Re: [PATCH 3/4] add ksm kernel shared memory driver

Jonathan Corbet
2008-11-11T22:26:22Z

On Wed, 12 Nov 2008 00:17:39 +0200
Izik Eidus <ieidus@redhat.com> wrote:

> >> +static int ksm_dev_open(struct inode *inode, struct file *filp)
> >> +{
> >> + try_module_get(THIS_MODULE);
> >> + return 0;
> >> +}
> >> +
> >> +static int ksm_dev_release(struct inode *inode, struct file *filp)
> >> +{
> >> + module_put(THIS_MODULE);
> >> + return 0;
> >> +}
> >> +
> >> +static struct file_operations ksm_chardev_ops = {
> >> + .open = ksm_dev_open,
> >> + .release = ksm_dev_release,
> >> + .unlocked_ioctl = ksm_dev_ioctl,
> >> + .compat_ioctl = ksm_dev_ioctl,
> >> +};
> >>
> >
> > Why do you roll your own module reference counting? Is there a
> > reason you don't just set .owner and let the VFS handle it?
> >
>
> Yes, I am taking get_task_mm() if the module will be removed before i
> free the mms, things will go wrong

But...if you set .owner, the VFS will do the try_module_get() *before*
calling into your module (as an added bonus, it will actually check the
return value too). All you've succeeded in doing here is adding a
microscopic race to the module reference counting; otherwise the end
result is the same.

jon