[PATCH 1/4] mm: add new mmgrab() helper

[PATCH 1/4] mm: add new mmgrab() helper

[Vegard Nossum]

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.)

Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
 arch/alpha/kernel/smp.c                  |  2 +-
 arch/arc/kernel/smp.c                    |  2 +-
 arch/arm/kernel/smp.c                    |  2 +-
 arch/arm64/kernel/smp.c                  |  2 +-
 arch/blackfin/mach-common/smp.c          |  2 +-
 arch/hexagon/kernel/smp.c                |  2 +-
 arch/ia64/kernel/setup.c                 |  2 +-
 arch/m32r/kernel/setup.c                 |  2 +-
 arch/metag/kernel/smp.c                  |  2 +-
 arch/mips/kernel/traps.c                 |  2 +-
 arch/mn10300/kernel/smp.c                |  2 +-
 arch/parisc/kernel/smp.c                 |  2 +-
 arch/powerpc/kernel/smp.c                |  2 +-
 arch/s390/kernel/processor.c             |  2 +-
 arch/score/kernel/traps.c                |  2 +-
 arch/sh/kernel/smp.c                     |  2 +-
 arch/sparc/kernel/leon_smp.c             |  2 +-
 arch/sparc/kernel/smp_64.c               |  2 +-
 arch/sparc/kernel/sun4d_smp.c            |  2 +-
 arch/sparc/kernel/sun4m_smp.c            |  2 +-
 arch/sparc/kernel/traps_32.c             |  2 +-
 arch/sparc/kernel/traps_64.c             |  2 +-
 arch/tile/kernel/smpboot.c               |  2 +-
 arch/x86/kernel/cpu/common.c             |  4 ++--
 arch/xtensa/kernel/smp.c                 |  2 +-
 drivers/gpu/drm/amd/amdkfd/kfd_process.c |  2 +-
 drivers/gpu/drm/i915/i915_gem_userptr.c  |  2 +-
 drivers/infiniband/hw/hfi1/file_ops.c    |  2 +-
 fs/proc/base.c                           |  4 ++--
 fs/userfaultfd.c                         |  2 +-
 include/linux/sched.h                    | 22 ++++++++++++++++++++++
 kernel/exit.c                            |  2 +-
 kernel/futex.c                           |  2 +-
 kernel/sched/core.c                      |  4 ++--
 mm/khugepaged.c                          |  2 +-
 mm/ksm.c                                 |  2 +-
 mm/mmu_context.c                         |  2 +-
 mm/mmu_notifier.c                        |  2 +-
 mm/oom_kill.c                            |  4 ++--
 virt/kvm/kvm_main.c                      |  2 +-
 40 files changed, 65 insertions(+), 43 deletions(-)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index 46bf263c3153..acb4b146a607 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -144,7 +144,7 @@ smp_callin(void)
 		alpha_mv.smp_callin();
 
 	/* All kernel threads share the same mm context.  */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	/* inform the notifiers about the new cpu */
diff --git a/arch/arc/kernel/smp.c b/arch/arc/kernel/smp.c
index 88674d972c9d..9cbc7aba3ede 100644
--- a/arch/arc/kernel/smp.c
+++ b/arch/arc/kernel/smp.c
@@ -125,7 +125,7 @@ void start_kernel_secondary(void)
 	setup_processor();
 
 	atomic_inc(&mm->mm_users);
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	current->active_mm = mm;
 	cpumask_set_cpu(cpu, mm_cpumask(mm));
 
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 7dd14e8395e6..c6514ce0fcbc 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -371,7 +371,7 @@ asmlinkage void secondary_start_kernel(void)
 	 * reference and switch to it.
 	 */
 	cpu = smp_processor_id();
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	current->active_mm = mm;
 	cpumask_set_cpu(cpu, mm_cpumask(mm));
 
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index cb87234cfcf2..959e41196cba 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -222,7 +222,7 @@ 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_count);
+	mmgrab(mm);
 	current->active_mm = mm;
 
 	/*
diff --git a/arch/blackfin/mach-common/smp.c b/arch/blackfin/mach-common/smp.c
index 23c4ef5f8bdc..bc5617ef7128 100644
--- a/arch/blackfin/mach-common/smp.c
+++ b/arch/blackfin/mach-common/smp.c
@@ -308,7 +308,7 @@ void secondary_start_kernel(void)
 
