Re: [PATCHv2 4/4] thp: rewrite freeze_page()/unfreeze_page() with generic rmap walkers

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Mon, Mar 07, 2016 at 02:57:18PM +0300, Kirill A. Shutemov wrote:
> freeze_page() and unfreeze_page() helpers evolved in rather complex
> beasts. It would be nice to cut complexity of this code.
> 
> This patch rewrites freeze_page() using standard try_to_unmap().
> unfreeze_page() is rewritten with remove_migration_ptes().
> 
> The result is much simpler.
> 
> But the new variant is somewhat slower for PTE-mapped THPs.
> Current helpers iterates over VMAs the compound page is mapped to, and
> then over ptes within this VMA. New helpers iterates over small page,
> then over VMA the small page mapped to, and only then find relevant pte.
> 
> We have short cut for PMD-mapped THP: we directly install migration
> entries on PMD split.
> 
> I don't think the slowdown is critical, considering how much simpler
> result is and that split_huge_page() is quite rare nowadays. It only
> happens due memory pressure or migration.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  include/linux/huge_mm.h |  10 ++-
>  mm/huge_memory.c        | 201 +++++++-----------------------------------------
>  mm/rmap.c               |   9 ++-
>  3 files changed, 40 insertions(+), 180 deletions(-)

Andrew, could you please fold patch below into this one.