > len)
+			partlen = len;
+
+		/*
+		** calculate bitnr of written chunk of cowblock
+		*/
+		cowblock = offset >> MUSHIFT;
+
+		mapnum   = CALCMAP (cowblock);
+		mapbyte  = CALCBYTE(cowblock);
+		mapbit   = CALCBIT (cowblock);
+
+		if (*(*(cowdev->mapcache+mapnum)+mapbyte) & (1<<mapbit))
+			continue;	/* already written before */
+
+	       	/*
+		** if the block is written for the first time,
+		** the corresponding bit should be set in the bitmap
+		*/
+		*(*(cowdev->mapcache+mapnum)+mapbyte) |= (1<<mapbit);
+
+		cowdev->nrcowblocks++;
+
+		DEBUGP(DCOW"cowloop - bitupdate blk=%ld map=%ld "
+		        "byte=%ld bit=%ld\n",
+			cowblock, mapnum, mapbyte, mapbit);
+
+		/*
+		** check if the cowhead in the cowfile is currently
+		** marked clean; if so, mark it dirty and flush it
+		*/
+		if ( !(cowdev->cowhead->flags &= COWDIRTY)) {
+			cowdev->cowhead->flags	|= COWDIRTY;
+
+			cowlo_writecowraw(cowdev, cowdev->cowhead,
+							MAPUNIT, (loff_t)0);
+		}
+
+		/*
+		** if the written datablock contained binary zeroes,
+		** the bitmap block should be marked to be flushed to disk
+		** (blocks containing all zeroes cannot be recovered by
+		** the cowrepair-program later on if cowloop is not properly
+		** removed via rmmod)
+		*/
+		if ( memcmp(buf, allzeroes, partlen) ) /* not all zeroes? */
+			continue;                      /* no flush needed */
+
+		/*
+		** calculate positions of bitmap block to be flushed
+		** - pointer of bitmap block in memory
+		** - offset  of bitmap block in cowfile
+		*/
+		tmpptr    = *(cowdev->mapcache+mapnum) + (mapbyte & (~MUMASK));
+		tmpoffset = (loff_t) MAPUNIT + mapnum * MAPCHUNKSZ +
+		                                       (mapbyte & (~MUMASK));
+
+		/*
+		** flush a bitmap block at the moment that all bits have
+		** been set in that block, i.e. at the moment that we
+		** switch to another bitmap block
+		*/
+		if ( (mapoffset != 0) && (mapoffset != tmpoffset) ) {
+			if (cowlo_writecowraw(cowdev, mapptr, MAPUNIT,
+							mapoffset) < 0) {
+				printk(KERN_WARNING
+				       "cowloop - write-failure on bitmap - "
+				       "blk=%ld map=%ld byte=%ld bit=%ld\n",
+				  	cowblock, mapnum, mapbyte, mapbit);
+			}
+
+			DEBUGP(DCOW"cowloop - bitmap blk written %lld\n",
+								mapoffset);
+		}
+
+		/*
+		** remember offset in cowfile and offset in memory
+		** for bitmap to be flushed; flushing will be done
+		** as soon as all updates in this bitmap block have
+		** been done
+		*/
+		mapoffset = tmpoffset;
+		mapptr    = tmpptr;
+	}
+
+	/*
+	** any new block written containing binary zeroes?
+	*/
+	if (mapoffset) {
+		if (cowlo_writecowraw(cowdev, mapptr, MAPUNIT, mapoffset) < 0) {
+			printk(KERN_WARNING
+			       "cowloop - write-failure on bitmap - "
+			       "blk=%ld map=%ld byte=%ld bit=%ld\n",
+			       cowblock, mapnum, mapbyte, mapbit);
+		}
+
+		DEBUGP(DCOW"cowloop - bitmap block written %lld\n", mapoffset);
+	}
+
+	return rv;
+}
+
+/*
+** write cowfile from an absolute offset
+**
+** return-value: similar to user-mode write
+*/
+static long int
+cowlo_writecowraw(struct cowloop_device *cowdev,
+					void *buf, int len, loff_t offset)
+{
+	long int	rv;
+	mm_segment_t	old_fs;
+	loff_t		saveoffset = offset;
+
+	DEBUGP(DCOW"cowloop - writecowraw called\n");
+
+	/*
+	** be sure that cowfile is opened for read-write
+	*/
+	if ( !(cowdev->state & COWRWCOWOPEN) ) {
+		 printk(KERN_WARNING
+		        "cowloop - write request to cowfile refused\n");
+
+		return -EBADF;
+	}
+
+	/*
+	** issue low level write
+	*/
+        old_fs = get_fs();
+	set_fs( get_ds() );
+	rv = cowdev->cowfp->f_op->write(cowdev->cowfp, buf, len, &offset);
+        set_fs(old_fs);
+
+	if (rv < len) {
+		printk(KERN_WARNING
+		       "cowloop - write-failure %ld on cowfile"
+		       "- offset=%lld len=%d\n", rv, saveoffset, len);
+	}
+
+	cowdev->cowwrites++;
+	return rv;
+}
+
+
+/*
+** readproc-function: called when the corresponding /proc-file is read
+*/
+static int
+cowlo_readproc(char *buf, char **start, off_t pos, int cnt, int *eof, void *p)
+{
+	struct cowloop_device *cowdev = p;
+
+	revision[sizeof revision - 3] = '\0';
+
+	return sprintf(buf,
+		"   cowloop version: %9s\n\n"
+		"      device state: %s%s%s%s\n"
+		"   number of opens: %9d\n"
+		"     pid of thread: %9d\n\n"
+		"    read-only file: %9s\n"
+		"          rdoreads: %9lu\n\n"
+		"copy-on-write file: %9s\n"
+		"     state cowfile: %9s\n"
+		"     bitmap-blocks: %9lu (of %d bytes)\n"
+		"  cowblocks in use: %9lu (of %d bytes)\n"
+		"          cowreads: %9lu\n"
+		"         cowwrites: %9lu\n",
+			&revision[11],
+
+			cowdev->state & COWDEVOPEN   ? "devopen "   : "",
+			cowdev->state & COWRWCOWOPEN ? "cowopenrw " : "",
+			cowdev->state & COWRDCOWOPEN ? "cowopenro " : "",
+			cowdev->state & COWWATCHDOG  ? "watchdog "  : "",
+
+			cowdev->opencnt,
+			cowdev->pid,
+			cowdev->rdoname,
+			cowdev->rdoreads,
+			cowdev->cowname,
+			cowdev->cowhead->flags & COWDIRTY ? "dirty":"clean",
+			cowdev->mapsize >> MUSHIFT, MAPUNIT,
+			cowdev->nrcowblocks, MAPUNIT,
+			cowdev->cowreads,
+			cowdev->cowwrites);
+}
+
+/*****************************************************************************/
+/* Setup and destroy cowdevices                                              */
+/*****************************************************************************/
+
+/*
+** open and prepare a cowdevice (rdofile and cowfile) and allocate bitmaps
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_openpair(char *rdof, char *cowf, int autorecover, int minor)
+{
+	long int		rv;
+	struct cowloop_device	*cowdev = cowdevall[minor];
+	struct kstatfs		ks;
+
+	down(&cowdevlock);
+
+	/*
+	** requested device exists?
