linux-kernel - [patch 5/10] mm: debug write deadlocks

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Date:	Sat, 13 Jan 2007 04:25:02 +0100 (CET)
From:	Nick Piggin <npiggin@...e.de>
To:	Linux Memory Management <linux-mm@...ck.org>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Filesystems <linux-fsdevel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>, Andrew Morton <akpm@...l.org>
Subject: [patch 5/10] mm: debug write deadlocks

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 is useful for demonstration and testing purposes, but is removed in a
subsequent patch.

Signed-off-by: Nick Piggin <npiggin@...e.de>

Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -1894,6 +1894,7 @@ 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
@@ -1901,6 +1902,7 @@ 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) {
-
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