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>

1 files changed, 10 insertions, 10 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e1b6f4e13b91..9539d6654bb9 100644
--- a/mm/huge_memory.c
+++ b/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;