Fix use of make_unique in Arena::AllocateNewBlock (#11012)

Summary:
The change to `make_unique<char[]>` in https://github.com/facebook/rocksdb/issues/10810 inadvertently started initializing data in Arena blocks, which could lead to increased memory use due to (at least on our implementation) force-mapping pages as a result. This change goes back to `new char[]` while keeping all the other good parts of https://github.com/facebook/rocksdb/issues/10810.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11012

Test Plan: unit test added (fails on Linux before fix)

Reviewed By: anand1976

Differential Revision: D41658893

Pulled By: pdillinger

fbshipit-source-id: 267b7dccfadaeeb1be767d43c602a6abb0e71cd0

memory/arena.cc

@@ -143,9 +143,10 @@ char* Arena::AllocateAligned(size_t bytes, size_t huge_page_size,
 }
 
 char* Arena::AllocateNewBlock(size_t block_bytes) {
-  auto uniq = std::make_unique<char[]>(block_bytes);
-  char* block = uniq.get();
-  blocks_.push_back(std::move(uniq));
+  // NOTE: std::make_unique zero-initializes the block so is not appropriate
+  // here
+  char* block = new char[block_bytes];
+  blocks_.push_back(std::unique_ptr<char[]>(block));
 
   size_t allocated_size;
 #ifdef ROCKSDB_MALLOC_USABLE_SIZE

memory/arena_test.cc

@@ -256,6 +256,36 @@ TEST(MmapTest, AllocateLazyZeroed) {
   }
 }
 
+TEST_F(ArenaTest, UnmappedAllocation) {
+  // Verify that it's possible to get unmapped pages in large allocations,
+  // for memory efficiency and to ensure we don't accidentally waste time &
+  // space initializing the memory.
+  constexpr size_t kBlockSize = 2U << 20;
+  Arena arena(kBlockSize);
+
+  // The allocator might give us back recycled memory for a while, but
+  // shouldn't last forever.
+  for (int i = 0;; ++i) {
+    char* p = arena.Allocate(kBlockSize);
+
+    // Start counting page faults
+    PopMinorPageFaultCount();
+
+    // Overwrite the whole allocation
+    for (size_t j = 0; j < kBlockSize; ++j) {
+      p[j] = static_cast<char>(j & 255);
+    }
+
+    size_t faults = PopMinorPageFaultCount();
+    if (faults >= kBlockSize * 3 / 4 / port::kPageSize) {
+      // Most of the access generated page faults => GOOD
+      break;
+    }
+    // Should have succeeded after enough tries
+    ASSERT_LT(i, 1000);
+  }
+}
+
 }  // namespace ROCKSDB_NAMESPACE
 
 int main(int argc, char** argv) {