 	/* Attach the new idle task to the global mm. */
 	atomic_inc(&mm->mm_users);
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	current->active_mm = mm;
 
 	preempt_disable();
diff --git a/arch/hexagon/kernel/smp.c b/arch/hexagon/kernel/smp.c
index 983bae7d2665..c02a6455839e 100644
--- a/arch/hexagon/kernel/smp.c
+++ b/arch/hexagon/kernel/smp.c
@@ -162,7 +162,7 @@ void start_secondary(void)
 	);
 
 	/*  Set the memory struct  */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	cpu = smp_processor_id();
diff --git a/arch/ia64/kernel/setup.c b/arch/ia64/kernel/setup.c
index 7ec7acc844c2..ecbff47b01f1 100644
--- a/arch/ia64/kernel/setup.c
+++ b/arch/ia64/kernel/setup.c
@@ -992,7 +992,7 @@ cpu_init (void)
 	 */
 	ia64_setreg(_IA64_REG_CR_DCR,  (  IA64_DCR_DP | IA64_DCR_DK | IA64_DCR_DX | IA64_DCR_DR
 					| IA64_DCR_DA | IA64_DCR_DD | IA64_DCR_LC));
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	BUG_ON(current->mm);
 
diff --git a/arch/m32r/kernel/setup.c b/arch/m32r/kernel/setup.c
index 136c69f1fb8a..b18bc0bd6544 100644
--- a/arch/m32r/kernel/setup.c
+++ b/arch/m32r/kernel/setup.c
@@ -403,7 +403,7 @@ void __init cpu_init (void)
 	printk(KERN_INFO "Initializing CPU#%d\n", cpu_id);
 
 	/* Set up and load the per-CPU TSS and LDT */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	if (current->mm)
 		BUG();
diff --git a/arch/metag/kernel/smp.c b/arch/metag/kernel/smp.c
index bad13232de51..af9cff547a19 100644
--- a/arch/metag/kernel/smp.c
+++ b/arch/metag/kernel/smp.c
@@ -345,7 +345,7 @@ asmlinkage void secondary_start_kernel(void)
 	 * reference and switch to it.
 	 */
 	atomic_inc(&mm->mm_users);
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	current->active_mm = mm;
 	cpumask_set_cpu(cpu, mm_cpumask(mm));
 	enter_lazy_tlb(mm, current);
diff --git a/arch/mips/kernel/traps.c b/arch/mips/kernel/traps.c
index 3905003dfe2b..e50b0e0ca44c 100644
--- a/arch/mips/kernel/traps.c
+++ b/arch/mips/kernel/traps.c
@@ -2177,7 +2177,7 @@ void per_cpu_trap_init(bool is_boot_cpu)
 	if (!cpu_data[cpu].asid_cache)
 		cpu_data[cpu].asid_cache = asid_first_version(cpu);
 
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	BUG_ON(current->mm);
 	enter_lazy_tlb(&init_mm, current);
diff --git a/arch/mn10300/kernel/smp.c b/arch/mn10300/kernel/smp.c
index 426173c4b0b9..e65b5cc2fa67 100644
--- a/arch/mn10300/kernel/smp.c
+++ b/arch/mn10300/kernel/smp.c
@@ -589,7 +589,7 @@ static void __init smp_cpu_init(void)
 	}
 	printk(KERN_INFO "Initializing CPU#%d\n", cpu_id);
 
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	BUG_ON(current->mm);
 
diff --git a/arch/parisc/kernel/smp.c b/arch/parisc/kernel/smp.c
index 75dab2871346..67b452b41ff6 100644
--- a/arch/parisc/kernel/smp.c
+++ b/arch/parisc/kernel/smp.c
@@ -279,7 +279,7 @@ smp_cpu_init(int cpunum)
 	set_cpu_online(cpunum, true);
 
 	/* Initialise the idle task for this CPU */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	BUG_ON(current->mm);
 	enter_lazy_tlb(&init_mm, current);
diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 9c6f3fd58059..42b82364c782 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -707,7 +707,7 @@ void start_secondary(void *unused)
 	unsigned int cpu = smp_processor_id();
 	int i, base;
 
