mm: replace some BUG_ONs by VM_BUG_ONs

The swap code is over-provisioned with BUG_ONs on assorted page flags,
mostly dating back to 2.3.  They're good documentation, and guard against
developer error, but a waste of space on most systems: change them to
VM_BUG_ONs, conditional on CONFIG_DEBUG_VM.  Just delete the PagePrivate
ones: they're later, from 2.5.69, but even less interesting now.

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Reviewed-by: Christoph Lameter <cl@linux-foundation.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_io.c

@@ -125,8 +125,8 @@ int swap_readpage(struct file *file, struct page *page)
 	struct bio *bio;
 	int ret = 0;
 
-	BUG_ON(!PageLocked(page));
-	BUG_ON(PageUptodate(page));
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(PageUptodate(page));
 	bio = get_swap_bio(GFP_KERNEL, page_private(page), page,
 				end_swap_bio_read);
 	if (bio == NULL) {

mm/swap_state.c

@@ -72,10 +72,10 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
 {
 	int error;
 
-	BUG_ON(!PageLocked(page));
-	BUG_ON(PageSwapCache(page));
-	BUG_ON(PagePrivate(page));
-	BUG_ON(!PageSwapBacked(page));
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(PageSwapCache(page));
+	VM_BUG_ON(!PageSwapBacked(page));
+
 	error = radix_tree_preload(gfp_mask);
 	if (!error) {
 		page_cache_get(page);
@@ -108,10 +108,9 @@ int add_to_swap_cache(struct page *page, swp_entry_t entry, gfp_t gfp_mask)
  */
 void __delete_from_swap_cache(struct page *page)
 {
-	BUG_ON(!PageLocked(page));
-	BUG_ON(!PageSwapCache(page));
-	BUG_ON(PageWriteback(page));
-	BUG_ON(PagePrivate(page));
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(!PageSwapCache(page));
+	VM_BUG_ON(PageWriteback(page));
 
 	radix_tree_delete(&swapper_space.page_tree, page_private(page));
 	set_page_private(page, 0);
@@ -134,8 +133,8 @@ int add_to_swap(struct page * page, gfp_t gfp_mask)
 	swp_entry_t entry;
 	int err;
 
-	BUG_ON(!PageLocked(page));
-	BUG_ON(!PageUptodate(page));
+	VM_BUG_ON(!PageLocked(page));
+	VM_BUG_ON(!PageUptodate(page));
 
 	for (;;) {
 		entry = get_swap_page();

mm/swapfile.c

@@ -333,7 +333,7 @@ int can_share_swap_page(struct page *page)
 {
 	int count;
 
-	BUG_ON(!PageLocked(page));
+	VM_BUG_ON(!PageLocked(page));
 	count = page_mapcount(page);
 	if (count <= 1 && PageSwapCache(page))
 		count += page_swapcount(page);
@@ -350,8 +350,7 @@ static int remove_exclusive_swap_page_count(struct page *page, int count)
 	struct swap_info_struct * p;
 	swp_entry_t entry;
 
-	BUG_ON(PagePrivate(page));
-	BUG_ON(!PageLocked(page));
+	VM_BUG_ON(!PageLocked(page));
 
 	if (!PageSwapCache(page))
 		return 0;
@@ -432,7 +431,6 @@ void free_swap_and_cache(swp_entry_t entry)
 	if (page) {
 		int one_user;
 
-		BUG_ON(PagePrivate(page));
 		one_user = (page_count(page) == 2);
 		/* Only cache user (+us), or swap space full? Free it! */
 		/* Also recheck PageSwapCache after page is locked (above) */
@@ -1209,7 +1207,7 @@ int page_queue_congested(struct page *page)
 {
 	struct backing_dev_info *bdi;
 
-	BUG_ON(!PageLocked(page));	/* It pins the swap_info_struct */
+	VM_BUG_ON(!PageLocked(page));	/* It pins the swap_info_struct */
 
 	if (PageSwapCache(page)) {
 		swp_entry_t entry = { .val = page_private(page) };