This patch introduces new ioctl command LOOP_UPDATE_SIZE
which enables to resize online mounted loop device.
EXAMPLE
=======
# Make 10MB disk image
# dd if=/dev/zero of=image bs=1k count=10k
# mkfs.ext3 -j -F image
# Mount
# mkdir loop
# mount -o loop=/dev/loop/0,debug -t ext3 image loop
# Check disk size
# df -h loop
Filesystem Size Used Avail Use% Mounted on
/home/mita/looback-test/image
9.7M 1.1M 8.2M 12% /home/mita/looback-test/loop
# Extend disk image to 20MB
# dd if=/dev/zero of=appendix bs=1k count=10k
# cat appendix >> image
# Resize
# gcc -o loop-update loop-update.c
# ./loop-update /dev/loop/0
# ext2online -d -v image
# Check disk size again
# df -h loop
Filesystem Size Used Avail Use% Mounted on
/home/mita/looback-test/image
20M 1.1M 18M 6% /home/mita/looback-test/loop
loop-update.c
=============
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#define LOOP_UPDATE_SIZE 0x4C07
int main(int argc, char **argv)
{
int fd;
int err;
if (argc < 2) {
fprintf(stderr, "usage: loop-update loop_device\n");
exit(1);
}
fd = open(argv[1], O_RDWR);
if (fd < 0) {
perror(argv[1]);
exit(1);
}
err = ioctl(fd, LOOP_UPDATE_SIZE, NULL);
if (err) {
perror(argv[1]);
exit(1);
}
}
CC: Jens Axboe <[email protected]>
CC: "Theodore Ts'o" <[email protected]>
Signed-off-by: Akinobu Mita <[email protected]>
--- 2.6-mm/include/linux/loop.h.orig 2006-05-23 12:52:54.000000000 +0800
+++ 2.6-mm/include/linux/loop.h 2006-05-23 12:59:14.000000000 +0800
@@ -158,5 +158,6 @@ int loop_unregister_transfer(int number)
#define LOOP_SET_STATUS64 0x4C04
#define LOOP_GET_STATUS64 0x4C05
#define LOOP_CHANGE_FD 0x4C06
+#define LOOP_UPDATE_SIZE 0x4C07
#endif
--- 2.6-mm/drivers/block/loop.c.orig 2006-05-23 12:51:11.000000000 +0800
+++ 2.6-mm/drivers/block/loop.c 2006-05-23 13:50:58.000000000 +0800
@@ -1108,7 +1108,8 @@ loop_set_status64(struct loop_device *lo
}
static int
-loop_get_status_old(struct loop_device *lo, struct loop_info __user *arg) {
+loop_get_status_old(struct loop_device *lo, struct loop_info __user *arg)
+{
struct loop_info info;
struct loop_info64 info64;
int err = 0;
@@ -1126,7 +1127,8 @@ loop_get_status_old(struct loop_device *
}
static int
-loop_get_status64(struct loop_device *lo, struct loop_info64 __user *arg) {
+loop_get_status64(struct loop_device *lo, struct loop_info64 __user *arg)
+{
struct loop_info64 info64;
int err = 0;
@@ -1140,6 +1142,17 @@ loop_get_status64(struct loop_device *lo
return err;
}
+static int loop_update_size(struct loop_device *lo)
+{
+ int err = figure_loop_size(lo);
+
+ if (!err)
+ i_size_write(lo->lo_device->bd_inode,
+ (loff_t) get_capacity(disks[lo->lo_number]) << 9);
+
+ return err;
+}
+
static int lo_ioctl(struct inode * inode, struct file * file,
unsigned int cmd, unsigned long arg)
{
@@ -1169,6 +1182,9 @@ static int lo_ioctl(struct inode * inode
case LOOP_GET_STATUS64:
err = loop_get_status64(lo, (struct loop_info64 __user *) arg);
break;
+ case LOOP_UPDATE_SIZE:
+ err = loop_update_size(lo);
+ break;
default:
err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
}
Akinobu Mita <[email protected]> wrote:
>
> This patch introduces new ioctl command LOOP_UPDATE_SIZE
> which enables to resize online mounted loop device.
>
> EXAMPLE
> =======
> # Make 10MB disk image
> # dd if=/dev/zero of=image bs=1k count=10k
> # mkfs.ext3 -j -F image
>
> # Mount
> # mkdir loop
> # mount -o loop=/dev/loop/0,debug -t ext3 image loop
>
> # Check disk size
> # df -h loop
> Filesystem Size Used Avail Use% Mounted on
> /home/mita/looback-test/image
> 9.7M 1.1M 8.2M 12% /home/mita/looback-test/loop
>
> # Extend disk image to 20MB
> # dd if=/dev/zero of=appendix bs=1k count=10k
> # cat appendix >> image
>
> # Resize
> # gcc -o loop-update loop-update.c
> # ./loop-update /dev/loop/0
> # ext2online -d -v image
>
> # Check disk size again
> # df -h loop
> Filesystem Size Used Avail Use% Mounted on
> /home/mita/looback-test/image
> 20M 1.1M 18M 6% /home/mita/looback-test/loop
<tries to remember how loop works>
> +static int loop_update_size(struct loop_device *lo)
> +{
> + int err = figure_loop_size(lo);
> +
> + if (!err)
> + i_size_write(lo->lo_device->bd_inode,
> + (loff_t) get_capacity(disks[lo->lo_number]) << 9);
> +
> + return err;
> +}
> +
> static int lo_ioctl(struct inode * inode, struct file * file,
> unsigned int cmd, unsigned long arg)
> {
> @@ -1169,6 +1182,9 @@ static int lo_ioctl(struct inode * inode
> case LOOP_GET_STATUS64:
> err = loop_get_status64(lo, (struct loop_info64 __user *) arg);
> break;
> + case LOOP_UPDATE_SIZE:
> + err = loop_update_size(lo);
> + break;
> default:
> err = lo->ioctl ? lo->ioctl(lo, cmd, arg) : -EINVAL;
> }
I don't immediately see any show-stoppers here.
Note that this interface will allow the loop "device" to be larger than the
backing file (in fact that's already the case). Just ftruncate the backing
file to a shorter size. Everything should still work after that has
happened - the VFS will just extend the file again once the loop driver
writes to it outside i_size.
Given that, and given that your code is a bit racy anyway, I don't think
the interface should be "resize the device to match the backing file's
size". I think the interface should be "resize the loop device to this
loff_t". That's a superset of what you have there, and it permits the
device size to be larger than or smaller than the backing file.
Also, one really should take i_mutex when altering an i_size. Probably it
doesn't make much difference here, but that's the rule.
Please ensure that the loop driver is well tested with a device size which
is both smaller than and larger than the backing file, and that it's tested
for both do_lo_send_aops()-based and do_lo_send_write()-based backing
filesystems, thanks.