Clean up and clarify PartitionAlloc

Source/wtf/PartitionAlloc.h

Index: Source/wtf/PartitionAlloc.h
diff --git a/Source/wtf/PartitionAlloc.h b/Source/wtf/PartitionAlloc.h
index e2e68733ed2ff1a35b4217e13f446a5d3c62b97a..1db9ab68245ce91cc9bd38dcd2886a8fc35b3603 100644
--- a/Source/wtf/PartitionAlloc.h
+++ b/Source/wtf/PartitionAlloc.h
@@ -141,10 +141,10 @@ static const size_t kMaxPartitionPagesPerSlotSpan = 4;
 
 // To avoid fragmentation via never-used freelist entries, we hand out partition
 // freelist sections gradually, in units of the dominant system page size.
-// What we're actually doing is avoiding filling the full partition page
-// (typically 16KB) will freelist pointers right away. Writing freelist
-// pointers will fault and dirty a private page, which is very wasteful if we
-// never actually store objects there.
+// What we're actually doing is avoiding filling the full partition page (16 KB)

[Jens Widell, 2015/09/03 13:11:45]

I guess this comment meant to discuss the "bucket pages" (my own term, not used
in the code) that are multiples of partition pages, and indeed "typically 16 KB"
(i.e. a single partition page) at least for smaller bucket sizes. "Bucket pages"
are logically what PartitionBucket::activePagesHead point to.

In other words, |X x partition page| makes up the storage for a "bucket page" of
which |Y x bucket slot size| will be used, thus leaving |Z x system page size|
memory permanently unused. (Z might be zero.)

And the restricted freelist filling discussed here avoids filling up the whole
bucket page, by touching as few system pages as possible. It doesn't care
specifically about partition page boundaries.

+// with freelist pointers right away. Writing freelist pointers will fault and
+// dirty a private page, which is very wasteful if we never actually store
+// objects there.
 static const size_t kNumSystemPagesPerPartitionPage = kPartitionPageSize / kSystemPageSize;
 static const size_t kMaxSystemPagesPerSlotSpan = kNumSystemPagesPerPartitionPage * kMaxPartitionPagesPerSlotSpan;