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

[Minchan Kim]

Hi Huang,

On Mon, Feb 12, 2018 at 04:12:27PM +0800, 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.

Is it always true for every FSes, even upcoming FSes?
IOW, do we have any strict rule FS folks must use RCU(i.e., call_rcu)
to destroy inode?

Let's cc linux-fs.

> 
> 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>
> ---
>  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
>