Message-ID: <20161216091417.GB13940@dhcp22.suse.cz>
Date:   Fri, 16 Dec 2016 10:14:17 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vegard Nossum <vegard.nossum@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Rik van Riel <riel@...hat.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/4] mm: add new mmgrab() helper

On Fri 16-12-16 09:21:59, 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.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>

Yes this make sense to me. I usually do not like wrappers around simple
atomic operations but the api will be more symmetric (and cscope will
work nicer as a bonus).

While you are there a comment explaining where mmgrab should be used
would be nice

/** 
 * mmgrab: pins the mm_struct
 *
 * Make sure that the mm struct will not get freed even after the owner
 * 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 the mm struct for longer/unbound
 * time.
 */

or something along those lines.

Anyway
Acked-by: Michal Hocko <mhocko@...e.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                    | 5 +++++
>  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, 48 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 8507703dabe4..61969ea29654 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -214,7 +214,7 @@ 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;
>  
>  	set_my_cpu_offset(per_cpu_offset(smp_processor_id()));
> 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 81d0808085e6..ec9bc100895c 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 4094a51b1970..0dbbe40012ef 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 cc9e980c68ec..b580da4582e1 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1555,7 +1555,7 @@ 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);
> @@ -1606,7 +1606,7 @@ 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 c6f780f5abc9..f21ca404af79 100644
> --- a/drivers/gpu/drm/i915/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/i915_gem_userptr.c
> @@ -341,7 +341,7 @@ 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 ca651ac00660..0b8ccacae8b3 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -795,7 +795,7 @@ 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);
>  		}
> @@ -1093,7 +1093,7 @@ 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 85959d8324df..ffa9c7cbc5fa 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -1304,7 +1304,7 @@ 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 e9c009dc3a4a..31ae1f49eebb 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2872,6 +2872,11 @@ static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig)
>   */
>  extern struct mm_struct * mm_alloc(void);
>  
> +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 3076f3089919..b12753840050 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -500,7 +500,7 @@ 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 2c4be467fecd..cbe6056c17c1 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 154fd689fe02..ee1fb0070544 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2877,7 +2877,7 @@ 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);
> @@ -7667,7 +7667,7 @@ 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 87e1a7ca3846..1343271a18f1 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 7f9ee2929cfe..43914b981691 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -613,7 +613,7 @@ 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
> 

-- 
Michal Hocko
SUSE Labs

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