mm: debug write deadlocks

Allow CONFIG_DEBUG_VM to switch off the prefaulting logic, to simulate the
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@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/mm/filemap.c b/mm/filemap.c
index f74d05201862..0c54fc9b8e3d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1878,6 +1878,7 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
                 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
@@ -1885,6 +1886,7 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
                  * up-to-date.
                  */
                 fault_in_pages_readable(buf, maxlen);
+#endif
 
                 page = __grab_cache_page(mapping,index,&cached_page,&lru_pvec);
                 if (!page) {