[patch 5/6] mm: debug write deadlocks

Nick Piggin
2006-10-13T16:44:42Z

Allow CONFIG_DEBUG_VM to switch off the prefaulting logic, to simulate the
difficult race where the page may be unmapped before calling copy_from_user.
Makes the race much easier to hit.

This probably needn't go upstream.

Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -1895,6 +1895,7 @@ generic_file_buffered_write(struct kiocb
if (maxlen > bytes)
maxlen = bytes;

+#ifndef CONFIG_DEBUG_VM
/*
* Bring in the user page that we will copy from _first_.
* Otherwise there's a nasty deadlock on copying from the
@@ -1902,6 +1903,7 @@ generic_file_buffered_write(struct kiocb
* up-to-date.
*/
fault_in_pages_readable(buf, maxlen);
+#endif

page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
if (!page) {