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

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Date:   Thu, 10 Nov 2016 09:09:05 +0000
From:   Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        Pavel Emelyanov <xemul@...allels.com>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Balbir Singh <bsingharora@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Naoya Horiguchi" <nao.horiguchi@...il.com>
Subject: Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp
 freezing

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