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Sun, 8 Apr 2018 20:34:01 -0400 From: Sasha Levin To: "stable@vger.kernel.org" , "linux-kernel@vger.kernel.org" CC: Mel Gorman , "Huang, Ying" , Jan Kara , Andrew Morton , Linus Torvalds , Sasha Levin Subject: [PATCH AUTOSEL for 4.9 262/293] mm: pin address_space before dereferencing it while isolating an LRU page Thread-Topic: [PATCH AUTOSEL for 4.9 262/293] mm: pin address_space before dereferencing it while isolating an LRU page Thread-Index: AQHTz5lhj1H0JiIFD0C/sMrEjAABXQ== Date: Mon, 9 Apr 2018 00:26:19 +0000 Message-ID: <20180409002239.163177-262-alexander.levin@microsoft.com> References: <20180409002239.163177-1-alexander.levin@microsoft.com> In-Reply-To: <20180409002239.163177-1-alexander.levin@microsoft.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [52.168.54.252] x-ms-publictraffictype: Email x-microsoft-exchange-diagnostics: 1;DM5PR2101MB0984;7:sP2Y6LDjXV4IrpSWdF0NcdmxDqwRA12LL9fBF0ez2cWBHsw19WvqUTlO2wrrlyZt2Cus0k/Q99DnLiAeO/FDNYEg2G+QSIn/5Y0p6pzA6q8Gz3fRWf6YehR6UTEb9/Ij8frAGSkXT3Y5VbggE3E5O/nNsgK2TsVtX5qkmyraZPSndu2gx3GvmxSeVuncvCZjFzdc1mwt4C2fVnof34EY3NC8qbdHFldq6o4RRu2Ceyq7GnMRm2rXVGGZfv2NNFf8;20:DxqMBI4kF/FtfwSd1GCm28a2+UlNgMpvJxWelSYCopW9f9Nc2Fw3Tjsy2JGAVDbt+T6B6wI30QvixQ5Xzj/sMiP832JvAS6RZyAs3T09FruBtXSarjpCJRybH1Psnpsh+/58gJAmf5E4QaCstFBpI6XCiukLA4xwPeezjT77uQk= x-ms-office365-filtering-ht: Tenant X-MS-Office365-Filtering-Correlation-Id: ee1775ba-9d4e-4db5-ef4b-08d59db19514 x-microsoft-antispam: UriScan:;BCL:0;PCL:0;RULEID:(7020095)(4652020)(48565401081)(5600026)(4604075)(3008032)(4534165)(4627221)(201703031133081)(201702281549075)(2017052603328)(7193020);SRVR:DM5PR2101MB0984; 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x-microsoft-antispam-message-info: 57UHByP8S8MJsNC5dPIMULmZWIk0NREW1C7WXvpOuvzFvQOnx3JdKwjrC3brLg8FYVYnLzOY/KkuIsG5YwHZji4i2AZXQDxfIgXiFSxt8AeXuJi3jxCk+bPPFtiviwsDZIkEWv5Iff07CmwE9r2hzKRD1m7IwoDtCEvxT8Fu3YMytcJ5ePpZd+Ee0Q4iG5xAUn9aQrdeHlXMeXyCa6qDfmN/w61SkrcCIuI/e1C7en6tms8QYyoNuGQuM+SXdWoZn3YgmRWX+lM6h9QF2zXI8BEPREwHfzeG5trnQ2qZSJKibDviYD/yri2kWpJ7P6roBX2rj8HfSP8ZepuV3na9H3LVrP1PA9pODwkTPZ4nMaeuejz9075leeWSSB4pGejDUGkLD5pNNDMxl0NSaHAI3rxtWR8+auNubPoOtyz5b50= spamdiagnosticoutput: 1:99 spamdiagnosticmetadata: NSPM Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: microsoft.com X-MS-Exchange-CrossTenant-Network-Message-Id: ee1775ba-9d4e-4db5-ef4b-08d59db19514 X-MS-Exchange-CrossTenant-originalarrivaltime: 09 Apr 2018 00:26:19.3632 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47 X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR2101MB0984 Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: From: Mel Gorman [ Upstream commit 69d763fc6d3aee787a3e8c8c35092b4f4960fa5d ] 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 =3D page_mapping(= page); page->mapping =3D 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@techsingular= ity.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 Signed-off-by: Sasha Levin --- mm/vmscan.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index cdd5c3b5c357..d012c13d96f7 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1374,6 +1374,7 @@ int __isolate_lru_page(struct page *page, isolate_mod= e_t mode) =20 if (PageDirty(page)) { struct address_space *mapping; + bool migrate_dirty; =20 /* ISOLATE_CLEAN means only clean pages */ if (mode & ISOLATE_CLEAN) @@ -1382,10 +1383,19 @@ int __isolate_lru_page(struct page *page, isolate_m= ode_t mode) /* * 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 =3D page_mapping(page); - if (mapping && !mapping->a_ops->migratepage) + migrate_dirty =3D mapping && mapping->a_ops->migratepage; + unlock_page(page); + if (!migrate_dirty) return ret; } } --=20 2.15.1