+	*/
+	if (minor >= maxcows) {
+		up(&cowdevlock);
+		return -ENODEV;
+	}
+
+	/*
+	** requested device already assigned to cowdevice?
+	*/
+	if (cowdev->state & COWDEVOPEN) {
+		up(&cowdevlock);
+		return -EBUSY;
+	}
+
+	/*
+	** initialize administration
+	*/
+	memset(cowdev, 0, sizeof *cowdev);
+
+	spin_lock_init     (&cowdev->rqlock);
+	init_waitqueue_head(&cowdev->waitq);
+	init_waitqueue_head(&cowdev->watchq);
+
+	/*
+	** open the read-only file
+	*/
+	DEBUGP(DCOW"cowloop - call openrdo....\n");
+
+	if ( (rv = cowlo_openrdo(cowdev, rdof)) ) {
+		cowlo_undo_openrdo(cowdev);
+		up(&cowdevlock);
+		return rv;
+	}
+
+	/*
+	** open the cowfile
+	*/
+	DEBUGP(DCOW"cowloop - call opencow....\n");
+
+	if ( (rv = cowlo_opencow(cowdev, cowf, autorecover)) ) {
+		cowlo_undo_openrdo(cowdev);
+		cowlo_undo_opencow(cowdev);
+		up(&cowdevlock);
+		return rv;
+	}
+
+	/*
+	** administer total and available size of filesystem holding cowfile
+	*/
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,18))
+		if (vfs_statfs(cowdev->cowfp->f_dentry, &ks)==0){
+#else
+		if (vfs_statfs(cowdev->cowfp->f_dentry->d_inode->i_sb, &ks)==0){
+#endif
+		cowdev->blksize  = ks.f_bsize;
+		cowdev->blktotal = ks.f_blocks;
+		cowdev->blkavail = ks.f_bavail;
+	} else {
+		cowdev->blksize  = 1024;	/* avoid division by zero */
+	}
+
+	/*
+	** flush the (recovered) bitmaps and cowhead to the cowfile
+	*/
+	DEBUGP(DCOW"cowloop - call cowsync....\n");
+
+	cowlo_sync();
+
+	/*
+	** allocate gendisk for the cow device
+	*/
+	DEBUGP(DCOW"cowloop - alloc disk....\n");
+
+	if ((cowdev->gd = alloc_disk(1)) == NULL) {
+		printk(KERN_WARNING
+		       "cowloop - unable to alloc_disk for cowloop\n");
+
+		cowlo_undo_openrdo(cowdev);
+		cowlo_undo_opencow(cowdev);
+		up(&cowdevlock);
+		return -ENOMEM;
+	}
+
+	cowdev->gd->major        = COWMAJOR;
+	cowdev->gd->first_minor  = minor;
+	cowdev->gd->minors       = 1;
+	cowdev->gd->fops         = &cowlo_fops;
+	cowdev->gd->private_data = cowdev;
+	sprintf(cowdev->gd->disk_name, "%s%d", DEVICE_NAME, minor);
+
+	/* in .5 Kb units */
+	set_capacity(cowdev->gd, (cowdev->numblocks*(MAPUNIT/512)));
+
+	DEBUGP(DCOW"cowloop - init request queue....\n");
+
+	if ((cowdev->rqueue = blk_init_queue(cowlo_request, &cowdev->rqlock))
+								== NULL) {
+		printk(KERN_WARNING
+		       "cowloop - unable to get request queue for cowloop\n");
+
+		del_gendisk(cowdev->gd);
+		cowlo_undo_openrdo(cowdev);
+		cowlo_undo_opencow(cowdev);
+		up(&cowdevlock);
+		return -EINVAL;
+	}
+
+	blk_queue_hardsect_size(cowdev->rqueue, cowdev->blocksz);
+	cowdev->gd->queue = cowdev->rqueue;
+
+	/*
+	** start kernel thread to handle requests
+	*/
+	DEBUGP(DCOW"cowloop - kickoff daemon....\n");
+
+	cowdev->pid = kernel_thread((int (*)(void *))cowlo_daemon, cowdev, 0);
+
+	/*
+	** create a file below directory /proc/cow for this new cowdevice
+	*/
+	if (cowlo_procdir) {
+		char 	tmpname[64];
+
+		sprintf(tmpname, "%d", minor);
+
+		create_proc_read_entry(tmpname, 0 , cowlo_procdir,
+						cowlo_readproc, cowdev);
+	}
+
+	cowdev->state	|= COWDEVOPEN;
+
+	cowdev->rdoname = rdof;
+	cowdev->cowname = cowf;
+
+	/*
+	** enable the new disk; this triggers the first request!
