Add support for MAP_SHARED to the proxy library for Linux only

Signed-off-by: Lukasz Dorau <lukasz.dorau@intel.com>

Author: ldorau

README.md

@@ -228,6 +228,10 @@ In case of Linux it can be done without any code changes using the `LD_PRELOAD`
 $ LD_PRELOAD=/usr/lib/libumf_proxy.so myprogram
 ```
 
+The memory used by the proxy memory allocator is mmap'ed:
+1) with the `MAP_PRIVATE` flag by default or
+2) with the `MAP_SHARED` flag only if the `UMF_PROXY` environment variable contains the `page.disposition=shared` string.
+
 #### Windows
 
 In case of Windows it requires:

src/proxy_lib/proxy_lib.c

@@ -111,6 +111,13 @@ void proxy_lib_create_common(void) {
         umfOsMemoryProviderParamsDefault();
     enum umf_result_t umf_result;
 
+#ifndef _WIN32
+    if (util_env_var_has_str("UMF_PROXY", "page.disposition=shared")) {
+        LOG_DEBUG("proxy_lib: using the MAP_SHARED flag");
+        os_params.flag = UMF_MEM_MAP_SHARED;
+    }
+#endif
+
     umf_result = umfMemoryProviderCreate(umfOsMemoryProviderOps(), &os_params,
                                          &OS_memory_provider);
     if (umf_result != UMF_RESULT_SUCCESS) {