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.

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)
  {
          ...
          /* PMD-mapped THP migration entry */
          if (!pvmw.pte && (flags & TTU_MIGRATION)) {
              if (!PageAnon(page))
                  continue;

              set_pmd_migration_entry(&pvmw, page);
              continue;
          }
	  ...
  }

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().

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Naoya Horiguchi <[email protected]>
Signed-off-by: Zi Yan <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: David Nellans <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/rmap.h

@@ -93,8 +93,9 @@ enum ttu_flags {
 	TTU_BATCH_FLUSH		= 0x40,	/* Batch TLB flushes where possible
 					 * and caller guarantees they will
 					 * do a final flush if necessary */
-	TTU_RMAP_LOCKED		= 0x80	/* do not grab rmap lock:
+	TTU_RMAP_LOCKED		= 0x80,	/* do not grab rmap lock:
 					 * caller holds it */
+	TTU_SPLIT_FREEZE	= 0x100,		/* freeze pte under splitting thp */
 };
 
 #ifdef CONFIG_MMU

mm/huge_memory.c

@@ -2210,7 +2210,7 @@ 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;
 
 	unmap_success = try_to_unmap(page, ttu_flags);
 	VM_BUG_ON_PAGE(!unmap_success, page);

mm/rmap.c

@@ -1348,7 +1348,7 @@ static bool 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);
 	}
 
 	/*
@@ -1445,7 +1445,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
 			 */
 			dec_mm_counter(mm, mm_counter(page));
 		} else if (IS_ENABLED(CONFIG_MIGRATION) &&
-				(flags & TTU_MIGRATION)) {
+				(flags & (TTU_MIGRATION|TTU_SPLIT_FREEZE))) {
 			swp_entry_t entry;
 			pte_t swp_pte;
 			/*
@@ -1575,7 +1575,8 @@ bool 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)