[patch 05/41] 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.

Cc: Linux Memory Management <redacted>
Cc: Linux Filesystems <redacted>
Signed-off-by: Nick Piggin <redacted>

 mm/filemap.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6/mm/filemap.c
===================================================================

--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c

@@ -1940,6 +1940,7 @@ 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

@@ -1947,6 +1948,7 @@ 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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