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	smp_store_cpu_info(cpu);
diff --git a/arch/s390/kernel/processor.c b/arch/s390/kernel/processor.c
index 9e60ef144d03..b8fc689c9e2a 100644
--- a/arch/s390/kernel/processor.c
+++ b/arch/s390/kernel/processor.c
@@ -73,7 +73,7 @@ void cpu_init(void)
 	get_cpu_id(id);
 	if (machine_has_cpu_mhz)
 		update_cpu_mhz(NULL);
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	BUG_ON(current->mm);
 	enter_lazy_tlb(&init_mm, current);
diff --git a/arch/score/kernel/traps.c b/arch/score/kernel/traps.c
index 5cea1e750cec..6f6e5a39d147 100644
--- a/arch/score/kernel/traps.c
+++ b/arch/score/kernel/traps.c
@@ -336,7 +336,7 @@ void __init trap_init(void)
 	set_except_vector(18, handle_dbe);
 	flush_icache_range(DEBUG_VECTOR_BASE_ADDR, IRQ_VECTOR_BASE_ADDR);
 
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	cpu_cache_init();
 }
diff --git a/arch/sh/kernel/smp.c b/arch/sh/kernel/smp.c
index 38e7860845db..ee379c699c08 100644
--- a/arch/sh/kernel/smp.c
+++ b/arch/sh/kernel/smp.c
@@ -178,7 +178,7 @@ asmlinkage void start_secondary(void)
 	struct mm_struct *mm = &init_mm;
 
 	enable_mmu();
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	atomic_inc(&mm->mm_users);
 	current->active_mm = mm;
 #ifdef CONFIG_MMU
diff --git a/arch/sparc/kernel/leon_smp.c b/arch/sparc/kernel/leon_smp.c
index 71e16f2241c2..b99d33797e1d 100644
--- a/arch/sparc/kernel/leon_smp.c
+++ b/arch/sparc/kernel/leon_smp.c
@@ -93,7 +93,7 @@ void leon_cpu_pre_online(void *arg)
 			     : "memory" /* paranoid */);
 
 	/* Attach to the address space of init_task. */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	while (!cpumask_test_cpu(cpuid, &smp_commenced_mask))
diff --git a/arch/sparc/kernel/smp_64.c b/arch/sparc/kernel/smp_64.c
index 8182f7caf5b1..c1d2bed22961 100644
--- a/arch/sparc/kernel/smp_64.c
+++ b/arch/sparc/kernel/smp_64.c
@@ -122,7 +122,7 @@ void smp_callin(void)
 	current_thread_info()->new_child = 0;
 
 	/* Attach to the address space of init_task. */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	/* inform the notifiers about the new cpu */
diff --git a/arch/sparc/kernel/sun4d_smp.c b/arch/sparc/kernel/sun4d_smp.c
index 9d98e5002a09..7b55c50eabe5 100644
--- a/arch/sparc/kernel/sun4d_smp.c
+++ b/arch/sparc/kernel/sun4d_smp.c
@@ -93,7 +93,7 @@ void sun4d_cpu_pre_online(void *arg)
 	show_leds(cpuid);
 
 	/* Attach to the address space of init_task. */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	local_ops->cache_all();
diff --git a/arch/sparc/kernel/sun4m_smp.c b/arch/sparc/kernel/sun4m_smp.c
index 278c40abce82..633c4cf6fdb0 100644
--- a/arch/sparc/kernel/sun4m_smp.c
+++ b/arch/sparc/kernel/sun4m_smp.c
@@ -59,7 +59,7 @@ void sun4m_cpu_pre_online(void *arg)
 			     : "memory" /* paranoid */);
 
 	/* Attach to the address space of init_task. */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	while (!cpumask_test_cpu(cpuid, &smp_commenced_mask))
diff --git a/arch/sparc/kernel/traps_32.c b/arch/sparc/kernel/traps_32.c
index 4f21df7d4f13..ecddac5a4c96 100644
--- a/arch/sparc/kernel/traps_32.c
+++ b/arch/sparc/kernel/traps_32.c
@@ -448,7 +448,7 @@ void trap_init(void)
 		thread_info_offsets_are_bolixed_pete();
 
