Subject: Re: [PATCH v12 21/31] mm: Introduce find_vma_rcu()

From: Jerome Glisse <[email-masked]>
Date: Mon, 22 Apr 2019 16:57:21 -0400

On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote:
> This allows to search for a VMA structure without holding the mmap_sem.
> 
> The search is repeated while the mm seqlock is changing and until we found
> a valid VMA.
> 
> While under the RCU protection, a reference is taken on the VMA, so the
> caller must call put_vma() once it not more need the VMA structure.
> 
> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a
> reference is taken to the VMA by calling get_vma().
> 
> When removing a VMA from the MM RB tree, the VMA is not release immediately
> but at the end of the RCU grace period through vm_rcu_put(). This ensures
> that the VMA remains allocated until the end the RCU grace period.
> 
> Since the vm_file pointer, if valid, is released in put_vma(), there is no
> guarantee that the file pointer will be valid on the returned VMA.
> 
> Signed-off-by: Laurent Dufour <[email-masked]>

Minor comments about comment (i love recursion :)) see below.

Reviewed-by: Jérôme Glisse <[email-masked]>

> ---
>  include/linux/mm_types.h |  1 +
>  mm/internal.h            |  5 ++-
>  mm/mmap.c                | 76 ++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 78 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 6a6159e11a3f..9af6694cb95d 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -287,6 +287,7 @@ struct vm_area_struct {
>  
>  #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>  	atomic_t vm_ref_count;
> +	struct rcu_head vm_rcu;
>  #endif
>  	struct rb_node vm_rb;
>  
> diff --git a/mm/internal.h b/mm/internal.h
> index 302382bed406..1e368e4afe3c 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -55,7 +55,10 @@ static inline void put_vma(struct vm_area_struct *vma)
>  		__free_vma(vma);
>  }
>  
> -#else
> +extern struct vm_area_struct *find_vma_rcu(struct mm_struct *mm,
> +					   unsigned long addr);
> +
> +#else /* CONFIG_SPECULATIVE_PAGE_FAULT */
>  
>  static inline void get_vma(struct vm_area_struct *vma)
>  {
> diff --git a/mm/mmap.c b/mm/mmap.c
> index c106440dcae7..34bf261dc2c8 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -179,6 +179,18 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
>  {
>  	write_sequnlock(&mm->mm_seq);
>  }
> +
> +static void __vm_rcu_put(struct rcu_head *head)
> +{
> +	struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
> +						  vm_rcu);
> +	put_vma(vma);
> +}
> +static void vm_rcu_put(struct vm_area_struct *vma)
> +{
> +	VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
> +	call_rcu(&vma->vm_rcu, __vm_rcu_put);
> +}
>  #else
>  static inline void mm_write_seqlock(struct mm_struct *mm)
>  {
> @@ -190,6 +202,8 @@ static inline void mm_write_sequnlock(struct mm_struct *mm)
>  
>  void __free_vma(struct vm_area_struct *vma)
>  {
> +	if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
> +		VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);
>  	mpol_put(vma_policy(vma));
>  	vm_area_free(vma);
>  }
> @@ -197,11 +211,24 @@ void __free_vma(struct vm_area_struct *vma)
>  /*
>   * Close a vm structure and free it, returning the next.
>   */
> -static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
> +static struct vm_area_struct *__remove_vma(struct vm_area_struct *vma)
>  {
>  	struct vm_area_struct *next = vma->vm_next;
>  
>  	might_sleep();
> +	if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT) &&
> +	    !RB_EMPTY_NODE(&vma->vm_rb)) {
> +		/*
> +		 * If the VMA is still linked in the RB tree, we must release
> +		 * that reference by calling put_vma().
> +		 * This should only happen when called from exit_mmap().
> +		 * We forcely clear the node to satisfy the chec in
                                                        ^
Typo: chec -> check

> +		 * __free_vma(). This is safe since the RB tree is not walked
> +		 * anymore.
> +		 */
> +		RB_CLEAR_NODE(&vma->vm_rb);
> +		put_vma(vma);
> +	}
>  	if (vma->vm_ops && vma->vm_ops->close)
>  		vma->vm_ops->close(vma);
>  	if (vma->vm_file)
> @@ -211,6 +238,13 @@ static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
>  	return next;
>  }
>  
> +static struct vm_area_struct *remove_vma(struct vm_area_struct *vma)
> +{
> +	if (IS_ENABLED(CONFIG_SPECULATIVE_PAGE_FAULT))
> +		VM_BUG_ON_VMA(!RB_EMPTY_NODE(&vma->vm_rb), vma);

Adding a comment here explaining the BUG_ON so people can understand
what is wrong if that happens. For instance:

/*
 * remove_vma() should be call only once a vma have been remove from the rbtree
 * at which point the vma->vm_rb is an empty node. The exception is when vmas
 * are destroy through exit_mmap() in which case we do not bother updating the
 * rbtree (see comment in __remove_vma()).
 */

> +	return __remove_vma(vma);
> +}
> +
>  static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long flags,
>  		struct list_head *uf);
>  SYSCALL_DEFINE1(brk, unsigned long, brk)
> @@ -475,7 +509,7 @@ static inline void vma_rb_insert(struct vm_area_struct *vma,
>  
>  	/* All rb_subtree_gap values must be consistent prior to insertion */
>  	validate_mm_rb(root, NULL);
> -
> +	get_vma(vma);
>  	rb_insert_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
>  }
>  
> @@ -491,6 +525,14 @@ static void __vma_rb_erase(struct vm_area_struct *vma, struct mm_struct *mm)
>  	mm_write_seqlock(mm);
>  	rb_erase_augmented(&vma->vm_rb, root, &vma_gap_callbacks);
>  	mm_write_sequnlock(mm);	/* wmb */
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +	/*
> +	 * Ensure the removal is complete before clearing the node.
> +	 * Matched by vma_has_changed()/handle_speculative_fault().
> +	 */
> +	RB_CLEAR_NODE(&vma->vm_rb);
> +	vm_rcu_put(vma);
> +#endif
>  }
>  
>  static __always_inline void vma_rb_erase_ignore(struct vm_area_struct *vma,
> @@ -2331,6 +2373,34 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>  
>  EXPORT_SYMBOL(find_vma);
>  
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +/*
> + * Like find_vma() but under the protection of RCU and the mm sequence counter.
> + * The vma returned has to be relaesed by the caller through the call to
> + * put_vma()
> + */
> +struct vm_area_struct *find_vma_rcu(struct mm_struct *mm, unsigned long addr)
> +{
> +	struct vm_area_struct *vma = NULL;
> +	unsigned int seq;
> +
> +	do {
> +		if (vma)
> +			put_vma(vma);
> +
> +		seq = read_seqbegin(&mm->mm_seq);
> +
> +		rcu_read_lock();
> +		vma = find_vma(mm, addr);
> +		if (vma)
> +			get_vma(vma);
> +		rcu_read_unlock();
> +	} while (read_seqretry(&mm->mm_seq, seq));
> +
> +	return vma;
> +}
> +#endif
> +
>  /*
>   * Same as find_vma, but also return a pointer to the previous VMA in *pprev.
>   */
> @@ -3231,7 +3301,7 @@ void exit_mmap(struct mm_struct *mm)
>  	while (vma) {
>  		if (vma->vm_flags & VM_ACCOUNT)
>  			nr_accounted += vma_pages(vma);
> -		vma = remove_vma(vma);
> +		vma = __remove_vma(vma);
>  	}
>  	vm_unacct_memory(nr_accounted);
>  }
> -- 
> 2.21.0
>