Hello Roy > I fail to understand the difference between task->mm and > task->active_mm. I've noticed that upon forking a task, both mm and > active_mm get the same memory descriptor. Well, here is my understanding. task_struct->mm points to memory descriptor which is unique to each process (unless they are on the same thread group, forked with CLONE_VM). active_mm points to the *actual* memory descriptor used by the process when it is executed. So why it is separated? IMHO the reason is to identify which process is kernel thread (doesn't own a process address space) and which one is normal process (owns a process address space). As you can see on functions related with context switching, by checking task_struct->mm, the scheduler can decide whether it is going to switch onto kernel thread or not. if it is NULL, then the process doesn't have process address space, in other word this is a kernel thread. But you also aware that even kernel threads don't acess user space memory, it still needs to access kernel space. because kernel space is 100% identical for every process, kernel thread can freely use memory descriptor (mm) owned by previously running process. All the kernel thread needed is page tables referencing toward virtual address bigger than PAGE_OFFSET, other are simply ignored by it is assumed that kernel thread doesn't need to access user space (perhaps it is somehow can be abused?) Hope it helps answering your question regards Mulyadi -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