+	*/
+	DEBUGP(DCOW"cowloop - call add_disk....\n");
+
+	add_disk(cowdev->gd);
+
+	up(&cowdevlock);
+	return 0;
+}
+
+/*
+** close a cowdevice (pair of rdofile/cowfile) and release memory
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_closepair(struct cowloop_device *cowdev)
+{
+	int minor;
+
+	down(&cowdevlock);
+
+	/*
+	** if cowdevice is not activated at all, refuse
+	*/
+	if ( !(cowdev->state & COWDEVOPEN) ) {
+		up(&cowdevlock);
+		return -ENODEV;
+	}
+
+	/*
+	** if this cowdevice is still open, refuse
+	*/
+	if (cowdev->opencnt > 0) {
+		up(&cowdevlock);
+		return -EBUSY;
+	}
+
+	up(&cowdevlock);
+
+	/*
+	** wakeup watcher (if any)
+	*/
+	if (cowdev->state & COWWATCHDOG) {
+		cowdev->watchthresh = cowdev->blkavail;
+		wake_up_interruptible(&cowdev->watchq);
+	}
+
+	/*
+	** wakeup kernel-thread to be able to exit
+	** and wait until it has exited
+	*/
+	cowdev->closedown = 1;
+	cowdev->qfilled   = 1;
+	wake_up_interruptible(&cowdev->waitq);
+
+       	while (cowdev->pid)
+               	schedule();
+
+	del_gendisk(cowdev->gd);  /* revert the alloc_disk() */
+	put_disk(cowdev->gd);     /* revert the add_disk()   */
+
+	if (cowlo_procdir) {
+		char 	tmpname[64];
+
+		for (minor = 0; minor < maxcows; minor++) {
+			if (cowdev == cowdevall[minor]) break;
+		}
+		sprintf(tmpname, "%d", minor);
+
+		remove_proc_entry(tmpname, cowlo_procdir);
+	}
+
+	blk_cleanup_queue(cowdev->rqueue);
+
+	/*
+	** release memory for filenames if these names have
+	** been allocated dynamically
+	*/
+	if ( (cowdev->cowname) && (cowdev->cowname != cowfile))
+		kfree(cowdev->cowname);
+
+	if ( (cowdev->rdoname) && (cowdev->rdoname != rdofile))
+		kfree(cowdev->rdoname);
+
+	cowlo_undo_openrdo(cowdev);
+	cowlo_undo_opencow(cowdev);
+
+	cowdev->state &= ~COWDEVOPEN;
+
+	return 0;
+}
+
+/*
+** open the read-only file
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_openrdo(struct cowloop_device *cowdev, char *rdof)
+{
+	struct file	*f;
+	struct inode	*inode;
+	long int	i, nrval;
+
+	DEBUGP(DCOW"cowloop - openrdo called\n");
+
+	/*
+	** open the read-only file
+	*/
+        if(*rdof == '\0') {
+     	     	printk(KERN_ERR
+		       "cowloop - specify name for read-only file\n\n");
+         	return -EINVAL;
+        }
+
+	f = filp_open(rdof, O_RDONLY|O_LARGEFILE, 0);
+
+	if ( (f == NULL) || IS_ERR(f) ) {
+		printk(KERN_ERR
+		       "cowloop - open of rdofile %s failed\n", rdof);
+		return -EINVAL;
+	}
+
+	cowdev->rdofp = f;
+
+	inode = f->f_dentry->d_inode;
+
+	if ( !S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode) ) {
+		printk(KERN_ERR
+		       "cowloop - %s not regular file or blockdev\n", rdof);
+		return -EINVAL;
+	}
+
+	DEBUGP(DCOW"cowloop - determine size rdo....\n");
+
+	/*
+	** determine block-size and total size of read-only file
+	*/
+	if (S_ISREG(inode->i_mode)) {
+		/*
+		** read-only file is a regular file
+		*/
+		cowdev->blocksz   = 512;	/* other value fails */
+		cowdev->numblocks = inode->i_size >> MUSHIFT;
+
+		if (inode->i_size & MUMASK) {
+			printk(KERN_WARNING
+			       "cowloop - rdofile %s truncated to multiple "
+			       "of %d bytes\n", rdof, MAPUNIT);
+		}
+
+		DEBUGP(DCOW"cowloop - RO=regular: numblocks=%d, blocksz=%d\n",
+			cowdev->numblocks, cowdev->blocksz);
+	} else {
+		/*
+		** read-only file is a block device
+		*/
+		cowdev->belowdev  = inode->i_bdev;
+		cowdev->belowgd   = cowdev->belowdev->bd_disk; /* gendisk */
+
+		if (cowdev->belowdev->bd_part) {
+			cowdev->numblocks = cowdev->belowdev->bd_part->nr_sects
+								/ (MAPUNIT/512);
+		}
+
+		if (cowdev->belowgd) {
+			cowdev->belowq = cowdev->belowgd->queue;
+
+			if (cowdev->numblocks == 0) {
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,27))
+				cowdev->numblocks = get_capacity(cowdev->belowgd)
+                         					/ (MAPUNIT/512);
+#else
+				cowdev->numblocks = cowdev->belowgd->capacity
+                         					/ (MAPUNIT/512);
+#endif
+			}
+		}
+
+
+		if (cowdev->belowq)
+			cowdev->blocksz = cowdev->belowq->hardsect_size;
+
+		if (cowdev->blocksz == 0)
+			cowdev->blocksz = BLOCK_SIZE; /* default 2^10 */
+
+		DEBUGP(DCOW"cowloop - numblocks=%d, "
+		           "blocksz=%d, belowgd=%p, belowq=%p\n",
+		  	cowdev->numblocks, cowdev->blocksz,
+			cowdev->belowgd, cowdev->belowq);
+
+		DEBUGP(DCOW"cowloop - belowdev.bd_block_size=%d\n",
+		  	cowdev->belowdev->bd_block_size);
+	}
+
+	if (cowdev->numblocks == 0) {
+		printk(KERN_ERR "cowloop - %s has no contents\n", rdof);
+		return -EINVAL;
+	}
+
+	/*
+	** reserve space in memory as generic I/O buffer
+	*/
+	cowdev->iobuf  = kmalloc(MAPUNIT, GFP_KERNEL);
