Minchan Kim <minchan@xxxxxxxxxx> writes: > 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 This only checks the value of page->mapping, not deference page->mapping. So it should be safe. Best Regards, Huang, Ying > I think caller should lock the page to protect entire operation, which > have been used more widely to pin a address_space. > > Thanks.
