runtime: track all heap arenas in a slice

Currently, there's no efficient way to iterate over the Go heap. We're
going to need this for fast free page sweeping, so this CL adds a
slice of all allocated heap arenas. This will also be useful for
generational GC.

For #18155.

Change-Id: I58d126cfb9c3f61b3125d80b74ccb1b2169efbcc
Reviewed-on: https://go-review.googlesource.com/c/138076
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/src/runtime/malloc.go b/src/runtime/malloc.go
index 12fa744052..e827dbae93 100644
--- a/src/runtime/malloc.go
+++ b/src/runtime/malloc.go
@@ -641,6 +641,27 @@ mapped:
 			}
 		}
 
+		// Add the arena to the arenas list.
+		if len(h.allArenas) == cap(h.allArenas) {
+			size := 2 * uintptr(cap(h.allArenas)) * sys.PtrSize
+			if size == 0 {
+				size = physPageSize
+			}
+			newArray := (*notInHeap)(persistentalloc(size, sys.PtrSize, &memstats.gc_sys))
+			if newArray == nil {
+				throw("out of memory allocating allArenas")
+			}
+			oldSlice := h.allArenas
+			*(*notInHeapSlice)(unsafe.Pointer(&h.allArenas)) = notInHeapSlice{newArray, len(h.allArenas), int(size / sys.PtrSize)}
+			copy(h.allArenas, oldSlice)
+			// Do not free the old backing array because
+			// there may be concurrent readers. Since we
+			// double the array each time, this can lead
+			// to at most 2x waste.
+		}
+		h.allArenas = h.allArenas[:len(h.allArenas)+1]
+		h.allArenas[len(h.allArenas)-1] = ri
+
 		// Store atomically just in case an object from the
 		// new heap arena becomes visible before the heap lock
 		// is released (which shouldn't happen, but there's