Enable only 1 `PartitionAllocZeroFill` optimization.

`SlowPathAlloc` has 3 tactics for getting new memory. Of these, only the 1st
(calling `PartitionDirectMap`) seems to most obviously get a fresh, zeroed page
from the OS. I think it is safe to optimize in that branch. It might be safe on
some platforms to optimize in the other 2 branches, but it's less obviously
safe, and we've had trouble on at least macOS. This CL enables the 1st branch
but turns off the optimization in the 2nd 2 branches.

Bug: 890752
Cq-Include-Trybots: luci.chromium.try:mac_optional_gpu_tests_rel
Change-Id: Ibd9ff599ba78aeab47c2369c472af71a4f71d600
Reviewed-on: https://chromium-review.googlesource.com/1256211Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Chris Palmer <palmer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596297}

base/allocator/partition_allocator/partition_bucket.cc

@@ -478,11 +478,7 @@ void* PartitionBucket::SlowPathAlloc(PartitionRootBase* root,
       PartitionExcessiveAllocationSize();
     }
     new_page = PartitionDirectMap(root, flags, size);
-#if !defined(OS_MACOSX)
-    // TODO(https://crbug.com/890752): Remove this when we figure out and fix
-    // whatever is breaking on macOS.
     *is_already_zeroed = true;
-#endif
   } else if (LIKELY(this->SetNewActivePage())) {
     // First, did we find an active page in the active pages list?
     new_page = this->active_pages_head;
@@ -514,11 +510,9 @@ void* PartitionBucket::SlowPathAlloc(PartitionRootBase* root,
       void* addr = PartitionPage::ToPointer(new_page);
       root->RecommitSystemPages(addr, new_page->bucket->get_bytes_per_span());
       new_page->Reset();
-#if !defined(OS_MACOSX)
-      // TODO(https://crbug.com/890752): Remove this when we figure out and fix
-      // whatever is breaking on macOS.
-      *is_already_zeroed = true;
-#endif
+      // TODO(https://crbug.com/890752): Optimizing here might cause pages to
+      // not be zeroed.
+      // *is_already_zeroed = true;
     }
     DCHECK(new_page);
   } else {
@@ -528,11 +522,9 @@ void* PartitionBucket::SlowPathAlloc(PartitionRootBase* root,
     if (LIKELY(raw_pages != nullptr)) {
       new_page = PartitionPage::FromPointerNoAlignmentCheck(raw_pages);
       InitializeSlotSpan(new_page);
-#if !defined(OS_MACOSX)
-      // TODO(https://crbug.com/890752): Remove this when we figure out and fix
-      // whatever is breaking on macOS.
-      *is_already_zeroed = true;
-#endif
+      // TODO(https://crbug.com/890752): Optimizing here causes pages to not be
+      // zeroed on at least macOS.
+      // *is_already_zeroed = true;
     }
   }
 

base/allocator/partition_allocator/partition_root_base.h

@@ -142,11 +142,6 @@ ALWAYS_INLINE void* PartitionRootBase::AllocFromBucket(PartitionBucket* bucket,
   }
   PartitionCookieWriteValue(char_ret + kCookieSize + no_cookie_size);
 #else
-#if defined(OS_MACOSX)
-  // TODO(https://crbug.com/890752): Remove this when we figure out and fix
-  // whatever is breaking on macOS.
-  is_already_zeroed = false;
-#endif
   if (ret && zero_fill && !is_already_zeroed) {
     memset(ret, 0, size);
   }