Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp freezing

[Naoya Horiguchi]

On Thu, Nov 10, 2016 at 02:01:56PM +0530, Anshuman Khandual wrote:
> On 11/08/2016 05:01 AM, Naoya Horiguchi wrote:
> > TTU_MIGRATION is used to convert pte into migration entry until thp split
> > completes. This behavior conflicts with thp migration added later patches,
>
> Hmm, could you please explain why it conflicts with the PMD based
> migration without split ? Why TTU_MIGRATION cannot be used to
> freeze/hold on the PMD while it's being migrated ?

try_to_unmap() is used both for thp split (via freeze_page()) and page
migration (via __unmap_and_move()). In freeze_page(), ttu_flag given for
head page is like below (assuming anonymous thp):

    (TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
     TTU_MIGRATION | TTU_SPLIT_HUGE_PMD)

and ttu_flag given for tail pages is:

    (TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
     TTU_MIGRATION)

__unmap_and_move() calls try_to_unmap() with ttu_flag:

    (TTU_MIGRATION | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS)

Now I'm trying to insert a branch for thp migration at the top of
try_to_unmap_one() like below


  static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
                       unsigned long address, void *arg)
  {
          ...
          if (flags & TTU_MIGRATION) {
                  if (!PageHuge(page) && PageTransCompound(page)) {
                          set_pmd_migration_entry(page, vma, address);
                          goto out;
                  }
          }

, so try_to_unmap() for tail pages called by thp split can go into thp
migration code path (which converts *pmd* into migration entry), while
the expectation is to freeze thp (which converts *pte* into migration entry.)

I detected this failure as a "bad page state" error in a testcase where
split_huge_page() is called from queue_pages_pte_range().

Anyway, I'll add this explanation into the patch description in the next post.

Thanks,
Naoya Horiguchi