thp: copy_huge_pmd(): copy huge zero page

It's easy to copy huge zero page. Just set destination pmd to huge zero
page.

It's safe to copy huge zero page since we have none yet :-p

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@linux.intel.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d525bdc58be1a9e95eff5afb68f973eadb526276..afe8b3d034e212150b4f1dad3ee9c7f67ec7569f 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -709,6 +709,18 @@ static inline struct page *alloc_hugepage(int defrag)
 }
 #endif
 
+static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
+               struct vm_area_struct *vma, unsigned long haddr, pmd_t *pmd)
+{
+       pmd_t entry;
+       entry = pfn_pmd(huge_zero_pfn, vma->vm_page_prot);
+       entry = pmd_wrprotect(entry);
+       entry = pmd_mkhuge(entry);
+       set_pmd_at(mm, haddr, pmd, entry);
+       pgtable_trans_huge_deposit(mm, pgtable);
+       mm->nr_ptes++;
+}
+
 int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
                               unsigned long address, pmd_t *pmd,
                               unsigned int flags)
@@ -946,6 +958,11 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
                pte_free(dst_mm, pgtable);
                goto out_unlock;
        }
+       if (is_huge_zero_pmd(pmd)) {
+               set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd);
+               ret = 0;
+               goto out_unlock;
+       }
        if (unlikely(pmd_trans_splitting(pmd))) {
                /* split huge page running from under us */
                spin_unlock(&src_mm->page_table_lock);