Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

[Mike Rapoport <rppt-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>]

On Tue, May 30, 2017 at 09:44:08AM +0200, Michal Hocko wrote:
> On Wed 24-05-17 17:27:36, Mike Rapoport wrote:
> > On Wed, May 24, 2017 at 01:18:00PM +0200, Michal Hocko wrote:
> [...]
> > > Why cannot khugepaged simply skip over all VMAs which have userfault
> > > regions registered? This would sound like a less error prone approach to
> > > me.
> > 
> > khugepaged does skip over VMAs which have userfault. We could register the
> > regions with userfault before populating them to avoid collapses in the
> > transition period.
> 
> Why cannot you register only post-copy regions and "manually" copy the
> pre-copy parts?

We can register only post-copy regions, but this will cause VMA
fragmentation. Now we register the entire VMA with userfaultfd, no matter
how many pages were dirtied there since the pre-dump. If we register only
post-copy regions, we will split out the VMAs for those regions.
 
> > But then we'll have to populate these regions with
> > UFFDIO_COPY which adds quite an overhead.
> 
> How big is the performance impact?

I don't have the numbers handy, but for each post-copy range it means that
instead of memcpy() we will use ioctl(UFFDIO_COPY).

> -- 
> Michal Hocko
> SUSE Labs
 
--
Sincerely yours,
Mike.