[PATCH] enforce proper tlb flush in unmap_hugepage_range

Spotted by Hugh that hugetlb page is free'ed back to global pool before
performing any TLB flush in unmap_hugepage_range().  This potentially allow
threads to abuse free-alloc race condition.

The generic tlb gather code is unsuitable to use by hugetlb, I just open
coded a page gathering list and delayed put_page until tlb flush is
performed.

Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Ken Chen <kenneth.w.chen@intel.com>
Acked-by: William Irwin <wli@holomorphy.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/hugetlb.c

@@ -364,6 +364,8 @@ void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
 	pte_t *ptep;
 	pte_t pte;
 	struct page *page;
+	struct page *tmp;
+	LIST_HEAD(page_list);
 
 	WARN_ON(!is_vm_hugetlb_page(vma));
 	BUG_ON(start & ~HPAGE_MASK);
@@ -384,12 +386,16 @@ void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
 			continue;
 
 		page = pte_page(pte);
-		put_page(page);
+		list_add(&page->lru, &page_list);
 		add_mm_counter(mm, file_rss, (int) -(HPAGE_SIZE / PAGE_SIZE));
 	}
 
 	spin_unlock(&mm->page_table_lock);
 	flush_tlb_range(vma, start, end);
+	list_for_each_entry_safe(page, tmp, &page_list, lru) {
+		list_del(&page->lru);
+		put_page(page);
+	}
 }
 
 static int hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,