+
+	if (!cowdev->iobuf) {
+		printk(KERN_ERR
+		       "cowloop - cannot get space for buffer %d\n", MAPUNIT);
+		return -ENOMEM;
+	}
+
+	DEBUGP(DCOW"cowloop - determine fingerprint rdo....\n");
+
+	/*
+	** determine fingerprint for read-only file
+	** 	calculate fingerprint from first four datablocks
+	**	which do not contain binary zeroes
+	*/
+	for (i=0, cowdev->fingerprint=0, nrval=0;
+			(nrval < 4)&&(i < cowdev->numblocks); i++) {
+		int 		j;
+		unsigned char	cs;
+
+		/*
+		** read next block
+		*/
+		if (cowlo_readrdo(cowdev, cowdev->iobuf, MAPUNIT,
+						(loff_t)i << MUSHIFT) < 1)
+			break;
+
+		/*
+		** calculate fingerprint by adding all byte-values
+		*/
+		for (j=0, cs=0; j < MAPUNIT; j++)
+			cs += *(cowdev->iobuf+j);
+
+		if (cs == 0)	/* block probably contained zeroes */
+			continue;
+
+		/*
+		** shift byte-value to proper place in final fingerprint
+		*/
+		cowdev->fingerprint |= cs << (nrval*8);
+		nrval++;
+	}
+
+	return 0;
+}
+
+/*
+** undo memory allocs and file opens issued so far
+** related to the read-only file
+*/
+static void
+cowlo_undo_openrdo(struct cowloop_device *cowdev)
+{
+	if(cowdev->iobuf);
+		kfree(cowdev->iobuf);
+
+	if (cowdev->rdofp)
+  		filp_close(cowdev->rdofp, 0);
+}
+
+/*
+** open the cowfile
+**
+** returns:
+** 	0   - okay
+**    < 0   - error value
+*/
+static int
+cowlo_opencow(struct cowloop_device *cowdev, char *cowf, int autorecover)
+{
+	long int		i, rv;
+	int			minor;
+	unsigned long		nb;
+	struct file		*f;
+	struct inode		*inode;
+	loff_t			offset;
+	struct cowloop_device	*cowtmp;
+
+	DEBUGP(DCOW"cowloop - opencow called\n");
+
+	/*
+	** open copy-on-write file (read-write)
+	*/
+        if (cowf[0] == '\0') {
+          	printk(KERN_ERR
+                 "cowloop - specify name of copy-on-write file\n\n");
+         	return -EINVAL;
+        }
+
+	f = filp_open(cowf, O_RDWR|O_LARGEFILE, 0600);
+
+	if ( (f == NULL) || IS_ERR(f) ) {
+		/*
+		** non-existing cowfile: try to create
+		*/
+		f = filp_open(cowf, O_RDWR|O_CREAT|O_LARGEFILE, 0600);
+
+		if ( (f == NULL) || IS_ERR(f) ) {
+			printk(KERN_ERR
+		       	  "cowloop - failed to open file %s for read-write\n\n",
+		       						cowf);
+			return -EINVAL;
+	 	}
+	}
+
+	cowdev->cowfp = f;
+
+	inode = f->f_dentry->d_inode;
+
+	if (!S_ISREG(inode->i_mode)) {
+		printk(KERN_ERR "cowloop - %s is not regular file\n", cowf);
+		return -EINVAL;
+	}
+
+	/*
+	** check if this cowfile is already in use for another cowdevice
+	*/
+	for (minor = 0; minor < maxcows; minor++) {
+
+		cowtmp = cowdevall[minor];
+
+		if ( !(cowtmp->state & COWDEVOPEN) )
+			continue;
+
+		if (cowtmp == cowdev)
+			continue;
+
+		if (cowtmp->cowfp->f_dentry->d_inode == f->f_dentry->d_inode) {
+			printk(KERN_ERR
+			       "cowloop - %s: already in use as cow\n", cowf);
+			return -EBUSY;
+		}
+	}
+
+	/*
+	** mark cowfile open for read-write
+	*/
+	cowdev->state |= COWRWCOWOPEN;
+
+	/*
+	** calculate size (in bytes) for total bitmap in cowfile;
+	** when the size of the cowhead block is added, the start-offset
+	** for the modified data blocks can be found
+	*/
+	nb = cowdev->numblocks;
+
+	if (nb%8)		/* transform #bits to #bytes */
+		nb+=8;  	/* rounded if necessary      */
+	nb /= 8;
+
+	if (nb & MUMASK)	/* round up #bytes to MAPUNIT chunks */
+		cowdev->mapsize = ( (nb>>MUSHIFT) +1) << MUSHIFT;
+	else
+		cowdev->mapsize = nb;
+
+	/*
+	** reserve space in memory for the cowhead
+	*/
+	cowdev->cowhead = kmalloc(MAPUNIT, GFP_KERNEL);
+
+	if (!cowdev->cowhead) {
+		printk(KERN_ERR "cowloop - cannot get space for cowhead %d\n",
+								     MAPUNIT);
+		return -ENOMEM;
+	}
+
+	memset(cowdev->cowhead, 0, MAPUNIT);
+
+	DEBUGP(DCOW"cowloop - prepare cowhead....\n");
+
+	/*
+	** check if the cowfile exists or should be created
+	*/
+	if (inode->i_size != 0) {
+		/*
+		** existing cowfile: read the cow head
+		*/
+		if (inode->i_size < MAPUNIT) {
+			printk(KERN_ERR
+			       "cowloop - existing cowfile %s too small\n",
+				cowf);
+			return -EINVAL;
+		}
+
+		cowlo_readcowraw(cowdev, cowdev->cowhead, MAPUNIT, (loff_t) 0);
+
+		/*
+		** verify if the existing file is really a cowfile
+		*/
+		if (cowdev->cowhead->magic != COWMAGIC) {
+			printk(KERN_ERR
+			       "cowloop - cowfile %s has incorrect format\n",
+				cowf);
+			return -EINVAL;
+		}
+
+		/*
+		** verify the cowhead version of the cowfile
+		*/
+		if (cowdev->cowhead->version > COWVERSION) {
+			printk(KERN_ERR
+			       "cowloop - cowfile %s newer than this driver\n",
+				cowf);
+			return -EINVAL;
+		}
+
+		/*
+		** make sure that this is not a packed cowfile
