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

["Zhoujian (jay)" <jianjay.zhou@huawei.com>]

> -----Original Message-----
> From: Peter Feiner [mailto:pfeiner@google.com]
> Sent: Saturday, February 22, 2020 8:19 AM
> To: Junaid Shahid <junaids@google.com>
> Cc: Ben Gardon <bgardon@google.com>; Zhoujian (jay)
> <jianjay.zhou@huawei.com>; Peter Xu <peterx@redhat.com>;
> kvm@vger.kernel.org; qemu-devel@nongnu.org; pbonzini@redhat.com;
> dgilbert@redhat.com; quintela@redhat.com; Liujinsong (Paul)
> <liu.jinsong@huawei.com>; linfeng (M) <linfeng23@huawei.com>; wangxin (U)
> <wangxinxin.wang@huawei.com>; Huangweidong (C)
> <weidong.huang@huawei.com>
> Subject: Re: RFC: Split EPT huge pages in advance of dirty logging
> 
> On Fri, Feb 21, 2020 at 2:08 PM Junaid Shahid <junaids@google.com> 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.

Hi Peter Feiner,

May I ask any new updates about your plan? Sorry to disturb.

Regards,
Jay Zhou