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

Anthony Liguori
2009-04-02T02:36:49Z

Chris Wright wrote:
> * Anthony Liguori (anthony@codemonkey.ws) wrote:
>
>> The ioctl() interface is quite bad for what you're doing. You're
>> telling the kernel extra information about a VA range in userspace.
>> That's what madvise is for. You're tweaking simple read/write values of
>> kernel infrastructure. That's what sysfs is for.
>>
>
> I agree re: sysfs (brought it up myself before). As far as madvise vs.
> ioctl, the one thing that comes from the ioctl is fops->release to
> automagically unregister memory on exit.

This is precisely why ioctl() is a bad interface. fops->release isn't
tied to the process but rather tied to the open file. The file can stay
open long after the process exits either by a fork()'d child inheriting
the file descriptor or through something more sinister like SCM_RIGHTS.

In fact, a common mistake is to leak file descriptors by not closing
them when exec()'ing a process. Instead of just delaying a close, if
you rely on this behavior to unregister memory regions, you could
potentially have badness happen in the kernel if ksm attempted to access
an invalid memory region.

So you absolutely have to automatically unregister regions in something
other than the fops->release handler based on something that's tied to
the pid's life cycle.

Using an interface like madvise() would force the issue to be dealt with
properly from the start :-)

I'm often afraid of what sort of bugs we'd uncover in kvm if we passed
the fds around via SCM_RIGHTS and started poking around :-/

Regards,

Anthony Liguori

> This needs to be handled
> anyway if some -p pid is added to add a process after it's running,
> so less weight there.
>
> thanks,
> -chris
>