[PATCH] __set_page_dirty_nobuffers mappings

[Hugh Dickins]

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@xxxxxxxxxxx>
---

 mm/page-writeback.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

--- 2.6.9-rc1-bk12/mm/page-writeback.c	2004-09-05 12:58:41.000000000 +0100
+++ linux/mm/page-writeback.c	2004-09-05 17:07:14.513083224 +0100
@@ -582,12 +582,13 @@ int __set_page_dirty_nobuffers(struct pa
 
 	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,

-
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