Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

1.7.4

What operating system and processor architecture are you using (go env)?

linux amd64

The definition of mheap.busyList locate in mheap.go:943 as followed:

func (h *mheap) busyList(npages uintptr) *mSpanList {
if npages < uintptr(len(h.free)) {
return &h.busy[npages]
}
return &h.busylarge
}

When check if it can return the busy list using "len(free)", I think it's not proper.
Although the current version of Go set the same lenght of h.free and h.busy, it's different.

The same puzzle occurs at the definition of mheap.alloc_m @ mheap.go:592:
if large {
memstats.heap_objects++
atomic.Xadd64(&memstats.heap_live, int64(npage<<_PageShift))
// Swept spans are at the end of lists.
if s.npages < uintptr(len(h.free)) {
h.busy[s.npages].insertBack(s)
} else {
h.busylarge.insertBack(s)
}
}

So it's a mistake, or it's some design.