+		*/
+		if (cowdev->cowhead->flags & COWPACKED) {
+			printk(KERN_ERR
+			    "cowloop - packed cowfile %s not accepted\n", cowf);
+			return -EINVAL;
+		}
+
+		/*
+		** verify if the cowfile has been properly closed
+		*/
+		if (cowdev->cowhead->flags & COWDIRTY) {
+			/*
+			** cowfile was not properly closed;
+			** check if automatic recovery is required
+			** (actual recovery will be done later on)
+			*/
+			if (!autorecover) {
+				printk(KERN_ERR
+				       "cowloop - cowfile %s is dirty "
+				       "(not properly closed by rmmod?)\n",
+					cowf);
+				printk(KERN_ERR
+				       "cowloop - run cowrepair or specify "
+				       "'option=r' to recover\n");
+				return -EINVAL;
+			}
+		}
+
+		/*
+		** verify if the cowfile is really related to this rdofile
+		*/
+		if (cowdev->cowhead->rdoblocks != cowdev->numblocks) {
+			printk(KERN_ERR
+		       	       "cowloop - cowfile %s (size %lld) not related "
+		       	       "to rdofile (size %lld)\n",
+				cowf,
+				(long long)cowdev->cowhead->rdoblocks <<MUSHIFT,
+				(long long)cowdev->numblocks <<MUSHIFT);
+			return -EINVAL;
+		}
+
+		if (cowdev->cowhead->rdofingerprint != cowdev->fingerprint) {
+			printk(KERN_ERR
+		       	     "cowloop - cowfile %s not related to rdofile "
+			     " (fingerprint err - rdofile modified?)\n", cowf);
+			return -EINVAL;
+		}
+	} else {
+		/*
+		** new cowfile: determine the minimal size (cowhead+bitmap)
+		*/
+		offset = (loff_t) MAPUNIT + cowdev->mapsize - 1;
+
+		if ( cowlo_writecowraw(cowdev, "", 1, offset) < 1) {
+			printk(KERN_ERR
+			       "cowloop - cannot set cowfile to size %lld\n",
+				offset+1);
+			return -EINVAL;
+		}
+
+		/*
+		** prepare new cowhead
+		*/
+		cowdev->cowhead->magic		= COWMAGIC;
+		cowdev->cowhead->version	= COWVERSION;
+		cowdev->cowhead->mapunit	= MAPUNIT;
+		cowdev->cowhead->mapsize	= cowdev->mapsize;
+		cowdev->cowhead->rdoblocks	= cowdev->numblocks;
+		cowdev->cowhead->rdofingerprint	= cowdev->fingerprint;
+		cowdev->cowhead->cowused	= 0;
+
+		/*
+		** calculate start offset of data in cowfile,
+		** rounded up to multiple of 4K to avoid
+		** unnecessary disk-usage for written datablocks in
+		** the sparsed cowfile on e.g. 4K filesystems
+		*/
+		cowdev->cowhead->doffset =
+			((MAPUNIT+cowdev->mapsize+4095)>>12)<<12;
+	}
+
+	cowdev->cowhead->flags	= 0;
+
+	DEBUGP(DCOW"cowloop - reserve space bitmap....\n");
+
+	/*
+	** reserve space in memory for the entire bitmap and
+	** fill it with the bitmap-data from disk; the entire
+	** bitmap is allocated in several chunks because kmalloc
+	** has restrictions regarding the allowed size per kmalloc
+	*/
+	cowdev->mapcount = (cowdev->mapsize+MAPCHUNKSZ-1)/MAPCHUNKSZ;
+
+	/*
+	** the size of every bitmap chunk will be MAPCHUNKSZ bytes, except for
+	** the last bitmap chunk: calculate remaining size for this chunk
+	*/
+	if (cowdev->mapsize % MAPCHUNKSZ == 0)
+		cowdev->mapremain = MAPCHUNKSZ;
+	else
+		cowdev->mapremain = cowdev->mapsize % MAPCHUNKSZ;
+
+	/*
+	** allocate space to store all pointers for the bitmap-chunks
+	** (initialize area with zeroes to allow proper undo)
+	*/
+	cowdev->mapcache = kmalloc(cowdev->mapcount * sizeof(char *),
+								GFP_KERNEL);
+	if (!cowdev->mapcache) {
+		printk(KERN_ERR
+		       "cowloop - can not allocate space for bitmap ptrs\n");
+		return -ENOMEM;
+	}
+
+	memset(cowdev->mapcache, 0, cowdev->mapcount * sizeof(char *));
+
+	/*
+	** allocate space to store the bitmap-chunks themselves
+	*/
+	for (i=0; i < cowdev->mapcount; i++) {
+		if (i < (cowdev->mapcount-1))
+			*(cowdev->mapcache+i) = kmalloc(MAPCHUNKSZ, GFP_KERNEL);
+		else
+			*(cowdev->mapcache+i) = kmalloc(cowdev->mapremain,
+						                  GFP_KERNEL);
+
+		if (*(cowdev->mapcache+i) == NULL) {
+			printk(KERN_ERR "cowloop - no space for bitmapchunk %ld"
+					" totmapsz=%ld, mapcnt=%d mapunit=%d\n",
+					i, cowdev->mapsize, cowdev->mapcount,
+					MAPUNIT);
+			return -ENOMEM;
+		}
+	}
+
+	DEBUGP(DCOW"cowloop - read bitmap from cow....\n");
+
+	/*
+	** read the entire bitmap from the cowfile into the in-memory cache;
+	** count the number of blocks that are in use already
+	** (statistical purposes)
+	*/
+	for (i=0, offset=MAPUNIT; i < cowdev->mapcount;
+					i++, offset+=MAPCHUNKSZ) {
+		unsigned long	numbytes;
+
+		if (i < (cowdev->mapcount-1))
+			/*
+			** full bitmap chunk
+			*/
+			numbytes = MAPCHUNKSZ;
+		else
+			/*
+			** last bitmap chunk: might be partly filled
+			*/
+			numbytes = cowdev->mapremain;
+
+		cowlo_readcowraw(cowdev, *(cowdev->mapcache+i),
+							numbytes, offset);
