uml: activate_mm: remove the dead PF_BORROWED_MM check

use_mm() was changed to use switch_mm() instead of activate_mm(), since
then nobody calls (and nobody should call) activate_mm() with
PF_BORROWED_MM bit set.

As Jeff Dike pointed out, we can also remove the "old != new" check, it is
always true.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/aio.c

@@ -591,10 +591,6 @@ static void use_mm(struct mm_struct *mm)
 	atomic_inc(&mm->mm_count);
 	tsk->mm = mm;
 	tsk->active_mm = mm;
-	/*
-	 * Note that on UML this *requires* PF_BORROWED_MM to be set, otherwise
-	 * it won't work. Update it accordingly if you change it here
-	 */
 	switch_mm(active_mm, mm, tsk);
 	task_unlock(tsk);
 

include/asm-um/mmu_context.h

@@ -22,16 +22,10 @@ extern void force_flush_all(void);
 static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
 {
 	/*
-	 * This is called by fs/exec.c and fs/aio.c. In the first case, for an
-	 * exec, we don't need to do anything as we're called from userspace
-	 * and thus going to use a new host PID. In the second, we're called
-	 * from a kernel thread, and thus need to go doing the mmap's on the
-	 * host. Since they're very expensive, we want to avoid that as far as
-	 * possible.
+	 * This is called by fs/exec.c and sys_unshare()
+	 * when the new ->mm is used for the first time.
 	 */
-	if (old != new && (current->flags & PF_BORROWED_MM))
 	__switch_mm(&new->context.id);
-
 	arch_dup_mmap(old, new);
 }