1. A computer program product for facilitating collaborative memory management within a computing environment supporting pageable guests, said computer program product comprising: a non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: enabling, using an enablement control indicator, a collaborative memory management facility for a guest executing on a virtual machine of the computing environment, the guest being a pageable guest and the virtual machine being managed by a host of the computing environment; assigning a block of memory to the guest; associating host state information with the block of memory assigned to the guest, the associating host state information comprising: defining one or more available host memory states, wherein a host memory state describes a state of the block of memory assigned to the guest as it relates to the host; associating guest state information of the guest with the block of memory assigned to the guest, the associating guest state information comprising: defining one or more available guest memory states, wherein a guest memory state describes a state of the block of memory as it relates to the guest to which it is assigned; maintaining current guest and host state information in control blocks in host memory, the current guest and host state information comprising one of the available guest and host memory states indicating the current state of the block of memory assigned to the guest; and providing to the guest, absent host involvement and based on issuance of an extract and set storage attributes service instruction of the collaborative memory management facility, direct access to the current guest and host state information and optionally changing the guest state of the block of memory, wherein the current guest and host state information provided to the guest comprises a particular guest memory state indicating a particular meaning to the guest of contents of the block of memory assigned to the guest and is used in managing memory of the computing environment.