Re: [Qemu-devel] Re: [Qemu-commits] [COMMIT 3086844] Instead of writing a zero page, madvise it away

[Anthony Liguori <aliguori@us.ibm.com>]

Avi Kivity wrote:
> On 06/22/2009 07:25 PM, Anthony Liguori wrote:
>
>>
>> Xen had a similar issue.  This ends up biting people who overcommit 
>> their VMs via ballooning, live migration, and badness ensues.  At 
>> least for us, the error is swapping but madvise also avoids the issue 
>> by never consuming that memory to begin with.
>
> Right.  I'd love to do madvise() on the source node as well if we 
> fault in a page and find out it's zero, but the guest (and aio) is 
> still running and we might drop live data.  We need a 
> madvise(MADV_DONTNEED_IFZERO), or a mincore() flag that tells us if 
> the page exists (vs. swapped).  ksm would also do this, but it is 
> overkill for some applications.
>
> Note that the patch contains a small bug -- the kernel is allowed to 
> ignore the advise according to the manual page, so it's better to 
> memset() the memory before dropping it.

Hrm, that's not quite how I interpreted the man page.

"This call
  does not influence the semantics of the application (except in the case
  of  MADV_DONTNEED),  but  may influence its performance.  The kernel is
  free to ignore the advice."

MADV_DONTNEED is called out as changing the application semantics.  
Specifically, I think the kernel has to zero-fill even if it choose to 
ignore the advice.

I limited the guard to Linux specifically because I was unsure about 
that behavior but it would be good to clarify if anyone knows how.

-- 
Regards,

Anthony Liguori