+	}
+
+	/*
+	** if the cowfile was dirty and automatic recovery is required,
+	** reconstruct a proper bitmap in memory now
+	*/
+	if (cowdev->cowhead->flags & COWDIRTY) {
+		unsigned long long	blocknum;
+		char			databuf[MAPUNIT];
+		unsigned long		mapnum, mapbyte, mapbit;
+
+		printk(KERN_NOTICE "cowloop - recover dirty cowfile %s....\n",
+							cowf);
+
+		/*
+		** read all data blocks
+		*/
+		for (blocknum=0, rv=1, offset=0;
+			cowlo_readcow(cowdev, databuf, MAPUNIT, offset) > 0;
+			blocknum++, offset += MAPUNIT) {
+
+			/*
+			** if this datablock contains real data (not binary
+			** zeroes), set the corresponding bit in the bitmap
+			*/
+			if ( memcmp(databuf, allzeroes, MAPUNIT) == 0)
+				continue;
+
+			mapnum  = CALCMAP (blocknum);
+			mapbyte = CALCBYTE(blocknum);
+			mapbit  = CALCBIT (blocknum);
+
+			*(*(cowdev->mapcache+mapnum)+mapbyte) |= (1<<mapbit);
+		}
+
+		printk(KERN_NOTICE "cowloop - cowfile recovery completed\n");
+	}
+
+	/*
+	** count all bits set in the bitmaps for statistical purposes
+	*/
+	for (i=0, cowdev->nrcowblocks = 0; i < cowdev->mapcount; i++) {
+		long	numbytes;
+		char	*p;
+
+		if (i < (cowdev->mapcount-1))
+			numbytes = MAPCHUNKSZ;
+		else
+			numbytes = cowdev->mapremain;
+
+		p = *(cowdev->mapcache+i);
+
+		for (numbytes--; numbytes >= 0; numbytes--, p++) {
+			/*
+			** for only eight checks the following construction
+			** is faster than a loop-construction
+			*/
+			if ((*p) & 0x01)	cowdev->nrcowblocks++;
+			if ((*p) & 0x02)	cowdev->nrcowblocks++;
+			if ((*p) & 0x04)	cowdev->nrcowblocks++;
+			if ((*p) & 0x08)	cowdev->nrcowblocks++;
+			if ((*p) & 0x10)	cowdev->nrcowblocks++;
+			if ((*p) & 0x20)	cowdev->nrcowblocks++;
+			if ((*p) & 0x40)	cowdev->nrcowblocks++;
+			if ((*p) & 0x80)	cowdev->nrcowblocks++;
+		}
+	}
+
+	/*
+	** consistency-check for number of bits set in bitmap
+	*/
+	if ( !(cowdev->cowhead->flags & COWDIRTY) &&
+	    (cowdev->cowhead->cowused != cowdev->nrcowblocks) ) {
+		printk(KERN_ERR "cowloop - inconsistent cowfile admi\n");
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+/*
+** undo memory allocs and file opens issued so far
+** related to the cowfile
+*/
+static void
+cowlo_undo_opencow(struct cowloop_device *cowdev)
+{
+	int	i;
+
+	if (cowdev->mapcache) {
+		for (i=0; i < cowdev->mapcount; i++) {
+			if (*(cowdev->mapcache+i) != NULL)
+				kfree( *(cowdev->mapcache+i) );
+		}
+
+		kfree(cowdev->mapcache);
+	}
+
+	if (cowdev->cowhead)
+		kfree(cowdev->cowhead);
+
+	if ( (cowdev->state & COWCOWOPEN) && (cowdev->cowfp) )
+  		filp_close(cowdev->cowfp, 0);
+
+	/*
+	** mark cowfile closed
+	*/
+	cowdev->state &= ~COWCOWOPEN;
+}
+
+/*
+** flush the entire bitmap and the cowhead (clean) to the cowfile
+**
+** must be called with the cowdevices-lock set
+*/
+static void
+cowlo_sync(void)
+{
+	int			i, minor;
+	loff_t			offset;
+	struct cowloop_device	*cowdev;
+
+	for (minor=0; minor < maxcows;  minor++) {
+		cowdev = cowdevall[minor];
+		if ( ! (cowdev->state & COWRWCOWOPEN) )
+			continue;
+
+		for (i=0, offset=MAPUNIT; i < cowdev->mapcount;
+					i++, offset += MAPCHUNKSZ) {
+			unsigned long	numbytes;
+
+			if (i < (cowdev->mapcount-1))
+				/*
+				** full bitmap chunk
+				*/
+				numbytes = MAPCHUNKSZ;
+			else
+				/*
+				** last bitmap chunk: might be partly filled
+				*/
+				numbytes = cowdev->mapremain;
+
+			DEBUGP(DCOW
+			       "cowloop - flushing bitmap %2d (%3ld Kb)\n",
+							i, numbytes/1024);
+
+			if (cowlo_writecowraw(cowdev, *(cowdev->mapcache+i),
+						numbytes, offset) < numbytes) {
+				break;
+			}
+		}
+
+		/*
+		** flush clean up-to-date cowhead to cowfile
+		*/
+		cowdev->cowhead->cowused	 = cowdev->nrcowblocks;
+		cowdev->cowhead->flags		&= ~COWDIRTY;
+
+		DEBUGP(DCOW "cowloop - flushing cowhead (%3d Kb)\n",
+							MAPUNIT/1024);
+
+		cowlo_writecowraw(cowdev, cowdev->cowhead, MAPUNIT, (loff_t) 0);
+	}
+}
+
+/*****************************************************************************/
+/* Module loading/unloading                                                  */
+/*****************************************************************************/
+
+/*
+** called during insmod/modprobe
+*/
+static int __init
+cowlo_init_module(void)
+{
+	int	rv;
+	int	minor, uptocows;
+
+        revision[sizeof revision - 3] = '\0';
+
+        printk(KERN_NOTICE "cowloop - (C) 2009 ATComputing.nl - version: %s\n", &revision[11]);
+        printk(KERN_NOTICE "cowloop - info: www.ATComputing.nl/cowloop\n");
+
+	memset(allzeroes, 0, MAPUNIT);
+
+	/*
+	** Setup administration for all possible cowdevices.