 	/* Attach to the address space of init_task. */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 
 	/* NOTE: Other cpus have this done as they are started
diff --git a/arch/sparc/kernel/traps_64.c b/arch/sparc/kernel/traps_64.c
index 496fa926e1e0..83dd1331a30e 100644
--- a/arch/sparc/kernel/traps_64.c
+++ b/arch/sparc/kernel/traps_64.c
@@ -2764,6 +2764,6 @@ void __init trap_init(void)
 	/* Attach to the address space of init_task.  On SMP we
 	 * do this in smp.c:smp_callin for other cpus.
 	 */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 }
diff --git a/arch/tile/kernel/smpboot.c b/arch/tile/kernel/smpboot.c
index 6c0abaacec33..53ce940a5016 100644
--- a/arch/tile/kernel/smpboot.c
+++ b/arch/tile/kernel/smpboot.c
@@ -160,7 +160,7 @@ static void start_secondary(void)
 	__this_cpu_write(current_asid, min_asid);
 
 	/* Set up this thread as another owner of the init_mm */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	current->active_mm = &init_mm;
 	if (current->mm)
 		BUG();
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 729f92ba8224..5b88f78c40f5 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1498,7 +1498,7 @@ void cpu_init(void)
 	for (i = 0; i <= IO_BITMAP_LONGS; i++)
 		t->io_bitmap[i] = ~0UL;
 
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	me->active_mm = &init_mm;
 	BUG_ON(me->mm);
 	enter_lazy_tlb(&init_mm, me);
@@ -1549,7 +1549,7 @@ void cpu_init(void)
 	/*
 	 * Set up and load the per-CPU TSS and LDT
 	 */
-	atomic_inc(&init_mm.mm_count);
+	mmgrab(&init_mm);
 	curr->active_mm = &init_mm;
 	BUG_ON(curr->mm);
 	enter_lazy_tlb(&init_mm, curr);
diff --git a/arch/xtensa/kernel/smp.c b/arch/xtensa/kernel/smp.c
index fc4ad21a5ed4..9bf5cea3bae4 100644
--- a/arch/xtensa/kernel/smp.c
+++ b/arch/xtensa/kernel/smp.c
@@ -136,7 +136,7 @@ void secondary_start_kernel(void)
 	/* All kernel threads share the same mm context. */
 
 	atomic_inc(&mm->mm_users);
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	current->active_mm = mm;
 	cpumask_set_cpu(cpu, mm_cpumask(mm));
 	enter_lazy_tlb(mm, current);
diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_process.c b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
index ef7c8de7060e..ca5f2aa7232d 100644
--- a/drivers/gpu/drm/amd/amdkfd/kfd_process.c
+++ b/drivers/gpu/drm/amd/amdkfd/kfd_process.c
@@ -262,7 +262,7 @@ static void kfd_process_notifier_release(struct mmu_notifier *mn,
 	 * and because the mmu_notifier_unregister function also drop
 	 * mm_count we need to take an extra count here.
 	 */
-	atomic_inc(&p->mm->mm_count);
+	mmgrab(p->mm);
 	mmu_notifier_unregister_no_release(&p->mmu_notifier, p->mm);
 	mmu_notifier_call_srcu(&p->rcu, &kfd_process_destroy_delayed);
 }
diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index d068af2ec3a3..1f27529cb48e 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -334,7 +334,7 @@ i915_gem_userptr_init__mm_struct(struct drm_i915_gem_object *obj)
 		mm->i915 = to_i915(obj->base.dev);
 
 		mm->mm = current->mm;
-		atomic_inc(&current->mm->mm_count);
+		mmgrab(current->mm);
 
 		mm->mn = NULL;
 
diff --git a/drivers/infiniband/hw/hfi1/file_ops.c b/drivers/infiniband/hw/hfi1/file_ops.c
index bd786b7bd30b..2e1a6643a910 100644
--- a/drivers/infiniband/hw/hfi1/file_ops.c
+++ b/drivers/infiniband/hw/hfi1/file_ops.c
@@ -185,7 +185,7 @@ static int hfi1_file_open(struct inode *inode, struct file *fp)
 	if (fd) {
 		fd->rec_cpu_num = -1; /* no cpu affinity by default */
 		fd->mm = current->mm;
-		atomic_inc(&fd->mm->mm_count);
+		mmgrab(fd->mm);
 		fp->private_data = fd;
 	} else {
 		fp->private_data = NULL;
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 5ea836362870..32f04999d930 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -798,7 +798,7 @@ struct mm_struct *proc_mem_open(struct inode *inode, unsigned int mode)
 
