[PATCH] Allow auto-destruction of loop devices.

[PATCH] Allow auto-destruction of loop devices.

[David Woodhouse]

This allows a flag to be set on loop devices so that when they are
closed for the last time, they'll self-destruct.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 56e2304..7fae828 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -973,6 +973,10 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 	lo->transfer = xfer->transfer;
 	lo->ioctl = xfer->ioctl;
 
+	if ( (lo->lo_flags & LO_FLAGS_AUTOCLEAR) !=
+	     (info->lo_flags & LO_FLAGS_AUTOCLEAR))
+		lo->lo_flags ^= LO_FLAGS_AUTOCLEAR;
+
 	lo->lo_encrypt_key_size = info->lo_encrypt_key_size;
 	lo->lo_init[0] = info->lo_init[0];
 	lo->lo_init[1] = info->lo_init[1];
@@ -1331,6 +1335,10 @@ static int lo_release(struct inode *inode, struct file *file)
 
 	mutex_lock(&lo->lo_ctl_mutex);
 	--lo->lo_refcnt;
+
+	if ((lo->lo_flags & LO_FLAGS_AUTOCLEAR) && !lo->lo_refcnt)
+		loop_clr_fd(lo, inode->i_bdev);
+
 	mutex_unlock(&lo->lo_ctl_mutex);
 
 	return 0;
diff --git a/include/linux/loop.h b/include/linux/loop.h
index 26a0a10..46169a7 100644
--- a/include/linux/loop.h
+++ b/include/linux/loop.h
@@ -76,6 +76,7 @@ struct loop_device {
 enum {
 	LO_FLAGS_READ_ONLY	= 1,
 	LO_FLAGS_USE_AOPS	= 2,
+	LO_FLAGS_AUTOCLEAR	= 4,
 };
 
 #include <asm/posix_types.h>	/* for __kernel_old_dev_t */

-- 
dwmw2

Re: [PATCH] Allow auto-destruction of loop devices.

[Andrew Morton]

On Sun, 28 Oct 2007 19:08:31 -0400
David Woodhouse <dwmw2@infradead.org> wrote:

> This allows a flag to be set on loop devices so that when they are
> closed for the last time, they'll self-destruct.
> 

Why do we want to do this?

> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 56e2304..7fae828 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -973,6 +973,10 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
>  	lo->transfer = xfer->transfer;
>  	lo->ioctl = xfer->ioctl;
>  
> +	if ( (lo->lo_flags & LO_FLAGS_AUTOCLEAR) !=
> +	     (info->lo_flags & LO_FLAGS_AUTOCLEAR))
> +		lo->lo_flags ^= LO_FLAGS_AUTOCLEAR;
> +
>  	lo->lo_encrypt_key_size = info->lo_encrypt_key_size;
>  	lo->lo_init[0] = info->lo_init[0];
>  	lo->lo_init[1] = info->lo_init[1];
> @@ -1331,6 +1335,10 @@ static int lo_release(struct inode *inode, struct file *file)
>  
>  	mutex_lock(&lo->lo_ctl_mutex);
>  	--lo->lo_refcnt;
> +
> +	if ((lo->lo_flags & LO_FLAGS_AUTOCLEAR) && !lo->lo_refcnt)
> +		loop_clr_fd(lo, inode->i_bdev);
> +
>  	mutex_unlock(&lo->lo_ctl_mutex);
>  
>  	return 0;
> diff --git a/include/linux/loop.h b/include/linux/loop.h
> index 26a0a10..46169a7 100644
> --- a/include/linux/loop.h
> +++ b/include/linux/loop.h
> @@ -76,6 +76,7 @@ struct loop_device {
>  enum {
>  	LO_FLAGS_READ_ONLY	= 1,
>  	LO_FLAGS_USE_AOPS	= 2,
> +	LO_FLAGS_AUTOCLEAR	= 4,
>  };
>  
>  #include <asm/posix_types.h>	/* for __kernel_old_dev_t */
> 
> -- 
> dwmw2

Re: [PATCH] Allow auto-destruction of loop devices.

[Jan Engelhardt]

On Oct 30 2007 13:01, Andrew Morton wrote:
>On Sun, 28 Oct 2007 19:08:31 -0400
>David Woodhouse <dwmw2@infradead.org> wrote:
>
>> This allows a flag to be set on loop devices so that when they are
>> closed for the last time, they'll self-destruct.
>> 
>
>Why do we want to do this?

Off the top of my head, lazy unmounting comes to mind.
Think of this:

	mount foobar /mnt -o loop;
	cd /mnt;
	umount /mnt;

Currently, this leaves /dev/loop0 assigned, either chewing up loop
places or needlessy exposing objects meant to be rather not
accessible. (Think cryptoloop devices -- and pam_mount.) BTW, such a
flag would also really be useful for dm(-crypt).

Re: [PATCH] Allow auto-destruction of loop devices.

[David Woodhouse]

On Tue, 2007-10-30 at 13:01 -0700, Andrew Morton wrote:
> Why do we want to do this?

In general, so that we can automatically allocate loop devices (as with
losetup -f) and have them disappear when we're done with them.

In particular, right now, so that we can stop relying on the hackish
special-case in umount(8) which kills off loop devices which were set up
by 'mount -oloop'. That means we can stop putting crap in /etc/mtab
which doesn't belong there, which means it can be a symlink
to /proc/mounts, which means yet another writable file on the root
filesystem is eliminated and the 'stateless' folks get happier... and
OLPC trac #356 can be closed.

The mount(8) side of that is at
http://marc.info/?l=util-linux-ng&m=119362955431694&w=2

-- 
dwmw2

Re: [PATCH] Allow auto-destruction of loop devices.

[Andrew Morton]

On Tue, 30 Oct 2007 16:12:51 -0400
David Woodhouse <dwmw2@infradead.org> wrote:

> On Tue, 2007-10-30 at 13:01 -0700, Andrew Morton wrote:
> > Why do we want to do this?
> 
> In general, so that we can automatically allocate loop devices (as with
> losetup -f) and have them disappear when we're done with them.
> 
> In particular, right now, so that we can stop relying on the hackish
> special-case in umount(8) which kills off loop devices which were set up
> by 'mount -oloop'. That means we can stop putting crap in /etc/mtab
> which doesn't belong there, which means it can be a symlink
> to /proc/mounts, which means yet another writable file on the root
> filesystem is eliminated and the 'stateless' folks get happier... and
> OLPC trac #356 can be closed.
> 
> The mount(8) side of that is at
> http://marc.info/?l=util-linux-ng&m=119362955431694&w=2

ooh, I spy a changelog.  Thanks.