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

[Minchan Kim <minchan@kernel.org>]

Hi Jan,

On Mon, Feb 19, 2018 at 11:57:35AM +0100, Jan Kara wrote:
> 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@intel.com>
> > > 
> > > 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?

Yub, it would be much better. However, how does this patch fix the problem?
Although it can make only page_evictable safe, we could go with the page
further and finally uses page->mapping, again.
For instance,

shrink_active_list
	page_evictable();
	..
	page_referened()
		page_rmapping
			page->mapping

I think caller should lock the page to protect entire operation, which
have been used more widely to pin a address_space.

Thanks.

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