make mlock2wrapper public

Summary: As title. Expose the useful portable mlock2() wrapper.

Reviewed By: alikhtarov

Differential Revision: D20239849

fbshipit-source-id: 0fa776faf29ad69e551dfbe6eb69369560800a0e

folly/system/MemoryMapping.cpp

@@ -273,12 +273,8 @@ bool memOpInChunks(
   return true;
 }
 
-/**
- * mlock2 is Linux-only and exists since Linux 4.4
- * On Linux pre-4.4 and other platforms fail with ENOSYS.
- * glibc added the mlock2 wrapper in 2.27
- * https://lists.gnu.org/archive/html/info-gnu/2018-02/msg00000.html
- */
+} // namespace
+
 int mlock2wrapper(
     const void* addr,
     size_t len,
@@ -305,8 +301,6 @@ int mlock2wrapper(
 #endif
 }
 
-} // namespace
-
 bool MemoryMapping::mlock(LockMode mode, LockFlags flags) {
   size_t amountSucceeded = 0;
   locked_ = memOpInChunks(

folly/system/MemoryMapping.h

@@ -285,4 +285,12 @@ void alignedForwardMemcpy(void* dst, const void* src, size_t size);
  */
 void mmapFileCopy(const char* src, const char* dest, mode_t mode = 0666);
 
+/**
+ * mlock2 is Linux-only and exists since Linux 4.4
+ * On Linux pre-4.4 and other platforms fail with ENOSYS.
+ * glibc added the mlock2 wrapper in 2.27
+ * https://lists.gnu.org/archive/html/info-gnu/2018-02/msg00000.html
+ */
+int mlock2wrapper(const void* addr, size_t len, MemoryMapping::LockFlags flags);
+
 } // namespace folly