+        ** Note that their minor numbers go from 0 to MAXCOWS-1 inclusive
+        ** and minor == MAXCOWS-1 is reserved for the control device.
+	*/
+	if ((maxcows < 1) || (maxcows > MAXCOWS)) {
+		printk(KERN_WARNING
+		       "cowloop - maxcows exceeds maximum of %d\n", MAXCOWS);
+
+                maxcows = DFLCOWS;
+        }
+
+	/* allocate room for a table with a pointer to each cowloop_device: */
+        if ( (cowdevall = kmalloc(maxcows * sizeof(struct cowloop_device *),
+							GFP_KERNEL)) == NULL) {
+		printk(KERN_WARNING
+		        "cowloop - can not alloc table for %d devs\n", maxcows);
+		uptocows = 0;
+		rv = -ENOMEM;
+		goto error_out;
+	}
+	memset(cowdevall, 0, maxcows * sizeof(struct cowloop_device *));
+	/* then hook an actual cowloop_device struct to each pointer: */
+	for (minor=0; minor < maxcows; minor++) {
+		if ((cowdevall[minor] = kmalloc(sizeof(struct cowloop_device),
+						GFP_KERNEL)) == NULL) {
+			printk(KERN_WARNING
+		           "cowloop - can not alloc admin-struct for dev no %d\n", minor);
+
+			uptocows = minor; /* this is how far we got.... */
+			rv = -ENOMEM;
+			goto error_out;
+		}
+        	memset(cowdevall[minor], 0, sizeof(struct cowloop_device));
+	}
+	uptocows = maxcows; /* we got all devices */
+
+	sema_init(&cowdevlock, 1);
+
+	/*
+	** register cowloop module
+	*/
+	if ( register_blkdev(COWMAJOR, DEVICE_NAME) < 0) {
+		printk(KERN_WARNING
+		    "cowloop - unable to get major %d for cowloop\n", COWMAJOR);
+		rv = -EIO;
+		goto error_out;
+	}
+
+	/*
+	** create a directory below /proc to allocate a file
+	** for each cowdevice that is allocated later on
+	*/
+	cowlo_procdir = proc_mkdir("cow", NULL);
+
+	/*
+	** check if a cowdevice has to be opened during insmod/modprobe;
+	** two parameters should be specified then: rdofile= and cowfile=
+	*/
+	if( (rdofile[0] != '\0') && (cowfile[0] != '\0') ) {
+		char	*po = option;
+		int	wantrecover = 0;
+
+		/*
+		** check if automatic recovery is wanted
+		*/
+		while (*po) {
+			if (*po == 'r') {
+				wantrecover = 1;
+				break;
+                        }
+			po++;
+		}
+
+		/*
+		** open new cowdevice with minor number 0
+		*/
+		if ( (rv = cowlo_openpair(rdofile, cowfile, wantrecover, 0))) {
+			remove_proc_entry("cow", NULL);
+			unregister_blkdev(COWMAJOR, DEVICE_NAME);
+			goto error_out;
+		}
+        } else {
+		/*
+		** check if only one parameter has been specified
+		*/
+		if( (rdofile[0] != '\0') || (cowfile[0] != '\0') ) {
+			printk(KERN_ERR
+			       "cowloop - only one filename specified\n");
+			remove_proc_entry("cow", NULL);
+			unregister_blkdev(COWMAJOR, DEVICE_NAME);
+			rv = -EINVAL;
+			goto error_out;
+		}
+	}
+
+	/*
+	** allocate fake disk as control channel to handle the requests
+	** to activate and deactivate cowdevices dynamically
+	*/
+	if (!(cowctlgd = alloc_disk(1))) {
+		printk(KERN_WARNING
+		       "cowloop - unable to alloc_disk for cowctl\n");
+
+		remove_proc_entry("cow", NULL);
+		(void) cowlo_closepair(cowdevall[0]);
+		unregister_blkdev(COWMAJOR, DEVICE_NAME);
+		rv = -ENOMEM;
+		goto error_out;
+	}
+
+	spin_lock_init(&cowctlrqlock);
+	cowctlgd->major        = COWMAJOR;
+	cowctlgd->first_minor  = COWCTL;
+	cowctlgd->minors       = 1;
+	cowctlgd->fops         = &cowlo_fops;
+	cowctlgd->private_data = NULL;
+	/* the device has capacity 0, so there will be no q-requests */
+	cowctlgd->queue = blk_init_queue(NULL, &cowctlrqlock);
+	sprintf(cowctlgd->disk_name, "cowctl");
+	set_capacity(cowctlgd, 0);
+
+	add_disk(cowctlgd);
+
+        printk(KERN_NOTICE "cowloop - number of configured cowdevices: %d\n",
+								maxcows);
+	if (rdofile[0] != '\0') {
+	    printk(KERN_NOTICE "cowloop - initialized on rdofile=%s\n",
+								rdofile);
+	} else {
+	    printk(KERN_NOTICE "cowloop - initialized without rdofile yet\n");
+	}
