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

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  [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp freezing Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-11-08 00:40 +0100
    Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp  freezing Anshuman Khandual <khandual@linux.vnet.ibm.com> - 2016-11-10 09:40 +0100
      Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp  freezing Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> - 2016-11-10 10:20 +0100
        Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp  freezing Anshuman Khandual <khandual@linux.vnet.ibm.com> - 2016-11-11 04:20 +0100
    Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp  freezing "Kirill A. Shutemov" <kirill@shutemov.name> - 2016-11-11 12:20 +0100

#1516667 — [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp freezing

TTU_MIGRATION is used to convert pte into migration entry until thp split
completes. This behavior conflicts with thp migration added later patches,
so let's introduce a new TTU flag specifically for freezing.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
---
 include/linux/rmap.h | 1 +
 mm/huge_memory.c     | 2 +-
 mm/rmap.c            | 8 +++++---
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git v4.9-rc2-mmotm-2016-10-27-18-27/include/linux/rmap.h v4.9-rc2-mmotm-2016-10-27-18-27_patched/include/linux/rmap.h
index b46bb56..a2fa425 100644
--- v4.9-rc2-mmotm-2016-10-27-18-27/include/linux/rmap.h
+++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/include/linux/rmap.h
@@ -87,6 +87,7 @@ enum ttu_flags {
 	TTU_MUNLOCK = 4,		/* munlock mode */
 	TTU_LZFREE = 8,			/* lazy free mode */
 	TTU_SPLIT_HUGE_PMD = 16,	/* split huge PMD if any */
+	TTU_SPLIT_FREEZE = 32,		/* freeze pte under splitting thp */
 
 	TTU_IGNORE_MLOCK = (1 << 8),	/* ignore mlock */
 	TTU_IGNORE_ACCESS = (1 << 9),	/* don't age */
diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/huge_memory.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/huge_memory.c
index 2d1d6bb..0509d17 100644
--- v4.9-rc2-mmotm-2016-10-27-18-27/mm/huge_memory.c
+++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/huge_memory.c
@@ -1794,7 +1794,7 @@ static void freeze_page(struct page *page)
 	VM_BUG_ON_PAGE(!PageHead(page), page);
 
 	if (PageAnon(page))
-		ttu_flags |= TTU_MIGRATION;
+		ttu_flags |= TTU_SPLIT_FREEZE;
 
 	/* We only need TTU_SPLIT_HUGE_PMD once */
 	ret = try_to_unmap(page, ttu_flags | TTU_SPLIT_HUGE_PMD);
diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/rmap.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/rmap.c
index 1ef3640..a4be307 100644
--- v4.9-rc2-mmotm-2016-10-27-18-27/mm/rmap.c
+++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/rmap.c
@@ -1449,7 +1449,7 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 
 	if (flags & TTU_SPLIT_HUGE_PMD) {
 		split_huge_pmd_address(vma, address,
-				flags & TTU_MIGRATION, page);
+				flags & TTU_SPLIT_FREEZE, page);
 		/* check if we have anything to do after split */
 		if (page_mapcount(page) == 0)
 			goto out;
@@ -1527,7 +1527,8 @@ static int try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 		 * will take care of the rest.
 		 */
 		dec_mm_counter(mm, mm_counter(page));
-	} else if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION)) {
+	} else if (IS_ENABLED(CONFIG_MIGRATION) &&
+		   (flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))) {
 		swp_entry_t entry;
 		pte_t swp_pte;
 		/*
@@ -1649,7 +1650,8 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
 	 * locking requirements of exec(), migration skips
 	 * temporary VMAs until after exec() completes.
 	 */
-	if ((flags & TTU_MIGRATION) && !PageKsm(page) && PageAnon(page))
+	if ((flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))
+	    && !PageKsm(page) && PageAnon(page))
 		rwc.invalid_vma = invalid_migration_vma;
 
 	if (flags & TTU_RMAP_LOCKED)
-- 
2.7.0

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#1518744 — Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp freezing

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 ?

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#1518772 — 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

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#1519535 — Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp freezing

On 11/10/2016 02:39 PM, Naoya Horiguchi wrote:
> 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)

Right.

> 
> and ttu_flag given for tail pages is:
> 
>     (TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS | TTU_RMAP_LOCKED | \
>      TTU_MIGRATION)

Right.

> 
> __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.)

Got it.

> 
> 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.

Sure, thanks for the explanation.

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#1519702 — Re: [PATCH v2 03/12] mm: thp: introduce separate TTU flag for thp freezing

On Tue, Nov 08, 2016 at 08:31:48AM +0900, 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,
> so let's introduce a new TTU flag specifically for freezing.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov

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