mm: thp: fix flags for pmd migration when split

When splitting a huge migrating PMD, we'll transfer all the existing PMD
bits and apply them again onto the small PTEs.  However we are fetching
the bits unconditionally via pmd_soft_dirty(), pmd_write() or
pmd_yound() while actually they don't make sense at all when it's a
migration entry.  Fix them up.  Since at it, drop the ifdef together as
not needed.

Note that if my understanding is correct about the problem then if
without the patch there is chance to lose some of the dirty bits in the
migrating pmd pages (on x86_64 we're fetching bit 11 which is part of
swap offset instead of bit 2) and it could potentially corrupt the
memory of an userspace program which depends on the dirty bit.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Xu <[email protected]>
Reviewed-by: Konstantin Khlebnikov <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Dave Jiang <[email protected]>
Cc: "Aneesh Kumar K.V" <[email protected]>
Cc: Souptick Joarder <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: <[email protected]>	[4.14+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: xzpeter

mm/huge_memory.c

@@ -2144,23 +2144,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 	 */
 	old_pmd = pmdp_invalidate(vma, haddr, pmd);
 
-#ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
 	pmd_migration = is_pmd_migration_entry(old_pmd);
-	if (pmd_migration) {
+	if (unlikely(pmd_migration)) {
 		swp_entry_t entry;
 
 		entry = pmd_to_swp_entry(old_pmd);
 		page = pfn_to_page(swp_offset(entry));
-	} else
-#endif
+		write = is_write_migration_entry(entry);
+		young = false;
+		soft_dirty = pmd_swp_soft_dirty(old_pmd);
+	} else {
 		page = pmd_page(old_pmd);
+		if (pmd_dirty(old_pmd))
+			SetPageDirty(page);
+		write = pmd_write(old_pmd);
+		young = pmd_young(old_pmd);
+		soft_dirty = pmd_soft_dirty(old_pmd);
+	}
 	VM_BUG_ON_PAGE(!page_count(page), page);
 	page_ref_add(page, HPAGE_PMD_NR - 1);
-	if (pmd_dirty(old_pmd))
-		SetPageDirty(page);
-	write = pmd_write(old_pmd);
-	young = pmd_young(old_pmd);
-	soft_dirty = pmd_soft_dirty(old_pmd);
 
 	/*
 	 * Withdraw the table only after we mark the pmd entry invalid.