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.
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