Use mmap MAP_NOSYNC option to limit shared memory writes

mmap() is rarely used for shared memory, but when it is, this option is
useful, particularly on the BSDs.

Patch by Sean Chittenden

src/backend/storage/ipc/dsm_impl.c

@@ -368,7 +368,7 @@ dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
 
 	/* Map it. */
 	address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
-				   MAP_SHARED | MAP_HASSEMAPHORE, fd, 0);
+				   MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
 	if (address == MAP_FAILED)
 	{
 		int			save_errno;
@@ -960,7 +960,7 @@ dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size,
 
 	/* Map it. */
 	address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
-				   MAP_SHARED | MAP_HASSEMAPHORE, fd, 0);
+				   MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
 	if (address == MAP_FAILED)
 	{
 		int			save_errno;

src/include/portability/mem.h

@@ -30,6 +30,14 @@
 #define MAP_HASSEMAPHORE		0
 #endif
 
+/*
+ * BSD-derived systems use the MAP_NOSYNC flag to prevent dirty mmap(2)
+ * pages from being gratuitously flushed to disk.
+ */
+#ifndef MAP_NOSYNC
+#define MAP_NOSYNC			0
+#endif
+
 #define PG_MMAP_FLAGS			(MAP_SHARED|MAP_ANONYMOUS|MAP_HASSEMAPHORE)
 
 /* Some really old systems don't define MAP_FAILED. */