On Wed, Jan 03, 2018 at 08:42:15AM +0800, Huang, Ying wrote: > Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> writes: > > > On Tue, Jan 02, 2018 at 12:29:55PM +0100, Jan Kara wrote: > >> On Tue 02-01-18 10:21:03, Mel Gorman wrote: > >> > On Sat, Dec 23, 2017 at 10:36:53AM +0900, Minchan Kim wrote: > >> > > > code path. It appears that similar situation is possible for them too. > >> > > > > >> > > > The file cache pages will be delete from file cache address_space before > >> > > > address_space (embedded in inode) is freed. But they will be deleted > >> > > > from LRU list only when its refcount dropped to zero, please take a look > >> > > > at put_page() and release_pages(). While address_space will be freed > >> > > > after putting reference to all file cache pages. If someone holds a > >> > > > reference to a file cache page for quite long time, it is possible for a > >> > > > file cache page to be in LRU list after the inode/address_space is > >> > > > freed. > >> > > > > >> > > > And I found inode/address_space is freed witch call_rcu(). I don't know > >> > > > whether this is related to page_mapping(). > >> > > > > >> > > > This is just my understanding. > >> > > > >> > > Hmm, it smells like a bug of __isolate_lru_page. > >> > > > >> > > Ccing Mel: > >> > > > >> > > What locks protects address_space destroying when race happens between > >> > > inode trauncation and __isolate_lru_page? > >> > > > >> > > >> > I'm just back online and have a lot of catching up to do so this is a rushed > >> > answer and I didn't read the background of this. However the question is > >> > somewhat ambiguous and the scope is broad as I'm not sure which race you > >> > refer to. For file cache pages, I wouldnt' expect the address_space to be > >> > destroyed specifically as long as the inode exists which is the structure > >> > containing the address_space in this case. A page on the LRU being isolated > >> > in __isolate_lru_page will have an elevated reference count which will > >> > pin the inode until remove_mapping is called which holds the page lock > >> > while inode truncation looking at a page for truncation also only checks > >> > page_mapping under the page lock. Very broadly speaking, pages avoid being > >> > added back to an inode being freed by checking the I_FREEING state. > >> > >> So I'm wondering what prevents the following: > >> > >> CPU1 CPU2 > >> > >> truncate(inode) __isolate_lru_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 > >> > >> if (mapping && !mapping->a_ops->migratepage) > >> - we've dereferenced mapping which is potentially already free. > >> > > > > Hmm, possible if unlikely. > > > > Before delete_from_page_cache, we called truncate_cleanup_page so the > > page is likely to be !PageDirty or PageWriteback which gets skipped by > > the only caller that checks the mappping in __isolate_lru_page. The race > > is tiny but it does exist. One way of closing it is to check the mapping > > under the page lock which will prevent races with truncation. The > > overhead is minimal as the calling context (compaction) is quite a heavy > > operation anyway. > > > > I think another possible fix is to use call_rcu_sched() to free inode > (and address_space). Because __isolate_lru_page() will be called with > LRU spinlock held and IRQ disabled, call_rcu_sched() will wait > LRU spin_unlock and IRQ enabled. > Maybe, but in this particular case, I would prefer to go with something more conventional unless there is strong evidence that it's an improvement (which I doubt in this case given the cost of migration overall and the corner case of migrating a dirty page). -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