 		if (!IS_ERR_OR_NULL(mm)) {
 			/* ensure this mm_struct can't be freed */
-			atomic_inc(&mm->mm_count);
+			mmgrab(mm);
 			/* but do not pin its memory */
 			mmput(mm);
 		}
@@ -1096,7 +1096,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
 		if (p) {
 			if (atomic_read(&p->mm->mm_users) > 1) {
 				mm = p->mm;
-				atomic_inc(&mm->mm_count);
+				mmgrab(mm);
 			}
 			task_unlock(p);
 		}
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index d96e2f30084b..a02bbf5897e6 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1306,7 +1306,7 @@ static struct file *userfaultfd_file_create(int flags)
 	ctx->released = false;
 	ctx->mm = current->mm;
 	/* prevent the mm struct to be freed */
-	atomic_inc(&ctx->mm->mm_count);
+	mmgrab(ctx->mm);
 
 	file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
 				  O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a440cf178191..6ce46220bda2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2911,6 +2911,28 @@ static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig)
  */
 extern struct mm_struct * mm_alloc(void);
 
+/**
+ * mmgrab() - Pin a &struct mm_struct.
+ * @mm: The &struct mm_struct to pin.
+ *
+ * Make sure that @mm will not get freed even after the owning task
+ * exits. This doesn't guarantee that the associated address space
+ * will still exist later on and mmget_not_zero() has to be used before
+ * accessing it.
+ *
+ * This is a preferred way to to pin @mm for a longer/unbounded amount
+ * of time.
+ *
+ * Use mmdrop() to release the reference acquired by mmgrab().
+ *
+ * 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 mmgrab(struct mm_struct *mm)
+{
+	atomic_inc(&mm->mm_count);
+}
+
 /* mmdrop drops the mm and the page tables */
 extern void __mmdrop(struct mm_struct *);
 static inline void mmdrop(struct mm_struct *mm)
diff --git a/kernel/exit.c b/kernel/exit.c
index aacff8e2aec0..a0bfdace6a07 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -509,7 +509,7 @@ static void exit_mm(struct task_struct *tsk)
 		__set_task_state(tsk, TASK_RUNNING);
 		down_read(&mm->mmap_sem);
 	}
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	BUG_ON(mm != tsk->active_mm);
 	/* more a memory barrier than a real lock */
 	task_lock(tsk);
diff --git a/kernel/futex.c b/kernel/futex.c
index 9246d9f593d1..cc84e6bd6a4d 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -338,7 +338,7 @@ static inline bool should_fail_futex(bool fshared)
 
 static inline void futex_get_mm(union futex_key *key)
 {
-	atomic_inc(&key->private.mm->mm_count);
+	mmgrab(key->private.mm);
 	/*
 	 * Ensure futex_get_mm() implies a full barrier such that
 	 * get_futex_key() implies a full barrier. This is relied upon
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 966556ebdbb3..49d7076d93c3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2878,7 +2878,7 @@ 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);
@@ -7686,7 +7686,7 @@ 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);
 
 	/*
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index e32389a97030..4cf3b3af6313 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -420,7 +420,7 @@ int __khugepaged_enter(struct mm_struct *mm)
 	list_add_tail(&mm_slot->mm_node, &khugepaged_scan.mm_head);
 	spin_unlock(&khugepaged_mm_lock);
 
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	if (wakeup)
 		wake_up_interruptible(&khugepaged_wait);
 
diff --git a/mm/ksm.c b/mm/ksm.c
index 9ae6011a41f8..5a49aad9d87b 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1813,7 +1813,7 @@ int __ksm_enter(struct mm_struct *mm)
 	spin_unlock(&ksm_mmlist_lock);
 
