Re: [PATCH] mm/frontswap: fix frontswap_register_ops() race with swapon and swapoff

[Miaohe Lin]

Hi:
On 2021/4/5 18:20, Miaohe Lin wrote:
> frontswap_register_ops can race with swapon. Consider the following scene:

Any comment or suggestion? Or is this race window too theoretical to fix?
Thanks.

> 
> CPU1					CPU2
> ----					----
> frontswap_register_ops
>   fill bitmap a
>   ops->init
> 					sys_swapon
> 					  enable_swap_info
> 					    frontswap_init without new ops
>   add ops to frontswap_ops list
>   check if swap_active_head changed
> 					    add to swap_active_head
> 
> So the frontswap_ops init is missed on the new swap device. Consider the
> another scene:
> CPU1                                    CPU2
> ----                                    ----
> frontswap_register_ops
>   fill bitmap a
>   ops->init
>   add ops to frontswap_ops list
>                                         sys_swapon
>                                           enable_swap_info
>                                             frontswap_init with new ops
>                                             add to swap_active_head
>   check if swap_active_head changed
>   ops->init for new swap device [twice!]
> 
> The frontswap_ops init will be called two times on the new swap device this
> time. frontswap_register_ops can also race with swapoff. Consider the
> following scene:
> 
> CPU1                                    CPU2
> ----                                    ----
>                                         sys_swapoff
> 					  removed from swap_active_head
> frontswap_register_ops
>   fill bitmap a
>   ops->init without swap device
>   add ops to frontswap_ops list
>                                             invalidate_area with new ops
>   check if swap_active_head changed
> 
> We could call invalidate_area on a swap device under swapoff with frontswap
> is uninitialized yet. Fix all these by using swapon_mutex to guard against
> race with swapon and add swap_info_get_if_under_swapoff() to collect swap
> devices under swapoff.
> 
> Fixes: d1dc6f1bcf1e ("frontswap: allow multiple backends")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
>  include/linux/swapfile.h |  2 ++
>  mm/frontswap.c           | 40 +++++++++++++++++-----------------------
>  mm/swapfile.c            | 13 ++++++++++++-
>  3 files changed, 31 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/swapfile.h b/include/linux/swapfile.h
> index e06febf62978..7ae15d917828 100644
> --- a/include/linux/swapfile.h
> +++ b/include/linux/swapfile.h
> @@ -9,8 +9,10 @@
>  extern spinlock_t swap_lock;
>  extern struct plist_head swap_active_head;
>  extern struct swap_info_struct *swap_info[];
> +extern struct mutex swapon_mutex;
>  extern int try_to_unuse(unsigned int, bool, unsigned long);
>  extern unsigned long generic_max_swapfile_size(void);
>  extern unsigned long max_swapfile_size(void);
> +extern struct swap_info_struct *swap_info_get_if_under_swapoff(int type);
>  
>  #endif /* _LINUX_SWAPFILE_H */
> diff --git a/mm/frontswap.c b/mm/frontswap.c
> index 130e301c5ac0..c16bfc7550b5 100644
> --- a/mm/frontswap.c
> +++ b/mm/frontswap.c
> @@ -123,12 +123,26 @@ void frontswap_register_ops(struct frontswap_ops *ops)
>  
>  	bitmap_zero(a, MAX_SWAPFILES);
>  	bitmap_zero(b, MAX_SWAPFILES);
> -
> +	mutex_lock(&swapon_mutex);
>  	spin_lock(&swap_lock);
>  	plist_for_each_entry(si, &swap_active_head, list) {
>  		if (!WARN_ON(!si->frontswap_map))
>  			set_bit(si->type, a);
>  	}
> +	/*
> +	 * There might be some swap devices under swapoff, i.e. they are
> +	 * removed from swap_active_head but frontswap_invalidate_area()
> +	 * is not called yet due to swapon_mutex is held here. We must
> +	 * collect these swap devices and call ops->init on them or they
> +	 * might invalidate frontswap area while frontswap is uninitialized.
> +	 */
> +	for_each_clear_bit(i, a, MAX_SWAPFILES) {
> +		si = swap_info_get_if_under_swapoff(i);
> +		if (!si || !si->frontswap_map)
> +			continue;
> +		set_bit(si->type, b);
> +	}
> +	bitmap_or(a, a, b, MAX_SWAPFILES);
>  	spin_unlock(&swap_lock);
>  
>  	/* the new ops needs to know the currently active swap devices */
> @@ -144,29 +158,9 @@ void frontswap_register_ops(struct frontswap_ops *ops)
>  		ops->next = frontswap_ops;
>  	} while (cmpxchg(&frontswap_ops, ops->next, ops) != ops->next);
>  
> -	static_branch_inc(&frontswap_enabled_key);
> -
> -	spin_lock(&swap_lock);
> -	plist_for_each_entry(si, &swap_active_head, list) {
> -		if (si->frontswap_map)
> -			set_bit(si->type, b);
> -	}
> -	spin_unlock(&swap_lock);
> +	mutex_unlock(&swapon_mutex);
>  
> -	/*
> -	 * On the very unlikely chance that a swap device was added or
> -	 * removed between setting the "a" list bits and the ops init
> -	 * calls, we re-check and do init or invalidate for any changed
> -	 * bits.
> -	 */
> -	if (unlikely(!bitmap_equal(a, b, MAX_SWAPFILES))) {
> -		for (i = 0; i < MAX_SWAPFILES; i++) {
> -			if (!test_bit(i, a) && test_bit(i, b))
> -				ops->init(i);
> -			else if (test_bit(i, a) && !test_bit(i, b))
> -				ops->invalidate_area(i);
> -		}
> -	}
> +	static_branch_inc(&frontswap_enabled_key);
>  }
>  EXPORT_SYMBOL(frontswap_register_ops);
>  
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 149e77454e3c..ee736533717f 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -89,7 +89,7 @@ static DEFINE_SPINLOCK(swap_avail_lock);
>  
>  struct swap_info_struct *swap_info[MAX_SWAPFILES];
>  
> -static DEFINE_MUTEX(swapon_mutex);
> +DEFINE_MUTEX(swapon_mutex);
>  
>  static DECLARE_WAIT_QUEUE_HEAD(proc_poll_wait);
>  /* Activity counter to indicate that a swapon or swapoff has occurred */
> @@ -2958,6 +2958,17 @@ __weak unsigned long max_swapfile_size(void)
>  	return generic_max_swapfile_size();
>  }
>  
> +struct swap_info_struct *swap_info_get_if_under_swapoff(int type)
> +{
> +	struct swap_info_struct *si = swap_type_to_swap_info(type);
> +
> +	if (!si || !si->swap_map)
> +		return NULL;
> +	if ((si->flags & SWP_USED) && !(si->flags & SWP_WRITEOK))
> +		return si;
> +	return NULL;
> +}
> +
>  static unsigned long read_swap_header(struct swap_info_struct *p,
>  					union swap_header *swap_header,
>  					struct inode *inode)
>