Re: RFC: Split EPT huge pages in advance of dirty logging

[Peter Feiner]

On Fri, Feb 21, 2020 at 2:08 PM Junaid Shahid <address@hidden> wrote:
>
> On 2/20/20 9:34 AM, Ben Gardon wrote:
> >
> > FWIW, we currently do this eager splitting at Google for live
> > migration. When the log-dirty-memory flag is set on a memslot we
> > eagerly split all pages in the slot down to 4k granularity.
> > As Jay said, this does not cause crippling lock contention because the
> > vCPU page faults generated by write protection / splitting can be
> > resolved in the fast page fault path without acquiring the MMU lock.
> > I believe +Junaid Shahid tried to upstream this approach at some point
> > in the past, but the patch set didn't make it in. (This was before my
> > time, so I'm hoping he has a link.)
> > I haven't done the analysis to know if eager splitting is more or less
> > efficient with parallel slow-path page faults, but it's definitely
> > faster under the MMU lock.
> >
>
> I am not sure if we ever posted those patches upstream. Peter Feiner would 
> know for sure. One notable difference in what we do compared to the approach 
> outlined by Jay is that we don't rely on tdp_page_fault() to do the 
> splitting. So we don't have to create a dummy VCPU and the specialized split 
> function is also much faster.

We've been carrying these patches since 2015. I've never posted them.
Getting them in shape for upstream consumption will take some work. I
can look into this next week.

Peter