[PATCH] __set_page_dirty_nobuffers mappings

Marcelo noticed that the BUG_ON in __set_page_dirty_nobuffers doesn't make
much sense: it lost its way in 2.6.7, amidst so many page_mappings!

It's supposed to be checking that, although page->mapping may suddenly go NULL
from truncation, and although tmpfs swizzles page_mapping(page) between tmpfs
inode address_space and swapper_space, there's sufficient stabilization while
here in __set_page_dirty_nobuffers that the mapping after we locked
mapping->tree_lock is the same as the mapping before we locked
mapping->tree_lock i.e.  the lock we hold is the right one.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/page-writeback.c

@@ -580,12 +580,13 @@ int __set_page_dirty_nobuffers(struct page *page)
 
 	if (!TestSetPageDirty(page)) {
 		struct address_space *mapping = page_mapping(page);
+		struct address_space *mapping2;
 
 		if (mapping) {
 			spin_lock_irq(&mapping->tree_lock);
-			mapping = page_mapping(page);
-			if (page_mapping(page)) { /* Race with truncate? */
-				BUG_ON(page_mapping(page) != mapping);
+			mapping2 = page_mapping(page);
+			if (mapping2) { /* Race with truncate? */
+				BUG_ON(mapping2 != mapping);
 				if (!mapping->backing_dev_info->memory_backed)
 					inc_page_state(nr_dirty);
 				radix_tree_tag_set(&mapping->page_tree,