[CRIU] Combining pre-copy and post-copy

[Adrian Reber]

On Fri, Jul 29, 2016 at 03:18:56PM +0300, Pavel Emelyanov wrote:
> On 07/28/2016 04:11 PM, Mike Rapoport wrote:
> > On Wed, Jul 27, 2016 at 01:27:28PM +0200, Adrian Reber wrote:
> >> Thanks for getting the remote uffd support into criu-dev.
> >>
> >> I am now looking at combining pre-copy and post-copy. Luckily there is
> >> for each pagemap entry the status if the page is in a parent checkpoint
> >> or not. Which makes it easier to decide if the page should be handled by
> >> uffd or not.
> >>
> >> Unfortunately this information is not available in the restorer. The
> >> restorer only gets a list of VMAs and if the page should handled by uffd
> >> is decided by vma_entry_can_be_lazy()
> >>
> >>         return ((e->flags & MAP_ANONYMOUS) &&
> >>                 (e->flags & MAP_PRIVATE) &&
> >>                 !(vma_entry_is(e, VMA_AREA_VDSO)) &&
> >>                 !(vma_entry_is(e, VMA_AREA_VSYSCALL)));
> >>
> >> Which has no information about parents. What would be the best way to
> >> include the information if the page exists in a parent checkpoint and
> >> therefore should not be handled by uffd?
> > 
> > I think that the best way is to make restore and uffd use the information
> > in the pagemap to detect lazy and/or zero pages.

That was also my first idea. But at a second thought it seemed to coarse
and did want to lose possible pages because of relying on the pagemap
information only.

> > For instance, the restore_priv_vma_content function should skip the entire
> > lazy pagemap. And uffd initialization of relevant page sets should also
> > rely on the pagemap flags rather than on vma_entry_can_be_lazy().
> 
> But why do we need the vma for this? When #PF happens we can find the pagemap
> entry which has all we need.

The restorer only works with the VMA entries. It does not know anything
about pagemap entries (as far as I can tell).

Currently I am doing the following: I added an additional flag to the
VMA entry called lazy which I set if the page is lazy and if it is not
in a parent checkpoint. To do this I split existing VMA entries, if
necessary, to better match pagemap boundaries. This almost works I only
have problems that the criu server on the source side thinks these pages
are zeropages. They are marked as lazy in the pagemap, so that I have to
figure out what is going on.

I will post the patches for this once it works.

		Adrian