From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752686AbeADKZP (ORCPT + 1 other); Thu, 4 Jan 2018 05:25:15 -0500 Received: from outbound-smtp26.blacknight.com ([81.17.249.194]:46998 "EHLO outbound-smtp26.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752508AbeADKZN (ORCPT ); Thu, 4 Jan 2018 05:25:13 -0500 Date: Thu, 4 Jan 2018 10:25:12 +0000 From: Mel Gorman To: Andrew Morton Cc: "Huang, Ying" , Jan Kara , Minchan Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [PATCH] mm: Pin address_space before dereferencing it while isolating an LRU page Message-ID: <20180104102512.2qos3h5vqzeisrek@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline User-Agent: NeoMutt/20170912 (1.9.0) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: Minchan Kim asked the following question -- what locks protects address_space destroying when race happens between inode trauncation and __isolate_lru_page? Jan Kara clarified by describing the race as follows 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. The race is theoritically possible but unlikely. Before the delete_from_page_cache, truncate_cleanup_page is called 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. Even if the race occurs, a substantial amount of work has to happen during a tiny window with no preemption but it could potentially be done using a virtual machine to artifically slow one CPU or halt it during the critical window. This patch should eliminate the race with truncation by try-locking the page before derefencing mapping and aborting if the lock was not acquired. There was a suggestion from Huang Ying to use RCU as a side-effect to prevent mapping being freed. However, I do not like the solution as it's an unconventional means of preserving a mapping and it's not a context where rcu_read_lock is obviously protecting rcu data. Fixes: c82449352854 ("mm: compaction: make isolate_lru_page() filter-aware again") Signed-off-by: Mel Gorman --- mm/vmscan.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index c02c850ea349..61bf0bc60d96 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1433,14 +1433,24 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode) if (PageDirty(page)) { struct address_space *mapping; + bool migrate_dirty; /* * Only pages without mappings or that have a * ->migratepage callback are possible to migrate - * without blocking + * without blocking. However, we can be racing with + * truncation so it's necessary to lock the page + * to stabilise the mapping as truncation holds + * the page lock until after the page is removed + * from the page cache. */ + if (!trylock_page(page)) + return ret; + mapping = page_mapping(page); - if (mapping && !mapping->a_ops->migratepage) + migrate_dirty = mapping && mapping->a_ops->migratepage; + unlock_page(page); + if (!migrate_dirty) return ret; } }