diff --git a/cmake/introspection.cmake b/cmake/introspection.cmake
index 0efcf7c63c7..9223129761f 100644
--- a/cmake/introspection.cmake
+++ b/cmake/introspection.cmake
@@ -55,13 +55,6 @@ check_cxx_source_compiles("
 
 # Check for posix_fallocate().
 check_cxx_source_compiles("
-  // same as in src/util/fs_helpers.cpp
-  #ifdef __linux__
-  #ifdef _POSIX_C_SOURCE
-  #undef _POSIX_C_SOURCE
-  #endif
-  #define _POSIX_C_SOURCE 200112L
-  #endif // __linux__
   #include <fcntl.h>
 
   int main()
diff --git a/src/util/fs_helpers.cpp b/src/util/fs_helpers.cpp
index ec848155651..be7f1ee5a27 100644
--- a/src/util/fs_helpers.cpp
+++ b/src/util/fs_helpers.cpp
@@ -22,17 +22,6 @@
 #include <utility>
 
 #ifndef WIN32
-// for posix_fallocate, in cmake/introspection.cmake we check if it is present after this
-#ifdef __linux__
-
-#ifdef _POSIX_C_SOURCE
-#undef _POSIX_C_SOURCE
-#endif
-
-#define _POSIX_C_SOURCE 200112L
-
-#endif // __linux__
-
 #include <fcntl.h>
 #include <sys/resource.h>
 #include <unistd.h>