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

From: Minchan Kim
Date: Mon Feb 26 2018 - 00:20:23 EST


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@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?

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.