Question on AllocationPool::newRunImpl implementation

[mikechen3]

velox/velox/common/memory/AllocationPool.cpp

Lines 62 to 80 in babb1c3

	void AllocationPool::newRunImpl(memory::MachinePageCount numPages) {
	++currentRun_;
	if (currentRun_ >= allocation_.numRuns()) {
	if (allocation_.numRuns()) {
	allocations_.push_back(std::make_unique<memory::MappedMemory::Allocation>(
	std::move(allocation_)));
	}
	if (!mappedMemory_->allocate(
	std::max<int32_t>(kMinPages, numPages),
	owner_,
	allocation_,
	nullptr,
	numPages)) {
	throw std::bad_alloc();
	}
	currentRun_ = 0;
	}
	currentOffset_ = 0;
	}

Problem statement:
It seems in AllocationPool::newRunImpl, we try to point the currentRun_ variable to a run whose size is greater or equal than numPages parameter. However it seems when we don't enter the if (currentRun_ >= allocation_.numRuns()) { condition, we simply move the currentRun to point to the next run in allocation_.

Question:
How does next run in allocation_ guarantee to have size GE than numPages?

[mbasmanova]

@xiaoxmeng Meng, can you help answer this question?