[PATCH] swap 2/4 unsafe SwapCache check

[Hugh Dickins]

Recent testing has shown that the BUG() check in try_to_unuse is unsafe.
delete_from_swap_cache does final swap_free just after removing page
from swap cache, and add_to_swap_cache does swap_duplicate just before
putting page into swap cache, therefore swapin_readahead may resurrect
a dying swap entry and assign a new page to it.  That's fine, there's
no need to change this ordering; but it does mean that try_to_unuse's
page may have left the swap cache yet its swap_map count still be set.
That BUG() has done good service for swapoff sanity, but now abandon it.

--- 2.4.19-pre10/mm/swapfile.c	Tue Jun  4 13:54:20 2002
+++ linux/mm/swapfile.c	Tue Jun 11 19:02:30 2002
@@ -671,10 +671,7 @@
 		 * private" pages, but they are handled by tmpfs files.
 		 * Note shmem_unuse already deleted its from swap cache.
 		 */
-		swcount = *swap_map;
-		if ((swcount > 0) != PageSwapCache(page))
-			BUG();
-		if ((swcount > 1) && PageDirty(page)) {
+		if ((*swap_map > 1) && PageDirty(page) && PageSwapCache(page)) {
 			rw_swap_page(WRITE, page);
 			lock_page(page);
 		}