pagewalk: don't look up vma if walk->hugetlb_entry is unused

Currently, walk_page_range() calls find_vma() every page table for walk
iteration.  but it's completely unnecessary if walk->hugetlb_entry is
unused.  And we don't have to assume find_vma() is a lightweight
operation.  So this patch checks the walk->hugetlb_entry and avoids the
find_vma() call if possible.

This patch also makes some cleanups.  1) remove ugly uninitialized_var()
and 2) #ifdef in function body.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/pagewalk.c

@@ -126,7 +126,39 @@ static int walk_hugetlb_range(struct vm_area_struct *vma,
 
 	return 0;
 }
-#endif
+
+static struct vm_area_struct* hugetlb_vma(unsigned long addr, struct mm_walk *walk)
+{
+	struct vm_area_struct *vma;
+
+	/* We don't need vma lookup at all. */
+	if (!walk->hugetlb_entry)
+		return NULL;
+
+	VM_BUG_ON(!rwsem_is_locked(&walk->mm->mmap_sem));
+	vma = find_vma(walk->mm, addr);
+	if (vma && vma->vm_start <= addr && is_vm_hugetlb_page(vma))
+		return vma;
+
+	return NULL;
+}
+
+#else /* CONFIG_HUGETLB_PAGE */
+static struct vm_area_struct* hugetlb_vma(unsigned long addr, struct mm_walk *walk)
+{
+	return NULL;
+}
+
+static int walk_hugetlb_range(struct vm_area_struct *vma,
+			      unsigned long addr, unsigned long end,
+			      struct mm_walk *walk)
+{
+	return 0;
+}
+
+#endif /* CONFIG_HUGETLB_PAGE */
+
+
 
 /**
  * walk_page_range - walk a memory map's page tables with a callback
@@ -165,18 +197,17 @@ int walk_page_range(unsigned long addr, unsigned long end,
 
 	pgd = pgd_offset(walk->mm, addr);
 	do {
-		struct vm_area_struct *uninitialized_var(vma);
+		struct vm_area_struct *vma;
 
 		next = pgd_addr_end(addr, end);
 
-#ifdef CONFIG_HUGETLB_PAGE
 		/*
 		 * handle hugetlb vma individually because pagetable walk for
 		 * the hugetlb page is dependent on the architecture and
 		 * we can't handled it in the same manner as non-huge pages.
 		 */
-		vma = find_vma(walk->mm, addr);
-		if (vma && vma->vm_start <= addr && is_vm_hugetlb_page(vma)) {
+		vma = hugetlb_vma(addr, walk);
+		if (vma) {
 			if (vma->vm_end < next)
 				next = vma->vm_end;
 			/*
@@ -189,7 +220,7 @@ int walk_page_range(unsigned long addr, unsigned long end,
 			pgd = pgd_offset(walk->mm, next);
 			continue;
 		}
-#endif
+
 		if (pgd_none_or_clear_bad(pgd)) {
 			if (walk->pte_hole)
 				err = walk->pte_hole(addr, next, walk);