Re: kvm PCI assignment & VFIO ramblings

Joerg Roedel
2011-08-28T13:56:33Z

On Sun, Aug 28, 2011 at 04:14:00PM +0300, Avi Kivity wrote:
> On 08/26/2011 12:24 PM, Roedel, Joerg wrote:

>> The biggest problem with this approach is that it has to happen in the
>> context of the given process. Linux can't really modify an mm which
>> which belong to another context in a safe way.
>>
>
> Is use_mm() insufficient?

Yes, it introduces a set of race conditions when a process that already
has an mm wants to take over another processes mm temporarily (and when
use_mm is modified to actually provide this functionality). It is only
save when used from kernel-thread context.

One example:

Process A Process B Process C
. . .
. <-- takes A->mm .
. and assignes as B->mm .
. . --> Wants to take
. . B->mm, but gets
A->mm now

This can't be secured by a lock, because it introduces potential
A->B<-->B->A lock problem when two processes try to take each others mm.
It could probably be solved by a task->real_mm pointer, havn't thought
about this yet...

Joerg