[PA-E] Don't double-count fragmentation in memory-infra

This is expected to improve reported memory footprint according to the
various malloc metrics for perf bots, and *.Malloc.* for UMA. This does
not correspond to any real memory footprint change, and as a consequence
Private Memory Footprint (PMF) should not move.

Note that this metric regressed on some platforms when switching to
PartitionAlloc, because of this double-counting. However, since it was
known that this footprint was not directly comparable across different
allocators, it was not used as an evaluation criterion.

Change-Id: If7640e86499f3d497e9ea5904003e3dc0f9433ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3383815
Reviewed-by: Thiabaud Engelbrecht <thiabaud@google.com>
Reviewed-by: Siddhartha S <ssid@chromium.org>
Commit-Queue: Benoit Lize <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#959122}

base/trace_event/malloc_dump_provider.cc

@@ -176,13 +176,18 @@ bool MallocDumpProvider::OnMemoryDump(const MemoryDumpArgs& args,
   size_t allocated_objects_count = 0;
 #if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
   uint64_t syscall_count = 0;
+  uint64_t pa_only_resident_size;
+  uint64_t pa_only_allocated_objects_size;
 #endif
 
 #if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
   ReportPartitionAllocStats(pmd, args.level_of_detail, &total_virtual_size,
                             &resident_size, &allocated_objects_size,
                             &syscall_count);
 
+  pa_only_resident_size = resident_size;
+  pa_only_allocated_objects_size = allocated_objects_size;
+
   // Even when PartitionAlloc is used, WinHeap is still used as well, report
   // its statistics.
 #if OS_WIN
@@ -251,14 +256,28 @@ bool MallocDumpProvider::OnMemoryDump(const MemoryDumpArgs& args,
                           allocated_objects_count);
   }
 
-  if (resident_size > allocated_objects_size) {
+  int64_t waste = static_cast<int64_t>(resident_size) - allocated_objects_size;
+
+  // With PartitionAlloc, reported size under malloc/partitions is the resident
+  // size, so it already includes fragmentation. Meaning that "malloc/"'s size
+  // would double-count fragmentation if we report it under
+  // "malloc/metadata_fragmentation_caches" as well.
+  //
+  // Still report waste, as on some platforms, PartitionAlloc doesn't capture
+  // all of malloc()'s memory footprint.
+#if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)
+  int64_t pa_waste = static_cast<int64_t>(pa_only_resident_size) -
+                     pa_only_allocated_objects_size;
+  waste -= pa_waste;
+#endif
+
+  if (waste > 0) {
     // Explicitly specify why is extra memory resident. In mac and ios it
     // accounts for the fragmentation and metadata.
     MemoryAllocatorDump* other_dump =
         pmd->CreateAllocatorDump("malloc/metadata_fragmentation_caches");
     other_dump->AddScalar(MemoryAllocatorDump::kNameSize,
-                          MemoryAllocatorDump::kUnitsBytes,
-                          resident_size - allocated_objects_size);
+                          MemoryAllocatorDump::kUnitsBytes, waste);
   }
 
 #if BUILDFLAG(USE_PARTITION_ALLOC_AS_MALLOC)