pitrou commented on a change in pull request #12116:
URL: https://github.com/apache/arrow/pull/12116#discussion_r806972002



##########
File path: cpp/src/arrow/memory_pool.cc
##########
@@ -603,14 +643,109 @@ class BaseMemoryPoolImpl : public MemoryPool {
     stats_.UpdateAllocatedBytes(-size);
   }
 
-  void ReleaseUnused() override { Allocator::ReleaseUnused(); }
+ protected:
+  virtual Status AllocateImmutableZeros(int64_t size, uint8_t** out) {
+#ifdef USE_MMAP_FOR_IMMUTABLE_ZEROS
+    if (size > 0) {
+      *out = static_cast<uint8_t*>(mmap(
+          nullptr, size, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | 
MAP_NORESERVE, -1, 0));

Review comment:
       @jvanstraten You're right. So let's keep it like this and just explain 
why it is in a comment.




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