ksm: check for ERR_PTR from follow_page()

The follow_page() function can potentially return -EFAULT so I added
checks for this.

Also I silenced an uninitialized variable warning on my version of gcc
(version 4.3.2).
Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -365,7 +365,7 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
 	do {
 		cond_resched();
 		page = follow_page(vma, addr, FOLL_GET);
-		if (!page)
+		if (IS_ERR_OR_NULL(page))
 			break;
 		if (PageKsm(page))
 			ret = handle_mm_fault(vma->vm_mm, vma, addr,
@@ -447,7 +447,7 @@ static struct page *get_mergeable_page(struct rmap_item *rmap_item)
 		goto out;
 
 	page = follow_page(vma, addr, FOLL_GET);
-	if (!page)
+	if (IS_ERR_OR_NULL(page))
 		goto out;
 	if (PageAnon(page)) {
 		flush_anon_page(vma, page, addr);
@@ -1086,7 +1086,7 @@ struct rmap_item *unstable_tree_search_insert(struct rmap_item *rmap_item,
 		cond_resched();
 		tree_rmap_item = rb_entry(*new, struct rmap_item, node);
 		tree_page = get_mergeable_page(tree_rmap_item);
-		if (!tree_page)
+		if (IS_ERR_OR_NULL(tree_page))
 			return NULL;
 
 		/*
@@ -1294,7 +1294,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
 			if (ksm_test_exit(mm))
 				break;
 			*page = follow_page(vma, ksm_scan.address, FOLL_GET);
-			if (*page && PageAnon(*page)) {
+			if (!IS_ERR_OR_NULL(*page) && PageAnon(*page)) {
 				flush_anon_page(vma, *page, ksm_scan.address);
 				flush_dcache_page(*page);
 				rmap_item = get_next_rmap_item(slot,
@@ -1308,7 +1308,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
 				up_read(&mm->mmap_sem);
 				return rmap_item;
 			}
-			if (*page)
+			if (!IS_ERR_OR_NULL(*page))
 				put_page(*page);
 			ksm_scan.address += PAGE_SIZE;
 			cond_resched();
@@ -1367,7 +1367,7 @@ static struct rmap_item *scan_get_next_rmap_item(struct page **page)
 static void ksm_do_scan(unsigned int scan_npages)
 {
 	struct rmap_item *rmap_item;
-	struct page *page;
+	struct page *uninitialized_var(page);
 
 	while (scan_npages--) {
 		cond_resched();