Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

[Wei Wang]

On 02/18/2019 10:36 AM, Wei Wang wrote:
> On 02/15/2019 05:41 PM, David Hildenbrand wrote:
> > On 15.02.19 10:05, Wang, Wei W wrote:
> > > On Thursday, February 14, 2019 5:43 PM, David Hildenbrand wrote:
> > > > Yes indeed, that is the important bit. They must not be put pack to 
> > > > the
> > > > buddy before they have been processed by the hypervisor. But as the 
> > > > pages
> > > > are not in the buddy, no one allocating a page will stumble over 
> > > > such a page
> > > > and try to allocate it. Threads trying to allocate memory will 
> > > > simply pick
> > > > another buddy page instead of "busy waiting" for that page to be 
> > > > finished
> > > > reporting.
> > > What if a guest thread try to allocate some pages but the buddy 
> > > cannot satisfy
> > > because all the pages are isolated? Would it be the same case that 
> > > the guest thread
> > > gets blocked by waiting all the isolated pages to get madvised by 
> > > the host and
> > > returned to the guest buddy, or even worse, some guest threads get 
> > > killed due to oom?
> > Your question targets low memory situations in the guest. I think Nitesh
> > already answered parts of that question somewhere and I'll let him
> > answer it in detail, only a short comment from my side :)
> >
> > I can imagine techniques where the OOM killer can be avoided, but the
> > OOM handler will eventually kick in and handle it.
> >
> > In general your question is valid and we will have to think about a way
> > to avoid that from happening. However, in contrast to your approach
> > blocking on potentially every page that is being hinted, in Nitesh's
> > approach this would only happen when the guest is really low on memory.
> > And the question is in general, if a guest wants to hint if low on
> > memory ("safety buffer").
>
> I think we should forget that the guest is low on memory because
%s/should/shouldn't
> this approach takes all the pages off the list, not because the guest 
> really
> uses up the free memory.
>
> Guest allocating one page could also potentially be blocked until all 
> the pages
> (as opposed to one page) being madvised and returned to the guest buddy.
>
> Best,
> Wei