[patch 5/6] mm: debug write deadlocks

From: Nick Piggin <npiggin@suse.de>
To: Linux Memory Management <linux-mm@kvack.org>
Cc: Neil Brown <neilb@suse.de>, Andrew Morton <akpm@osdl.org>,
	Anton Altaparmakov <aia21@cam.ac.uk>,
	Chris Mason <chris.mason@oracle.com>,
	Linux Kernel <linux-kernel@vger.kernel.org>,
	Nick Piggin <npiggin@suse.de>
Subject: [patch 5/6] mm: debug write deadlocks
Date: Fri, 13 Oct 2006 18:44:42 +0200 (CEST)	[thread overview]
Message-ID: <20061013143606.15438.18462.sendpatchset@linux.site> (raw)
In-Reply-To: <20061013143516.15438.8802.sendpatchset@linux.site>

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) {
