Throw MMapException if mmap fail.

src/file_reader.cpp

@@ -146,11 +146,13 @@ mmapReadOnly(int fd, offset_type offset, size_type size)
 #endif
 
   const auto p = (char*)mmap(NULL, size, PROT_READ, MAP_FLAGS, fd, offset);
-  if (p == MAP_FAILED)
-    throw std::runtime_error(Formatter()
-                             << "Cannot mmap size " << size << " at off "
-                             << offset << " : " << strerror(errno));
-
+  if (p == MAP_FAILED) {
+    // mmap may fails for a lot of reason.
+    // Most of them (io error, too big size...) may not recoverable but some of
+    // them may be relative to mmap only and a "simple" read from the file would work.
+    // Let's throw a MMapException to fallback to read (and potentially fail again there).
+    throw MMapException();
+  }
   return p;
 }