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

From: Vegard Nossum <hidden>
Date: 2016-12-18 12:33:17
Also in: lkml
Subsystem: exec & binfmt api, elf, kernel virtual machine (kvm), memory management, memory management - core, memory management - swap, scheduler, superh, synopsys arc architecture, tensilica xtensa port (xtensa), the rest · Maintainers: Kees Cook, Paolo Bonzini, Andrew Morton, David Hildenbrand, Chris Li, Kairui Song, Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot, Yoshinori Sato, Rich Felker, John Paul Adrian Glaubitz, Vineet Gupta, Chris Zankel, Max Filippov, Linus Torvalds

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 <redacted>
---
 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

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