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

[Jan Kara]

Hi Minchan,

On Sun 18-02-18 18:22:45, Minchan Kim wrote:
> 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.

That's actually a good question. Pathname lookup relies on inodes being
protected by RCU so "normal" filesystems definitely need to use RCU freeing
of inodes. OTOH a filesystem could in theory refuse any attempt for RCU
pathname walk (in its .d_revalidate/.d_compare callback) and then get away
with freeing its inodes normally AFAICT. I don't see that happening
anywhere in the tree but in theory it is possible with some effort... But
frankly I don't see a good reason for that so all we should do is to
document that .destroy_inode needs to free the inode structure through RCU
if it uses page cache? Al?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR