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Subject[patch 5/9] 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 <>

Index: linux-2.6/mm/filemap.c
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -2103,6 +2103,7 @@ generic_file_buffered_write(struct kiocb
if (maxlen > bytes)
maxlen = bytes;

* Bring in the user page that we will copy from _first_.
* Otherwise there's a nasty deadlock on copying from the
@@ -2110,6 +2111,7 @@ generic_file_buffered_write(struct kiocb
* up-to-date.
fault_in_pages_readable(buf, maxlen);

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