Re: [PATCH v2 04/12] mm: thp: introduce CONFIG_ARCH_ENABLE_THP_MIGRATION

[Naoya Horiguchi]

On Mon, Nov 28, 2016 at 03:21:54PM +0100, Michal Hocko wrote:
> On Tue 08-11-16 08:31:49, Naoya Horiguchi wrote:
> > Introduces CONFIG_ARCH_ENABLE_THP_MIGRATION to limit thp migration
> > functionality to x86_64, which should be safer at the first step.
> 
> Please make sure to describe why this has to be arch specific and what
> are arches supposed to provide in order to enable this option.

OK, the below will be added in the future version:

  Thp migration is an arch-specific feature because it depends on the
  arch-dependent behavior of non-present format of page table entry.
  What you need to enable this option in other archs are:
  - to define arch-specific transformation functions like __pmd_to_swp_entry()
    and __swp_entry_to_pmd(),
  - to make sure that arch-specific page table walking code can properly handle
    !pmd_present case (gup_pmd_range() is a good example),
  - (if your archs enables CONFIG_HAVE_ARCH_SOFT_DIRTY,) to define soft dirty
    routines like pmd_swp_mksoft_dirty.

Thanks,
Naoya Horiguchi