thp: lazy huge zero page allocation

Instead of allocating huge zero page on hugepage_init() we can postpone it
until first huge zero page map. It saves memory if THP is not in use.

cmpxchg() is used to avoid race on huge_zero_pfn initialization.
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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/huge_memory.c

@@ -160,22 +160,24 @@ static int start_khugepaged(void)
 	return err;
 }
 
-static int __init init_huge_zero_page(void)
+static int init_huge_zero_pfn(void)
 {
 	struct page *hpage;
+	unsigned long pfn;
 
 	hpage = alloc_pages((GFP_TRANSHUGE | __GFP_ZERO) & ~__GFP_MOVABLE,
 			HPAGE_PMD_ORDER);
 	if (!hpage)
 		return -ENOMEM;
-
-	huge_zero_pfn = page_to_pfn(hpage);
+	pfn = page_to_pfn(hpage);
+	if (cmpxchg(&huge_zero_pfn, 0, pfn))
+		__free_page(hpage);
 	return 0;
 }
 
 static inline bool is_huge_zero_pfn(unsigned long pfn)
 {
-	return pfn == huge_zero_pfn;
+	return huge_zero_pfn && pfn == huge_zero_pfn;
 }
 
 static inline bool is_huge_zero_pmd(pmd_t pmd)
@@ -564,10 +566,6 @@ static int __init hugepage_init(void)
 	if (err)
 		return err;
 
-	err = init_huge_zero_page();
-	if (err)
-		goto out;
-
 	err = khugepaged_slab_init();
 	if (err)
 		goto out;
@@ -590,8 +588,6 @@ static int __init hugepage_init(void)
 
 	return 0;
 out:
-	if (huge_zero_pfn)
-		__free_page(pfn_to_page(huge_zero_pfn));
 	hugepage_exit_sysfs(hugepage_kobj);
 	return err;
 }
@@ -735,6 +731,10 @@ int do_huge_pmd_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 			return VM_FAULT_OOM;
 		if (!(flags & FAULT_FLAG_WRITE)) {
 			pgtable_t pgtable;
+			if (unlikely(!huge_zero_pfn && init_huge_zero_pfn())) {
+				count_vm_event(THP_FAULT_FALLBACK);
+				goto out;
+			}
 			pgtable = pte_alloc_one(mm, haddr);
 			if (unlikely(!pgtable))
 				return VM_FAULT_OOM;