+	return 0;
+
+error_out:
+	for (minor=0; minor < uptocows ; minor++) {
+		kfree(cowdevall[minor]);
+	}
+	kfree(cowdevall);
+	return rv;
+}
+
+/*
+** called during rmmod
+*/
+static void __exit
+cowlo_cleanup_module(void)
+{
+	int	minor;
+
+	/*
+	** flush bitmaps and cowheads to the cowfiles
+	*/
+	down(&cowdevlock);
+	cowlo_sync();
+	up(&cowdevlock);
+
+	/*
+	** close all cowdevices
+	*/
+	for (minor=0; minor < maxcows;  minor++)
+		(void) cowlo_closepair(cowdevall[minor]);
+
+#if (LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,23))
+	unregister_blkdev(COWMAJOR, DEVICE_NAME);
+#else
+	if (unregister_blkdev(COWMAJOR, DEVICE_NAME) != 0)
+		printk(KERN_WARNING "cowloop - cannot unregister blkdev\n");
+#endif
+
+	/*
+	** get rid of /proc/cow and unregister the driver
+	*/
+	remove_proc_entry("cow", NULL);
+
+	for (minor = 0; minor < maxcows; minor++) {
+		kfree(cowdevall[minor]);
+	}
+	kfree(cowdevall);
+
+	del_gendisk(cowctlgd);  /* revert the alloc_disk() */
+	put_disk   (cowctlgd);  /* revert the add_disk()   */
+	blk_cleanup_queue(cowctlgd->queue); /* cleanup the empty queue */
+
+	printk(KERN_NOTICE "cowloop - unloaded\n");
+}
+
+module_init(cowlo_init_module);
+module_exit(cowlo_cleanup_module);
diff --git a/drivers/staging/cowloop/cowloop.h b/drivers/staging/cowloop/cowloop.h
new file mode 100644
index 000000000000..bbd4a35ac667
--- /dev/null
+++ b/drivers/staging/cowloop/cowloop.h
@@ -0,0 +1,66 @@
+/*
+** DO NOT MODIFY THESE VALUES (would make old cowfiles unusable)
+*/
+#define	MAPUNIT		1024		/* blocksize for bit in bitmap       */
+#define	MUSHIFT		10		/* bitshift  for bit in bitmap       */
+#define	MUMASK		0x3ff		/* bitmask   for bit in bitmap       */
+
+#define	COWMAGIC	0x574f437f	/* byte-swapped '7f C O W'           */
+#define	COWDIRTY	0x01
+#define	COWPACKED	0x02
+#define	COWVERSION	1
+
+struct cowhead
+{
+	int		magic;		/* identifies a cowfile              */
+	short		version;	/* version of cowhead                */
+	short		flags;    	/* flags indicating status           */
+	unsigned long	mapunit;	/* blocksize per bit in bitmap       */
+	unsigned long	mapsize;	/* total size of bitmap (bytes)      */
+	unsigned long	doffset;	/* start-offset datablocks in cow    */
+	unsigned long	rdoblocks;	/* size of related read-only file    */
+	unsigned long	rdofingerprint;	/* fingerprint of read-only file     */
+	unsigned long	cowused;	/* number of datablocks used in cow  */
+};
+
+#define COWDEVDIR	"/dev/cow/"
+#define COWDEVICE	COWDEVDIR "%ld"
+#define COWCONTROL	COWDEVDIR "ctl"
+
+#define MAXCOWS		1024
+#define COWCTL		(MAXCOWS-1)	/* minor number of /dev/cow/ctl     */
+
+#define COWPROCDIR	"/proc/cow/"
+#define COWPROCFILE	COWPROCDIR "%d"
+
+/*
+** ioctl related stuff
+*/
+#define ANYDEV		((unsigned long)-1)
+
+struct cowpair
+{
+	unsigned char	*rdofile;	/* pathname of the rdofile           */
+	unsigned char	*cowfile;	/* pathname of the cowfile           */
+	unsigned short	rdoflen;	/* length of rdofile pathname        */
+	unsigned short	cowflen;	/* length of cowfile pathname        */
+	unsigned long	device;		/* requested/returned device number  */
+};
+
+struct cowwatch
+{
+	int      	flags;		/* request flags                     */
+	unsigned long	device;		/* requested device number           */
+	unsigned long	threshold;	/* continue if free Kb < threshold   */
+	unsigned long	totalkb;	/* ret: total filesystem size (Kb)   */
+	unsigned long	availkb;	/* ret: free  filesystem size (Kb)   */
+};
+
+#define	WATCHWAIT	0x01		/* block until threshold reached     */
+
+#define	COWSYNC		_IO  ('C', 1)
+#define	COWMKPAIR	_IOW ('C', 2, struct cowpair)
+#define	COWRMPAIR	_IOW ('C', 3, unsigned long)
+#define	COWWATCH	_IOW ('C', 4, struct cowwatch)
+#define	COWCLOSE	_IOW ('C', 5, unsigned long)
+#define	COWRDOPEN	_IOW ('C', 6, unsigned long)