From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753276AbeE3LRO (ORCPT ); Wed, 30 May 2018 07:17:14 -0400 Received: from shadbolt.e.decadent.org.uk ([88.96.1.126]:32843 "EHLO shadbolt.e.decadent.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752611AbeE3LHJ (ORCPT ); Wed, 30 May 2018 07:07:09 -0400 Content-Type: text/plain; charset="UTF-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit MIME-Version: 1.0 From: Ben Hutchings To: linux-kernel@vger.kernel.org, stable@vger.kernel.org CC: akpm@linux-foundation.org, "Mel Gorman" , "Huang, Ying" , "Jan Kara" , "Minchan Kim" , "Linus Torvalds" Date: Wed, 30 May 2018 11:52:41 +0100 Message-ID: X-Mailer: LinuxStableQueue (scripts by bwh) Subject: [PATCH 3.2 069/153] mm: pin address_space before dereferencing it while isolating an LRU page In-Reply-To: X-SA-Exim-Connect-IP: 2a02:8011:400e:2:6f00:88c8:c921:d332 X-SA-Exim-Mail-From: ben@decadent.org.uk X-SA-Exim-Scanned: No (on shadbolt.decadent.org.uk); SAEximRunCond expanded to false Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 3.2.102-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Mel Gorman commit 69d763fc6d3aee787a3e8c8c35092b4f4960fa5d upstream. 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 theoretically 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. Link: http://lkml.kernel.org/r/20180104102512.2qos3h5vqzeisrek@techsingularity.net Fixes: c82449352854 ("mm: compaction: make isolate_lru_page() filter-aware again") Signed-off-by: Mel Gorman Acked-by: Minchan Kim Cc: "Huang, Ying" Cc: Jan Kara Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds [bwh: Backported to 3.2: adjust context] Signed-off-by: Ben Hutchings --- mm/vmscan.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1085,6 +1085,7 @@ int __isolate_lru_page(struct page *page if (PageDirty(page)) { struct address_space *mapping; + bool migrate_dirty; /* ISOLATE_CLEAN means only clean pages */ if (mode & ISOLATE_CLEAN) @@ -1093,10 +1094,19 @@ int __isolate_lru_page(struct page *page /* * 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; } }