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
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