 	set_bit(MMF_VM_MERGEABLE, &mm->flags);
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 
 	if (needs_wakeup)
 		wake_up_interruptible(&ksm_thread_wait);
diff --git a/mm/mmu_context.c b/mm/mmu_context.c
index 6f4d27c5bb32..daf67bb02b4a 100644
--- a/mm/mmu_context.c
+++ b/mm/mmu_context.c
@@ -25,7 +25,7 @@ void use_mm(struct mm_struct *mm)
 	task_lock(tsk);
 	active_mm = tsk->active_mm;
 	if (active_mm != mm) {
-		atomic_inc(&mm->mm_count);
+		mmgrab(mm);
 		tsk->active_mm = mm;
 	}
 	tsk->mm = mm;
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index f4259e496f83..32bc9f2ff7eb 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -275,7 +275,7 @@ static int do_mmu_notifier_register(struct mmu_notifier *mn,
 		mm->mmu_notifier_mm = mmu_notifier_mm;
 		mmu_notifier_mm = NULL;
 	}
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 
 	/*
 	 * Serialize the update against mmu_notifier_unregister. A
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index ec9f11d4f094..ead093c6f2a6 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -660,7 +660,7 @@ static void mark_oom_victim(struct task_struct *tsk)
 
 	/* oom_mm is bound to the signal struct life time. */
 	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm))
-		atomic_inc(&tsk->signal->oom_mm->mm_count);
+		mmgrab(tsk->signal->oom_mm);
 
 	/*
 	 * Make sure that the task is woken up from uninterruptible sleep
@@ -877,7 +877,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 
 	/* Get a reference to safely compare mm after task_unlock(victim) */
 	mm = victim->mm;
-	atomic_inc(&mm->mm_count);
+	mmgrab(mm);
 	/*
 	 * We should send SIGKILL before setting TIF_MEMDIE in order to prevent
 	 * the OOM victim from depleting the memory reserves from the user
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index de102cae7125..3a61b572fa2c 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -616,7 +616,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
 		return ERR_PTR(-ENOMEM);
 
 	spin_lock_init(&kvm->mmu_lock);
-	atomic_inc(&current->mm->mm_count);
+	mmgrab(current->mm);
 	kvm->mm = current->mm;
 	kvm_eventfd_init(kvm);
 	mutex_init(&kvm->lock);
-- 
2.11.0.1.gaa10c3f

[PATCH 2/4] mm: add new mmget() helper

[Vegard Nossum]

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.)

Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
 arch/arc/kernel/smp.c           |  2 +-
 arch/blackfin/mach-common/smp.c |  2 +-
 arch/frv/mm/mmu-context.c       |  2 +-
 arch/metag/kernel/smp.c         |  2 +-
 arch/sh/kernel/smp.c            |  2 +-
 arch/xtensa/kernel/smp.c        |  2 +-
 include/linux/sched.h           | 21 +++++++++++++++++++++
 kernel/fork.c                   |  4 ++--
 mm/swapfile.c                   | 10 +++++-----
 virt/kvm/async_pf.c             |  2 +-
 10 files changed, 35 insertions(+), 14 deletions(-)

diff --git a/arch/arc/kernel/smp.c b/arch/arc/kernel/smp.c
index 9cbc7aba3ede..eec70cb71db1 100644
--- a/arch/arc/kernel/smp.c
+++ b/arch/arc/kernel/smp.c
@@ -124,7 +124,7 @@ 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 6ce46220bda2..9fc07aaf5c97 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2955,6 +2955,27 @@ 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 869b8ccc00bf..0e2aaa1837b3 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -994,7 +994,7 @@ 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;
@@ -1182,7 +1182,7 @@ 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 1c6e0321205d..914c31cc143c 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1401,7 +1401,7 @@ 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,
@@ -1450,7 +1450,7 @@ 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);
 		}
 
 		/*
@@ -1487,8 +1487,8 @@ 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) {
@@ -1511,7 +1511,7 @@ 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
-- 
2.11.0.1.gaa10c3f

[PATCH 3/4] mm: use mmget_not_zero() helper

[Vegard Nossum]

We already have the helper, we can convert the rest of the kernel
mechanically using:

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

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

Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
 drivers/gpu/drm/i915/i915_gem_userptr.c | 2 +-
 drivers/iommu/intel-svm.c               | 2 +-
 fs/proc/base.c                          | 4 ++--
 fs/proc/task_mmu.c                      | 4 ++--
 fs/proc/task_nommu.c                    | 2 +-
 kernel/events/uprobes.c                 | 2 +-
 mm/swapfile.c                           | 2 +-
 7 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_userptr.c b/drivers/gpu/drm/i915/i915_gem_userptr.c
index 1f27529cb48e..89be48ed7c77 100644
--- a/drivers/gpu/drm/i915/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
@@ -507,7 +507,7 @@ __i915_gem_userptr_get_pages_worker(struct work_struct *_work)
 			flags |= FOLL_WRITE;
 
