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>

2 files changed, 3 insertions, 13 deletions

diff --git a/fs/aio.c b/fs/aio.c
index b5253e77eb2f..0fb3117ddd93 100644
--- a/fs/aio.c
+++ b/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);
 
diff --git a/include/asm-um/mmu_context.h b/include/asm-um/mmu_context.h
index 6686fc524ca1..54f42e8b0105 100644
--- a/include/asm-um/mmu_context.h
+++ b/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);
-
+        __switch_mm(&new->context.id);
         arch_dup_mmap(old, new);
 }