mm/hugetlb.c: undo change to page mapcount in fault handler

Page mapcount should be updated only if we are sure that the page ends
up in the page table otherwise we would leak if we couldn't COW due to
reservations or if idx is out of bounds.

Signed-off-by: Hillf Danton <dhillf@gmail.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -2508,6 +2508,7 @@ static int hugetlb_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
 {
 	struct hstate *h = hstate_vma(vma);
 	int ret = VM_FAULT_SIGBUS;
+	int anon_rmap = 0;
 	pgoff_t idx;
 	unsigned long size;
 	struct page *page;
@@ -2562,14 +2563,13 @@ retry:
 			spin_lock(&inode->i_lock);
 			inode->i_blocks += blocks_per_huge_page(h);
 			spin_unlock(&inode->i_lock);
-			page_dup_rmap(page);
 		} else {
 			lock_page(page);
 			if (unlikely(anon_vma_prepare(vma))) {
 				ret = VM_FAULT_OOM;
 				goto backout_unlocked;
 			}
-			hugepage_add_new_anon_rmap(page, vma, address);
+			anon_rmap = 1;
 		}
 	} else {
 		/*
@@ -2582,7 +2582,6 @@ retry:
 			      VM_FAULT_SET_HINDEX(h - hstates);
 			goto backout_unlocked;
 		}
-		page_dup_rmap(page);
 	}
 
 	/*
@@ -2606,6 +2605,10 @@ retry:
 	if (!huge_pte_none(huge_ptep_get(ptep)))
 		goto backout;
 
+	if (anon_rmap)
+		hugepage_add_new_anon_rmap(page, vma, address);
+	else
+		page_dup_rmap(page);
 	new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
 				&& (vma->vm_flags & VM_SHARED)));
 	set_huge_pte_at(mm, address, ptep, new_pte);