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Date: Fri, 9 Sep 2022 15:56:06 +0200
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Subject: Re: [RFC PATCH RESEND 16/28] kernel/fork: assert no VMA readers
 during its destruction
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To: Suren Baghdasaryan <surenb@google.com>, akpm@linux-foundation.org
References: <20220901173516.702122-1-surenb@google.com>
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Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> Assert there are no holders of VMA lock for reading when it is about to be
> destroyed.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
>  include/linux/mm.h | 8 ++++++++
>  kernel/fork.c      | 2 ++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dc72be923e5b..0d9c1563c354 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -676,6 +676,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos)
>  	VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
>  }
>  
> +static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> +{
> +	VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
> +		      vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> +		      vma);
> +}
> +
>  #else /* CONFIG_PER_VMA_LOCK */
>  
>  static inline void vma_init_lock(struct vm_area_struct *vma) {}
> @@ -685,6 +692,7 @@ static inline bool vma_read_trylock(struct vm_area_struct *vma)
>  static inline void vma_read_unlock(struct vm_area_struct *vma) {}
>  static inline void vma_assert_locked(struct vm_area_struct *vma) {}
>  static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos) {}
> +static inline void vma_assert_no_reader(struct vm_area_struct *vma) {}
>  
>  #endif /* CONFIG_PER_VMA_LOCK */
>  
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 1872ad549fed..b443ba3a247a 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -487,6 +487,8 @@ static void __vm_area_free(struct rcu_head *head)
>  {
>  	struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
>  						  vm_rcu);
> +	/* The vma should either have no lock holders or be write-locked. */
> +	vma_assert_no_reader(vma);

I'm wondering if this can be hit in the case the thread freeing a VMA is
preempted before incrementing the mm ref count, like this:

VMA is about to be freed
write lock VMA
free vma -> call_rcu
..
<--- thread preempted
	rcu handler runs
	rcu calls __vm_area_free() <<<<<<
unlock mmap_lock and increase the mm seq count


>  	kmem_cache_free(vm_area_cachep, vma);
>  }
>  #endif