Access to User Buffers
When a user-mode thread makes an I/O request, it usually passes the address of a data buffer located in user space. Since user-mode addresses are referenced through the lower half (< 2 GB) of the page tables, a driver must cope with the possibility that the page tables will change before the request can be completed. This would occur if the driver code executed at Interrupt Context or Kernel-Mode Thread context. As previously discussed, the lower half of the page tables are changed with each process switch. Thus, code executing with an arbitrary page table state cannot assume that any user-mode address is valid.
Worse, the user buffer may be paged out of RAM and exist only on the system's swap file on disk. User memory ...
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