From 02d509cf44806e6974647725f8ce9dbbdc083d53 Mon Sep 17 00:00:00 2001
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Date: Thu, 29 Nov 2012 14:17:35 +1100
Subject: [PATCH] thp-copy_huge_pmd-copy-huge-zero-page-v6

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>
---
 mm/huge_memory.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index afe8b3d034e2..662ac5bea124 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -958,6 +958,11 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		pte_free(dst_mm, pgtable);
 		goto out_unlock;
 	}
+	/*
+	 * mm->pagetable lock is enough to be sure that huge zero pmd is not
+	 * under splitting since we don't split the page itself, only pmd to
+	 * a page table.
+	 */
 	if (is_huge_zero_pmd(pmd)) {
 		set_huge_zero_page(pgtable, dst_mm, vma, addr, dst_pmd);
 		ret = 0;
-- 
2.39.5