Re: [PATCH mm-unstable v1 2/3] mm/migrate.c: Prevent folio splitting from interacting with KSM

[xu.xin16]

> > >AFAIU, KSM and THP do often conflict with each other. THP tries hard to collapse
> > >a huge page (which may contain many zero pages). If KSM is enabled and part of
> > >that huge page is mergeable, it can easily be split by KSM, rendering THP's
> > >efforts futile.
> > >
> > >Therefore, in our actual production environment, we typically avoid making the
> > >same region both KSM mergeable and THP always.
> >
> > Right, some setups may choose to avoid using KSM and THP always on the
> > same region. But that is not something the kernel can assume :)
> >
> > David noted in the RFC that QEMU may use both MADV_HUGEPAGE and
> > MADV_MERGEABLE, while KSM can be enabled later system-wide.
> >
> > And I think Nico means something different from KSM spliting THPs in
> > general.
> >
> > KSM has been able to split THP before. the new part from b1f202060afe is
> > that a KSM-triggered split can also remap zero-filled subpages to the
> > shared zeropage, outside KSM's own use_zero_pages/pages_to_scan controls.
> >
> > Maybe the changelog could spell that out :)
> 
> Yeah maybe I didnt properly explain that :p
> 
> After some thought I still think the alternative approach i mentioned
> in the RFC may be better.
> 
> ie) prevent the zero-page merging that results from KSM splitting a
> folio. The check we add here is more general and will skip this
> zero-page merging with all MERGEABLE mappings, not just those causing
> the issue (the KSM splitting). The result is that even migrations, etc
> that are also MERGEABLE will be skipped needlessly.
> 
> If we use this approach we also don't need the first patch of the series.

Yeah, That's also what I want to suggest.