2 files changed, 13 insertions, 1 deletions

diff --git a/fs/aio.c b/fs/aio.c
index 06d7d4390fe7..4f641abac3c0 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -567,6 +567,10 @@ 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
+         */
         activate_mm(active_mm, mm);
         task_unlock(tsk);
 
diff --git a/include/asm-um/mmu_context.h b/include/asm-um/mmu_context.h
index 095bb627b96a..2edb4f1f789c 100644
--- a/include/asm-um/mmu_context.h
+++ b/include/asm-um/mmu_context.h
@@ -20,7 +20,15 @@ extern void force_flush_all(void);
 
 static inline void activate_mm(struct mm_struct *old, struct mm_struct *new)
 {
-        if (old != 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.
+         */
+        if (old != new && (current->flags & PF_BORROWED_MM))
                 force_flush_all();
 }