mm: add new mmget() helper

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

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

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/20161218123229.22952-2-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>

10 files changed, 35 insertions, 14 deletions

diff --git a/arch/arc/kernel/smp.c b/arch/arc/kernel/smp.c
index 695624181682..b8e8d3944481 100644
--- a/arch/arc/kernel/smp.c
+++ b/arch/arc/kernel/smp.c
@@ -139,7 +139,7 @@ void start_kernel_secondary(void)
         /* MMU, Caches, Vector Table, Interrupts etc */
         setup_processor();
 
-        atomic_inc(&mm->mm_users);
+        mmget(mm);
         mmgrab(mm);
         current->active_mm = mm;
         cpumask_set_cpu(cpu, mm_cpumask(mm));
diff --git a/arch/blackfin/mach-common/smp.c b/arch/blackfin/mach-common/smp.c
index bc5617ef7128..a2e6db2ce811 100644
--- a/arch/blackfin/mach-common/smp.c
+++ b/arch/blackfin/mach-common/smp.c
@@ -307,7 +307,7 @@ void secondary_start_kernel(void)
         local_irq_disable();
 
         /* Attach the new idle task to the global mm. */
-        atomic_inc(&mm->mm_users);
+        mmget(mm);
         mmgrab(mm);
         current->active_mm = mm;
 
diff --git a/arch/frv/mm/mmu-context.c b/arch/frv/mm/mmu-context.c
index 81757d55a5b5..3473bde77f56 100644
--- a/arch/frv/mm/mmu-context.c
+++ b/arch/frv/mm/mmu-context.c
@@ -188,7 +188,7 @@ int cxn_pin_by_pid(pid_t pid)
                 task_lock(tsk);
                 if (tsk->mm) {
                         mm = tsk->mm;
-                        atomic_inc(&mm->mm_users);
+                        mmget(mm);
                         ret = 0;
                 }
                 task_unlock(tsk);
diff --git a/arch/metag/kernel/smp.c b/arch/metag/kernel/smp.c
index af9cff547a19..c622293254e4 100644
--- a/arch/metag/kernel/smp.c
+++ b/arch/metag/kernel/smp.c
@@ -344,7 +344,7 @@ asmlinkage void secondary_start_kernel(void)
          * All kernel threads share the same mm context; grab a
          * reference and switch to it.
          */
-        atomic_inc(&mm->mm_users);
+        mmget(mm);
         mmgrab(mm);
         current->active_mm = mm;
         cpumask_set_cpu(cpu, mm_cpumask(mm));
diff --git a/arch/sh/kernel/smp.c b/arch/sh/kernel/smp.c
index ee379c699c08..edc4769b047e 100644
--- a/arch/sh/kernel/smp.c
+++ b/arch/sh/kernel/smp.c
@@ -179,7 +179,7 @@ asmlinkage void start_secondary(void)
 
         enable_mmu();
         mmgrab(mm);
-        atomic_inc(&mm->mm_users);
+        mmget(mm);
         current->active_mm = mm;
 #ifdef CONFIG_MMU
         enter_lazy_tlb(mm, current);
diff --git a/arch/xtensa/kernel/smp.c b/arch/xtensa/kernel/smp.c
index 9bf5cea3bae4..fcea72019df7 100644
--- a/arch/xtensa/kernel/smp.c
+++ b/arch/xtensa/kernel/smp.c
@@ -135,7 +135,7 @@ void secondary_start_kernel(void)
 
         /* All kernel threads share the same mm context. */
 
-        atomic_inc(&mm->mm_users);
+        mmget(mm);
         mmgrab(mm);
         current->active_mm = mm;
         cpumask_set_cpu(cpu, mm_cpumask(mm));
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 7cfa5546c840..4a28deb5f210 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2948,6 +2948,27 @@ static inline void mmdrop_async(struct mm_struct *mm)
         }
 }
 
+/**
+ * mmget() - Pin the address space associated with a &struct mm_struct.
+ * @mm: The address space to pin.
+ *
+ * Make sure that the address space of the given &struct mm_struct doesn't
+ * go away. This does not protect against parts of the address space being
+ * modified or freed, however.
+ *
+ * Never use this function to pin this address space for an
+ * unbounded/indefinite amount of time.
+ *
+ * Use mmput() to release the reference acquired by mmget().
+ *
+ * See also <Documentation/vm/active_mm.txt> for an in-depth explanation
+ * of &mm_struct.mm_count vs &mm_struct.mm_users.
+ */
+static inline void mmget(struct mm_struct *mm)
+{
+        atomic_inc(&mm->mm_users);
+}
+
 static inline bool mmget_not_zero(struct mm_struct *mm)
 {
         return atomic_inc_not_zero(&mm->mm_users);
diff --git a/kernel/fork.c b/kernel/fork.c
index 348fe73155bc..246bf9aaf9df 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1000,7 +1000,7 @@ struct mm_struct *get_task_mm(struct task_struct *task)
                 if (task->flags & PF_KTHREAD)
                         mm = NULL;
                 else
-                        atomic_inc(&mm->mm_users);
+                        mmget(mm);
         }
         task_unlock(task);
         return mm;
@@ -1188,7 +1188,7 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
         vmacache_flush(tsk);
 
         if (clone_flags & CLONE_VM) {
-                atomic_inc(&oldmm->mm_users);
+                mmget(oldmm);
                 mm = oldmm;
                 goto good_mm;
         }
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 2cac12cc9abe..7a0713b76668 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1671,7 +1671,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
          * that.
          */
         start_mm = &init_mm;
-        atomic_inc(&init_mm.mm_users);
+        mmget(&init_mm);
 
         /*
          * Keep on scanning until all entries have gone.  Usually,
@@ -1720,7 +1720,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
                 if (atomic_read(&start_mm->mm_users) == 1) {
                         mmput(start_mm);
                         start_mm = &init_mm;
-                        atomic_inc(&init_mm.mm_users);
+                        mmget(&init_mm);
                 }
 
                 /*
@@ -1757,8 +1757,8 @@ int try_to_unuse(unsigned int type, bool frontswap,
                         struct mm_struct *prev_mm = start_mm;
                         struct mm_struct *mm;
 
-                        atomic_inc(&new_start_mm->mm_users);
-                        atomic_inc(&prev_mm->mm_users);
+                        mmget(new_start_mm);
+                        mmget(prev_mm);
                         spin_lock(&mmlist_lock);
                         while (swap_count(*swap_map) && !retval &&
                                         (p = p->next) != &start_mm->mmlist) {
@@ -1781,7 +1781,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
 
                                 if (set_start_mm && *swap_map < swcount) {
                                         mmput(new_start_mm);
-                                        atomic_inc(&mm->mm_users);
+                                        mmget(mm);
                                         new_start_mm = mm;
                                         set_start_mm = 0;
                                 }
diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
index 3815e940fbea..2366177172f6 100644
--- a/virt/kvm/async_pf.c
+++ b/virt/kvm/async_pf.c
@@ -204,7 +204,7 @@ int kvm_setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, unsigned long hva,
         work->addr = hva;
         work->arch = *arch;
         work->mm = current->mm;
-        atomic_inc(&work->mm->mm_users);
+        mmget(work->mm);
         kvm_get_kvm(work->vcpu->kvm);
 
         /* this can't really happen otherwise gfn_to_pfn_async