type hit struct {
		npages, base, scav uintptr
	}
	type test struct {
		scav   map[ChunkIdx][]BitRange
		before map[ChunkIdx][]BitRange
		after  map[ChunkIdx][]BitRange
		hits   []hit
	}
	tests := map[string]test{
		"AllFree1": {
			before: map[ChunkIdx][]BitRange{
				BaseChunkIdx: {},
			},
			scav: map[ChunkIdx][]BitRange{
				BaseChunkIdx: {{0, 1}, {2, 2}},
			},
			hits: []hit{

	}
	type test struct {
		beforeAlloc map[ChunkIdx][]BitRange
		beforeScav  map[ChunkIdx][]BitRange
		hits        []PageCache // expected base addresses and patterns
		afterAlloc  map[ChunkIdx][]BitRange
		afterScav   map[ChunkIdx][]BitRange
	}
	tests := map[string]test{
		"AllFree": {
			beforeAlloc: map[ChunkIdx][]BitRange{
				BaseChunkIdx: {},
			},
			beforeScav: map[ChunkIdx][]BitRange{

	}
	type setup struct {
		beforeAlloc map[ChunkIdx][]BitRange
		beforeScav  map[ChunkIdx][]BitRange
		expect      []test
		afterScav   map[ChunkIdx][]BitRange
	}
	tests := map[string]setup{
		"AllFreeUnscavExhaust": {
			beforeAlloc: map[ChunkIdx][]BitRange{
				BaseChunkIdx:     {},
				BaseChunkIdx + 1: {},
				BaseChunkIdx + 2: {},
			},
			beforeScav: map[ChunkIdx][]BitRange{
				BaseChunkIdx:     {},

		// take the lock internally.
		lock(pp.mheapLock)
		pp.free(base, npages)
		unlock(pp.mheapLock)
	})
}
func (p *PageAlloc) Bounds() (ChunkIdx, ChunkIdx) {
	return ChunkIdx((*pageAlloc)(p).start), ChunkIdx((*pageAlloc)(p).end)
}
func (p *PageAlloc) Scavenge(nbytes uintptr) (r uintptr) {
	pp := (*pageAlloc)(p)
	systemstack(func() {
		r = pp.scavenge(nbytes, nil, true)
	})
	return

// space into chunks.
type chunkIdx uint

// chunkIndex returns the global index of the palloc chunk containing the
// pointer p.
func chunkIndex(p uintptr) chunkIdx {
	return chunkIdx((p - arenaBaseOffset) / pallocChunkBytes)
}

// chunkBase returns the base address of the palloc chunk at index ci.
func chunkBase(ci chunkIdx) uintptr {
	return uintptr(ci)*pallocChunkBytes + arenaBaseOffset

//
// Returns the number of bytes scavenged.
//
// Must run on the systemstack because it acquires p.mheapLock.
//
//go:systemstack
func (p *pageAlloc) scavengeOne(ci chunkIdx, searchIdx uint, max uintptr) uintptr {
	// Calculate the maximum number of pages to scavenge.
	//
	// This should be alignUp(max, pageSize) / pageSize but max can and will