Re: [PATCH] mm: Fix races between address_space dereference and free in page_evicatable

[Jan Kara]

On Mon 12-02-18 16:12:27, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> When page_mapping() is called and the mapping is dereferenced in
> page_evicatable() through shrink_active_list(), it is possible for the
> inode to be truncated and the embedded address space to be freed at
> the same time.  This may lead to the following race.
> 
> CPU1                                                CPU2
> 
> truncate(inode)                                     shrink_active_list()
>   ...                                                 page_evictable(page)
>   truncate_inode_page(mapping, page);
>     delete_from_page_cache(page)
>       spin_lock_irqsave(&mapping->tree_lock, flags);
>         __delete_from_page_cache(page, NULL)
>           page_cache_tree_delete(..)
>             ...                                         mapping = page_mapping(page);
>             page->mapping = NULL;
>             ...
>       spin_unlock_irqrestore(&mapping->tree_lock, flags);
>       page_cache_free_page(mapping, page)
>         put_page(page)
>           if (put_page_testzero(page)) -> false
> - inode now has no pages and can be freed including embedded address_space
> 
>                                                         mapping_unevictable(mapping)
> 							  test_bit(AS_UNEVICTABLE, &mapping->flags);
> - we've dereferenced mapping which is potentially already free.
> 
> Similar race exists between swap cache freeing and page_evicatable() too.
> 
> The address_space in inode and swap cache will be freed after a RCU
> grace period.  So the races are fixed via enclosing the page_mapping()
> and address_space usage in rcu_read_lock/unlock().  Some comments are
> added in code to make it clear what is protected by the RCU read lock.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>

The race looks real (although very unlikely) and the patch looks good to me.
You can add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  mm/vmscan.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d1c1e00b08bb..10a0f32a3f90 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -3886,7 +3886,13 @@ int node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned int order)
>   */
>  int page_evictable(struct page *page)
>  {
> -	return !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
> +	int ret;
> +
> +	/* Prevent address_space of inode and swap cache from being freed */
> +	rcu_read_lock();
> +	ret = !mapping_unevictable(page_mapping(page)) && !PageMlocked(page);
> +	rcu_read_unlock();
> +	return ret;
>  }
>  
>  #ifdef CONFIG_SHMEM
> -- 
> 2.15.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR