A method for data transfer includes receiving in an operating system of a host computer an instruction initiated by a user application running on the host processor identifying a page of virtual memory of the host computer that is to be used in receiving data in a message that is to be transmitted over a network to the host computer but has not yet been received by the host computer. In response to the instruction, the page is loaded into the memory, and upon receiving the message, the data are written to the loaded page.