 		ret = -EFAULT;
-		if (atomic_inc_not_zero(&mm->mm_users)) {
+		if (mmget_not_zero(mm)) {
 			down_read(&mm->mmap_sem);
 			while (pinned < npages) {
 				ret = get_user_pages_remote
diff --git a/drivers/iommu/intel-svm.c b/drivers/iommu/intel-svm.c
index cb72e0011310..51f2b228723f 100644
--- a/drivers/iommu/intel-svm.c
+++ b/drivers/iommu/intel-svm.c
@@ -579,7 +579,7 @@ static irqreturn_t prq_event_thread(int irq, void *d)
 		if (!svm->mm)
 			goto bad_req;
 		/* If the mm is already defunct, don't handle faults. */
-		if (!atomic_inc_not_zero(&svm->mm->mm_users))
+		if (!mmget_not_zero(svm->mm))
 			goto bad_req;
 		down_read(&svm->mm->mmap_sem);
 		vma = find_extend_vma(svm->mm, address);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 32f04999d930..ec7304f5117a 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -845,7 +845,7 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
 		return -ENOMEM;
 
 	copied = 0;
-	if (!atomic_inc_not_zero(&mm->mm_users))
+	if (!mmget_not_zero(mm))
 		goto free;
 
 	/* Maybe we should limit FOLL_FORCE to actual ptrace users? */
@@ -953,7 +953,7 @@ static ssize_t environ_read(struct file *file, char __user *buf,
 		return -ENOMEM;
 
 	ret = 0;
-	if (!atomic_inc_not_zero(&mm->mm_users))
+	if (!mmget_not_zero(mm))
 		goto free;
 
 	down_read(&mm->mmap_sem);
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 958f32545064..6c07c7813b26 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -167,7 +167,7 @@ static void *m_start(struct seq_file *m, loff_t *ppos)
 		return ERR_PTR(-ESRCH);
 
 	mm = priv->mm;
-	if (!mm || !atomic_inc_not_zero(&mm->mm_users))
+	if (!mm || !mmget_not_zero(mm))
 		return NULL;
 
 	down_read(&mm->mmap_sem);
@@ -1352,7 +1352,7 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
 	unsigned long end_vaddr;
 	int ret = 0, copied = 0;
 
-	if (!mm || !atomic_inc_not_zero(&mm->mm_users))
+	if (!mm || !mmget_not_zero(mm))
 		goto out;
 
 	ret = -EINVAL;
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 37175621e890..1ef97cfcf422 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -219,7 +219,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 		return ERR_PTR(-ESRCH);
 
 	mm = priv->mm;
-	if (!mm || !atomic_inc_not_zero(&mm->mm_users))
+	if (!mm || !mmget_not_zero(mm))
 		return NULL;
 
 	down_read(&mm->mmap_sem);
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 215871bda3a2..f164fe8ca5ff 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -741,7 +741,7 @@ build_map_info(struct address_space *mapping, loff_t offset, bool is_register)
 			continue;
 		}
 
-		if (!atomic_inc_not_zero(&vma->vm_mm->mm_users))
+		if (!mmget_not_zero(vma->vm_mm))
 			continue;
 
 		info = prev;
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 914c31cc143c..5502feef0a4a 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1493,7 +1493,7 @@ int try_to_unuse(unsigned int type, bool frontswap,
 			while (swap_count(*swap_map) && !retval &&
 					(p = p->next) != &start_mm->mmlist) {
 				mm = list_entry(p, struct mm_struct, mmlist);
-				if (!atomic_inc_not_zero(&mm->mm_users))
+				if (!mmget_not_zero(mm))
 					continue;
 				spin_unlock(&mmlist_lock);
 				mmput(prev_mm);
-- 
2.11.0.1.gaa10c3f

[PATCH 4/4] mm: clarify mm_struct.mm_{users,count} documentation

[Vegard Nossum]

