mac arm64: use the correct page size for this platform in PartitionAlloc

Apple CPUs since A9 use a 16kB physical page size, which is a change for
macOS on arm64 relative to the 4kB pages used on x86_64. PartitionAlloc
operates on pages directly, and must be aware of the larger pages on
the new platform.

If this same change were effective for macOS on x86_64, it would allow
x86_64 Chrome to run under binary translation (Rosetta) on arm64. Doing
so has potential observable impacts, including memory footprint and
performance. Run-time page size detection could be used to overcome
this, and would also provide insulation against future page size
changes, but that’s a larger and more invasive change. It’s likely that
we will adopt one of these strategies soon, but for now, this
arm64-only change is sufficient to run Chrome on arm64 Macs.

Bug: 1098899
Change-Id: I50b48728350b87a319433188306daad747741e54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2292831
Commit-Queue: Mark Mentovai <mark@chromium.org>
Auto-Submit: Mark Mentovai <mark@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787452}

base/allocator/partition_allocator/page_allocator_constants.h

@@ -14,6 +14,8 @@ namespace base {
 static constexpr size_t kPageAllocationGranularityShift = 16;  // 64KB
 #elif defined(_MIPS_ARCH_LOONGSON)
 static constexpr size_t kPageAllocationGranularityShift = 14;  // 16KB
+#elif defined(OS_MACOSX) && defined(ARCH_CPU_ARM64)
+static constexpr size_t kPageAllocationGranularityShift = 14;  // 16KB
 #else
 static constexpr size_t kPageAllocationGranularityShift = 12;  // 4KB
 #endif
@@ -32,6 +34,8 @@ static constexpr size_t kSystemPageSize = 16384;
 // and binaries compiled for 64KB are likely to work on 4KB systems,
 // 64KB is a good choice here.
 static constexpr size_t kSystemPageSize = 65536;
+#elif defined(OS_MACOSX) && defined(ARCH_CPU_ARM64)
+static constexpr size_t kSystemPageSize = 16384;
 #else
 static constexpr size_t kSystemPageSize = 4096;
 #endif

base/allocator/partition_allocator/partition_alloc_constants.h

@@ -34,6 +34,8 @@ namespace base {
 static const size_t kPartitionPageShift = 16;  // 64 KiB
 #elif defined(ARCH_CPU_PPC64)
 static const size_t kPartitionPageShift = 18;  // 256 KiB
+#elif defined (OS_MACOSX) && defined(ARCH_CPU_ARM64)
+static const size_t kPartitionPageShift = 16;  // 64 KiB
 #else
 static const size_t kPartitionPageShift = 14;  // 16 KiB
 #endif