On Thu, Jan 04, 2018 at 10:25:12AM +0000, Mel Gorman wrote:
> Minchan Kim asked the following question -- what locks protects
> address_space destroying when race happens between inode trauncation and
> __isolate_lru_page? Jan Kara clarified by describing the race as follows
>
> CPU1 CPU2
>
> truncate(inode) __isolate_lru_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
>
> if (mapping && !mapping->a_ops->migratepage)
> - we've dereferenced mapping which is potentially already free.
>
> The race is theoritically possible but unlikely. Before the
> delete_from_page_cache, truncate_cleanup_page is called so the page is
> likely to be !PageDirty or PageWriteback which gets skipped by the only
> caller that checks the mappping in __isolate_lru_page. Even if the race
> occurs, a substantial amount of work has to happen during a tiny window
> with no preemption but it could potentially be done using a virtual machine
> to artifically slow one CPU or halt it during the critical window.
>
> This patch should eliminate the race with truncation by try-locking the page
> before derefencing mapping and aborting if the lock was not acquired. There
> was a suggestion from Huang Ying to use RCU as a side-effect to prevent
> mapping being freed. However, I do not like the solution as it's an
> unconventional means of preserving a mapping and it's not a context where
> rcu_read_lock is obviously protecting rcu data.
>
> Fixes: c82449352854 ("mm: compaction: make isolate_lru_page() filter-aware again")
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
Thanks for the patch.
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