mm: add new mmgrab() helper

Apart from adding the helper function itself, the rest of the kernel is
converted mechanically using:

  git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)->mm_count);/mmgrab\(\1\);/'
  git grep -l 'atomic_inc.*mm_count' | xargs sed -i 's/atomic_inc(&\(.*\)\.mm_count);/mmgrab\(\&\1\);/'

This is needed for a later patch that hooks into the helper, but might
be a worthwhile cleanup on its own.

(Michal Hocko provided most of the kerneldoc comment.)

Link: http://lkml.kernel.org/r/20161218123229.22952-1-vegard.nossum@oracle.com
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e1ae6ac15eac..6ea1925ac5c0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2847,7 +2847,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
 
         if (!mm) {
                 next->active_mm = oldmm;
-                atomic_inc(&oldmm->mm_count);
+                mmgrab(oldmm);
                 enter_lazy_tlb(oldmm, next);
         } else
                 switch_mm_irqs_off(oldmm, mm, next);
@@ -6098,7 +6098,7 @@ void __init sched_init(void)
         /*
          * The boot idle thread does lazy MMU switching as well:
          */
-        atomic_inc(&init_mm.mm_count);
+        mmgrab(&init_mm);
         enter_lazy_tlb(&init_mm, current);
 
         /*