static inline int expand_downwards(struct vm_area_struct *vma,
				   unsigned long address)
{
	int error;

	/*
	 * We must make sure the anon_vma is allocated
	 * so that the anon_vma locking is not a noop.
	 */
	if (unlikely(anon_vma_prepare(vma)))
		return -ENOMEM;
	anon_vma_lock(vma);

	/*
	 * vma->vm_start/vm_end cannot change under us because the caller
	 * is required to hold the mmap_sem in read mode.  We need the
	 * anon_vma lock to serialize against concurrent expand_stacks.
	 */
	address &= PAGE_MASK;
	error = 0;

	/* Somebody else might have raced and expanded it already */
	if (address < vma->vm_start) {
		unsigned long size, grow;

		size = vma->vm_end - address;
		grow = (vma->vm_start - address) >> PAGE_SHIFT;

		error = acct_stack_growth(vma, size, grow);
		if (!error) {
			vma->vm_start = address;
			vma->vm_pgoff -= grow;
		}
	}
	anon_vma_unlock(vma);
	return error;
}
Unknown