Control whether a file data source memory-maps the file with an envir…

…onment variable (#17004)

Adds an environment variable, `LIBCUDF_MMAP_ENABLED`, to control whether we memory map the input file in the data source.

Authors:
  - Vukasin Milovanovic (https://github.com/vuule)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Tianyu Liu (https://github.com/kingcrimsontianyu)

URL: #17004

cpp/src/io/utilities/datasource.cpp

@@ -15,6 +15,7 @@
  */
 
 #include "file_io_utilities.hpp"
+#include "getenv_or.hpp"
 
 #include <cudf/detail/utilities/logger.hpp>
 #include <cudf/detail/utilities/vector_factories.hpp>
@@ -392,14 +393,21 @@ std::unique_ptr<datasource> datasource::create(std::string const& filepath,
                                                size_t offset,
                                                size_t max_size_estimate)
 {
-#ifdef CUFILE_FOUND
-  if (cufile_integration::is_always_enabled()) {
-    // avoid mmap as GDS is expected to be used for most reads
+  auto const use_memory_mapping = [] {
+    auto const policy = getenv_or("LIBCUDF_MMAP_ENABLED", std::string{"ON"});
+
+    if (policy == "ON") { return true; }
+    if (policy == "OFF") { return false; }
+
+    CUDF_FAIL("Invalid LIBCUDF_MMAP_ENABLED value: " + policy);
+  }();
+
+  if (use_memory_mapping) {
+    return std::make_unique<memory_mapped_source>(filepath.c_str(), offset, max_size_estimate);
+  } else {
+    // `file_source` reads the file directly, without memory mapping
     return std::make_unique<file_source>(filepath.c_str());
   }
-#endif
-  // Use our own memory mapping implementation for direct file reads
-  return std::make_unique<memory_mapped_source>(filepath.c_str(), offset, max_size_estimate);
 }
 
 std::unique_ptr<datasource> datasource::create(host_buffer const& buffer)