[PATCH] swap 3/4 unsafe Dirty check

[Hugh Dickins]

Todd reported swapoff kernel BUG at filemap.c:122 to LKML 24 May.
Other problems on that system may have contributed, but yes, despite
__delete_from_swap_cache doing ClearPageDirty before __remove_inode_page
to avoid the BUG(), a concurrent __free_pte might race to SetPageDirty.
So skip that oops in PageSwapCache case (please, if someone wants to use
unlikely or BUG_ON there, do so in your later patch).  Remove the prior
ClearPageDirty? maybe but not without deeper thought: let stay for now.

--- 2.4.19-pre10/mm/filemap.c	Tue Jun  4 13:54:19 2002
+++ linux/mm/filemap.c	Tue Jun 11 19:02:30 2002
@@ -119,7 +119,8 @@
  */
 void __remove_inode_page(struct page *page)
 {
-	if (PageDirty(page)) BUG();
+	if (PageDirty(page) && !PageSwapCache(page))
+		BUG();
 	remove_page_from_inode_queue(page);
 	remove_page_from_hash_queue(page);
 }