Re: [PATCH v6 1/2] mm: migration: fix migration of huge PMD shared pages

[Michal Hocko <mhocko@kernel.org>]

On Wed 29-08-18 17:11:07, Jerome Glisse wrote:
> On Wed, Aug 29, 2018 at 08:39:06PM +0200, Michal Hocko wrote:
> > On Wed 29-08-18 14:14:25, Jerome Glisse wrote:
> > > On Wed, Aug 29, 2018 at 10:24:44AM -0700, Mike Kravetz wrote:
> > [...]
> > > > What would be the best mmu notifier interface to use where there are no
> > > > start/end calls?
> > > > Or, is the best solution to add the start/end calls as is done in later
> > > > versions of the code?  If that is the suggestion, has there been any change
> > > > in invalidate start/end semantics that we should take into account?
> > > 
> > > start/end would be the one to add, 4.4 seems broken in respect to THP
> > > and mmu notification. Another solution is to fix user of mmu notifier,
> > > they were only a handful back then. For instance properly adjust the
> > > address to match first address covered by pmd or pud and passing down
> > > correct page size to mmu_notifier_invalidate_page() would allow to fix
> > > this easily.
> > > 
> > > This is ok because user of try_to_unmap_one() replace the pte/pmd/pud
> > > with an invalid one (either poison, migration or swap) inside the
> > > function. So anyone racing would synchronize on those special entry
> > > hence why it is fine to delay mmu_notifier_invalidate_page() to after
> > > dropping the page table lock.
> > > 
> > > Adding start/end might the solution with less code churn as you would
> > > only need to change try_to_unmap_one().
> > 
> > What about dependencies? 369ea8242c0fb sounds like it needs work for all
> > notifiers need to be updated as well.
> 
> This commit remove mmu_notifier_invalidate_page() hence why everything
> need to be updated. But in 4.4 you can get away with just adding start/
> end and keep around mmu_notifier_invalidate_page() to minimize disruption.

OK, this is really interesting. I was really worried to change the
semantic of the mmu notifiers in stable kernels because this is really
a hard to review change and high risk for anybody running those old
kernels. If we can keep the mmu_notifier_invalidate_page and wrap them
into the range scope API then this sounds like the best way forward.

So just to make sure we are at the same page. Does this sounds goo for
stable 4.4. backport? Mike's hugetlb pmd shared fixup can be applied on
top. What do you think?