What are the rules for managing MDLs and User Buffers? How do I substitute my own buffer in an IRP?
In all fairness, there are no “rules” for managing MDLs and user buffers. There are suggestions that we can offer based upon observed behavior of the file systems. First, we note that there are two basic sources of I/O operations for a file system – the applications layer, and other operating system components. For applications programs, most IRPs are still buffered. The operations for which this is not necessarily the case are those for which larger amounts of data are transferred. These are IRP_MJ_READ, IRP_MJ_WRITE, IRP_MJ_DIRECTORY_CONTROL, IRP_MJ_QUERY_EA, IRP_MJ_SET_EA, IRP_MJ_QUERY_QUOTA, IRP_MJ_SET_QUOTA, and the per-control code options of IRP_MJ_DEVICE_CONTROL, IRP_MJ_INTERNAL_DEVICE_CONTROL, and IRP_MJ_FILE_SYSTEM_CONTROL. If the Flags field in the device object specifies DO_DIRECT_IO then the buffer for IRP_MJ_READ, IRP_MJ_WRITE, IRP_MJ_DIRECTORY_CONTROL, IRP_MJ_QUERY_EA, IRP_MJ_SET_EA, IRP_MJ_QUERY_QUOTA, and IRP_MJ_SET_QUOTA, is specified as a memory descriptor list (MDL)
