Exit early in os::pd_pretouch_memory and generate a warning when user…

… turns on UseMadvPopulteWrite when not supported

src/hotspot/os/linux/os_linux.cpp

@@ -3040,33 +3040,34 @@ void os::pd_free_memory(char *addr, size_t bytes, size_t alignment_hint) {
 }
 
 size_t os::pd_pretouch_memory(void* first, void* last, size_t page_size) {
+  if (HugePages::thp_mode() != THPMode::always && UseTransparentHugePages) {
+    // No THP. Use the platform-independent pretouch memory code.
+    return page_size;
+  }
+
+  if (!UseMadvPopulateWrite) {
+    // Use small pages with the platform-independent pretouch memory code.
+    // When using THP, we need to always pre-touch using small pages as the
+    // OS will initially always use small pages.
+    return os::vm_page_size();
+  }
+
   const size_t len = pointer_delta(last, first, sizeof(char)) + page_size;
-  // Use madvise to pretouch on Linux when THP is used, and fallback to the
-  // common method if unsupported. THP can form right after madvise rather than
-  // being assembled later.
-  if (HugePages::thp_mode() == THPMode::always || UseTransparentHugePages) {
-    int err = 0;
-    if (UseMadvPopulateWrite &&
-        ::madvise(first, len, MADV_POPULATE_WRITE) == -1) {
-      err = errno;
-    }
-    if (!UseMadvPopulateWrite || err == EINVAL) { // Not to use or not supported
-      // When using THP we need to always pre-touch using small pages as the
-      // OS will initially always use small pages.
-      page_size = os::vm_page_size();
-    } else if (err == 0) {
-      page_size = 0;
-    }
-    if (UseMadvPopulateWrite) {
-      log_debug(gc, os)("Called madvise(" PTR_FORMAT ", " SIZE_FORMAT ", %d):"
-                        " error='%s' (errno=%d), when THPMode::always=%d and"
-                        " UseTransparentHugePages=%d",
-                        p2i(first), len, MADV_POPULATE_WRITE, os::strerror(err),
-                        err, (int)(HugePages::thp_mode() == THPMode::always),
-                        (int)UseTransparentHugePages);
-    }
+  if (::madvise(first, len, MADV_POPULATE_WRITE) != -1) {
+    // Succeeded
+    // 0 signals not to run the platform-independent pretouch memory code.
+    return 0;
   }
-  return page_size;
+
+  int err = errno;
+  log_debug(gc, os)("Called madvise(" PTR_FORMAT ", " SIZE_FORMAT ", %d):"
+                    " error='%s' (errno=%d), when THPMode::always=%d and"
+                    " UseTransparentHugePages=%d",
+                    p2i(first), len, MADV_POPULATE_WRITE, os::strerror(err),
+                    err, (int)(HugePages::thp_mode() == THPMode::always),
+                    (int)UseTransparentHugePages);
+
+  return os::vm_page_size();
 }
 
 void os::numa_make_global(char *addr, size_t bytes) {
@@ -4544,6 +4545,7 @@ void os::init(void) {
     (int(*)(pthread_t, const char*))dlsym(RTLD_DEFAULT, "pthread_setname_np");
 
   check_pax();
+
   Linux::version_init();
 
   os::Posix::init();
@@ -4823,18 +4825,18 @@ jint os::init_2(void) {
   }
 
   // Check the availability of MADV_POPULATE_WRITE.
-  if (FLAG_IS_DEFAULT(UseMadvPopulateWrite) && UseMadvPopulateWrite) {
+  if (UseMadvPopulateWrite) {
     // Some downstream kernels recognize MADV_POPULATE_WRITE_value as another
     // advice, so the check of versions is required here.
     // See https://github.com/oracle/linux-uek/issues/23
-    if (Linux::_release_major < 5 ||
-        (Linux::_release_major == 5 && Linux::_release_minor < 14)) {
-      FLAG_SET_DEFAULT(UseMadvPopulateWrite, false);
-    } else {
-      FLAG_SET_DEFAULT(UseMadvPopulateWrite, (::madvise(0, 0, MADV_POPULATE_WRITE) == 0));
-    }
+    bool supportMadvPopulateWrite =
+      (Linux::_release_major > 5 ||
+       (Linux::_release_major == 5 && Linux::_release_minor >= 14)) &&
+      (::madvise(0, 0, MADV_POPULATE_WRITE) == 0);
+    if (!FLAG_IS_DEFAULT(UseMadvPopulateWrite) && !supportMadvPopulateWrite)
+      warning("Platform is supposed not to support MADV_POPULATE_WRITE, "
+              "disabling using it to pretouch (-XX:-UseMadvPopulateWrite)");
   }
-  log_debug(gc, os)("UseMadvPopulateWrite=%d", (int)(UseMadvPopulateWrite ? 1 : 0));
 
   return JNI_OK;
 }