Add compiler flag to disable explicit msync() calls

Makefile

@@ -4,7 +4,7 @@ VPATH=src:tests/unit
 DESTDIR?=/
 PREFIX?=/usr/local/bin
 INSTALLDIR=$(DESTDIR)/$(PREFIX)
-	
+
 ifdef DEBUG
 	CFLAGS_EXTRA=-g -DDEBUG
 	LDFLAGS_EXTRA=-g
@@ -12,6 +12,11 @@ else
 	CFLAGS_EXTRA=-O2
 endif
 
+NO_MSYNC=1
+ifdef NO_MSYNC
+	CFLAGS_EXTRA += -DNO_MSYNC
+endif
+
 CFLAGS_EXTRA  += -fPIC --std=gnu99
 LDFLAGS_EXTRA += -Wl,--relax,--gc-sections -L$(LIB) -Wl,-rpath-link,$(LIB)
 

src/server/client.c

@@ -472,7 +472,7 @@ void client_reply_to_write( struct client* client, struct nbd_request request )
 		bitset_set_range(client->serve->allocation_map, request.from, request.len);
 	}
 
-	if (1) /* not sure whether this is necessary... */
+#ifndef NO_MSYNC
 	{
 		/* multiple of 4K page size */
 		uint64_t from_rounded = request.from & (!0xfff);
@@ -485,6 +485,7 @@ void client_reply_to_write( struct client* client, struct nbd_request request )
 			"msync failed %ld %ld", request.from, request.len
 		);
 	}
+#endif
 	client_write_reply( client, &request, 0);
 }