Re: kvm userspace: ksm support

[Jan Kiszka <jan.kiszka@siemens.com>]

Anthony Liguori wrote:
> Izik Eidus wrote:
>>
>> You mean: when we later call for other madvise calls, if it will
>> remove the MADV_MERGEABLE from that memory?
>> if yes, the answer is no, it should be still l left in the
>> vma->vm_flags...
> 
> Excellent.
> 
>>>
>>> I'd suggest doing the following in osdep.h too:
>>>
>>> #if !defined(MADV_MERGABLE)
>>> #define MADV_MERGABLE MADV_NORMAL
>>> #endif
>>>
>>> To avoid #ifdefs in .c files.
>>
>> I tried to follow the way DONTFORK madvise is working...
>>
>> So you say, just to throw this thing into osdep.h instead of that c file?
> 
> Yes.
> 
> I think the DONTFORK thing is a bit odd.  Of course we have
> MADV_DONTFORK if we're running KVM.  I'm not sure why that is there.
> 
> I also think that we could get away with getting rid of any checks for
> !sync_mmu() since that was introduced in 2.6.27.

The problem is that your host kernel also must have CONFIG_MMU_NOTIFIER
enabled - and that's not always the case.

> 
> Otherwise, you should technically avoid doing madvise() unless we have
> sync_mmu().
> 
> Regards,
> 
> Anthony Liguori

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux