Function vmem_xalloc_nextfit

freebsd/kern/subr_vmem.c:1112–1196

}

static int
vmem_xalloc_nextfit(vmem_t *vm, const vmem_size_t size, vmem_size_t align,
    const vmem_size_t phase, const vmem_size_t nocross, int flags,
    vmem_addr_t *addrp)
{
	struct vmem_btag *bt, *cursor, *next, *prev;
	int error;

	error = ENOMEM;
	VMEM_LOCK(vm);

	/*
	 * Make sure we have enough tags to complete the operation.
	 */
	if (bt_fill(vm, flags) != 0)
		goto out;

retry:
	/*
	 * Find the next free tag meeting our constraints.  If one is found,
	 * perform the allocation.
	 */
	for (cursor = &vm->vm_cursor, bt = TAILQ_NEXT(cursor, bt_seglist);
	    bt != cursor; bt = TAILQ_NEXT(bt, bt_seglist)) {
		if (bt == NULL)
			bt = TAILQ_FIRST(&vm->vm_seglist);
		if (bt->bt_type == BT_TYPE_FREE && bt->bt_size >= size &&
		    (error = vmem_fit(bt, size, align, phase, nocross,
		    VMEM_ADDR_MIN, VMEM_ADDR_MAX, addrp)) == 0) {
			vmem_clip(vm, bt, *addrp, size);
			break;
		}
	}

	/*
	 * Try to coalesce free segments around the cursor.  If we succeed, and
	 * have not yet satisfied the allocation request, try again with the
	 * newly coalesced segment.
	 */
	if ((next = TAILQ_NEXT(cursor, bt_seglist)) != NULL &&
	    (prev = TAILQ_PREV(cursor, vmem_seglist, bt_seglist)) != NULL &&
	    next->bt_type == BT_TYPE_FREE && prev->bt_type == BT_TYPE_FREE &&
	    prev->bt_start + prev->bt_size == next->bt_start) {
		prev->bt_size += next->bt_size;
		bt_remfree(vm, next);
		bt_remseg(vm, next);

		/*
		 * The coalesced segment might be able to satisfy our request.
		 * If not, we might need to release it from the arena.
		 */
		if (error == ENOMEM && prev->bt_size >= size &&
		    (error = vmem_fit(prev, size, align, phase, nocross,
		    VMEM_ADDR_MIN, VMEM_ADDR_MAX, addrp)) == 0) {
			vmem_clip(vm, prev, *addrp, size);
			bt = prev;
		} else
			(void)vmem_try_release(vm, prev, true);