Clarify documentation relating to mm_users and mm_count, and switch to
kernel-doc syntax.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
---
 include/linux/mm_types.h | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 08d947fc4c59..316c3e1fc226 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -407,8 +407,27 @@ struct mm_struct {
 	unsigned long task_size;		/* size of task vm space */
 	unsigned long highest_vm_end;		/* highest vma end address */
 	pgd_t * pgd;
-	atomic_t mm_users;			/* How many users with user space? */
-	atomic_t mm_count;			/* How many references to "struct mm_struct" (users count as 1) */
+
+	/**
+	 * @mm_users: The number of users including userspace.
+	 *
+	 * Use mmget()/mmget_not_zero()/mmput() to modify. When this drops
+	 * to 0 (i.e. when the task exits and there are no other temporary
+	 * reference holders), we also release a reference on @mm_count
+	 * (which may then free the &struct mm_struct if @mm_count also
+	 * drops to 0).
+	 */
+	atomic_t mm_users;
+
+	/**
+	 * @mm_count: The number of references to &struct mm_struct
+	 * (@mm_users count as 1).
+	 *
+	 * Use mmgrab()/mmdrop() to modify. When this drops to 0, the
+	 * &struct mm_struct is freed.
+	 */
+	atomic_t mm_count;
+
 	atomic_long_t nr_ptes;			/* PTE page table pages */
 #if CONFIG_PGTABLE_LEVELS > 2
 	atomic_long_t nr_pmds;			/* PMD page table pages */
-- 
2.11.0.1.gaa10c3f

Re: [PATCH 1/4] mm: add new mmgrab() helper

[Peter Zijlstra]

For all 4,

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Re: [PATCH 4/4] mm: clarify mm_struct.mm_{users,count} documentation

[Michal Hocko]

On Sun 18-12-16 13:32:29, Vegard Nossum wrote:
> Clarify documentation relating to mm_users and mm_count, and switch to
> kernel-doc syntax.

Looks good to me.
 
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
>  include/linux/mm_types.h | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 08d947fc4c59..316c3e1fc226 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -407,8 +407,27 @@ struct mm_struct {
>  	unsigned long task_size;		/* size of task vm space */
>  	unsigned long highest_vm_end;		/* highest vma end address */
>  	pgd_t * pgd;
> -	atomic_t mm_users;			/* How many users with user space? */
> -	atomic_t mm_count;			/* How many references to "struct mm_struct" (users count as 1) */
> +
> +	/**
> +	 * @mm_users: The number of users including userspace.
> +	 *
> +	 * Use mmget()/mmget_not_zero()/mmput() to modify. When this drops
> +	 * to 0 (i.e. when the task exits and there are no other temporary
> +	 * reference holders), we also release a reference on @mm_count
> +	 * (which may then free the &struct mm_struct if @mm_count also
> +	 * drops to 0).
> +	 */
> +	atomic_t mm_users;
> +
> +	/**
> +	 * @mm_count: The number of references to &struct mm_struct
> +	 * (@mm_users count as 1).
> +	 *
> +	 * Use mmgrab()/mmdrop() to modify. When this drops to 0, the
> +	 * &struct mm_struct is freed.
> +	 */
> +	atomic_t mm_count;
> +
>  	atomic_long_t nr_ptes;			/* PTE page table pages */
>  #if CONFIG_PGTABLE_LEVELS > 2
>  	atomic_long_t nr_pmds;			/* PMD page table pages */
> -- 
> 2.11.0.1.gaa10c3f
> 

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/4] mm: add new mmgrab() helper

[David Rientjes]

On Sun, 18 Dec 2016, Vegard Nossum wrote:

> 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.)
> 
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Acked-by: Michal Hocko <mhocko@suse.com>
> Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>

Acked-by: David Rientjes <rientjes@google.com>

for the series

Re: [PATCH 2/4] mm: add new mmget() helper

[Andrew Morton]

On Sun, 18 Dec 2016 13:32:27 +0100 Vegard Nossum <vegard.nossum@oracle.com> wrote:

> 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.
>
> ...
>

mmgrap() and mmget() naming is really quite confusing.

> --- a/arch/arc/kernel/smp.c
> +++ b/arch/arc/kernel/smp.c
> @@ -124,7 +124,7 @@ void start_kernel_secondary(void)
>  	/* MMU, Caches, Vector Table, Interrupts etc */
>  	setup_processor();
>  
> -	atomic_inc(&mm->mm_users);
> +	mmget(mm);

I wonder if mmuse() would be a bit clearer than mmget().  Although that
sounds like use_mm().