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.
