From patchwork Wed May 17 17:16:39 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ross Zwisler X-Patchwork-Id: 9731735 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id F099C6022E for ; Wed, 17 May 2017 17:21:18 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 01E3D28689 for ; Wed, 17 May 2017 17:21:19 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id EA9E6287D3; Wed, 17 May 2017 17:21:18 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=unavailable version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 7D31328689 for ; Wed, 17 May 2017 17:21:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753657AbdEQRSK (ORCPT ); Wed, 17 May 2017 13:18:10 -0400 Received: from mga11.intel.com ([192.55.52.93]:26481 "EHLO mga11.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752995AbdEQRQ5 (ORCPT ); Wed, 17 May 2017 13:16:57 -0400 Received: from fmsmga005.fm.intel.com ([10.253.24.32]) by fmsmga102.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 17 May 2017 10:16:56 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.38,355,1491289200"; d="scan'208";a="102912400" Received: from theros.lm.intel.com ([10.232.112.77]) by fmsmga005.fm.intel.com with ESMTP; 17 May 2017 10:16:55 -0700 From: Ross Zwisler To: Andrew Morton , linux-kernel@vger.kernel.org Cc: Ross Zwisler , "Darrick J. Wong" , Alexander Viro , Christoph Hellwig , Dan Williams , Dave Hansen , Jan Kara , Matthew Wilcox , linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@lists.01.org, "Kirill A . Shutemov" , Pawel Lebioda , Dave Jiang , Xiong Zhou , Eryu Guan , stable@vger.kernel.org Subject: [PATCH 2/2] dax: Fix race between colliding PMD & PTE entries Date: Wed, 17 May 2017 11:16:39 -0600 Message-Id: <20170517171639.14501-2-ross.zwisler@linux.intel.com> X-Mailer: git-send-email 2.9.4 In-Reply-To: <20170517171639.14501-1-ross.zwisler@linux.intel.com> References: <20170517171639.14501-1-ross.zwisler@linux.intel.com> Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP We currently have two related PMD vs PTE races in the DAX code. These can both be easily triggered by having two threads reading and writing simultaneously to the same private mapping, with the key being that private mapping reads can be handled with PMDs but private mapping writes are always handled with PTEs so that we can COW. Here is the first race: CPU 0 CPU 1 (private mapping write) __handle_mm_fault() create_huge_pmd() - FALLBACK handle_pte_fault() passes check for pmd_devmap() (private mapping read) __handle_mm_fault() create_huge_pmd() dax_iomap_pmd_fault() inserts PMD dax_iomap_pte_fault() does a PTE fault, but we already have a DAX PMD installed in our page tables at this spot. Here's the second race: CPU 0 CPU 1 (private mapping write) __handle_mm_fault() create_huge_pmd() - FALLBACK (private mapping read) __handle_mm_fault() passes check for pmd_none() create_huge_pmd() handle_pte_fault() dax_iomap_pte_fault() inserts PTE dax_iomap_pmd_fault() inserts PMD, but we already have a PTE at this spot. The core of the issue is that while there is isolation between faults to the same range in the DAX fault handlers via our DAX entry locking, there is no isolation between faults in the code in mm/memory.c. This means for instance that this code in __handle_mm_fault() can run: if (pmd_none(*vmf.pmd) && transparent_hugepage_enabled(vma)) { ret = create_huge_pmd(&vmf); But by the time we actually get to run the fault handler called by create_huge_pmd(), the PMD is no longer pmd_none() because a racing PTE fault has installed a normal PMD here as a parent. This is the cause of the 2nd race. The first race is similar - there is the following check in handle_pte_fault(): } else { /* See comment in pte_alloc_one_map() */ if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd)) return 0; So if a pmd_devmap() PMD (a DAX PMD) has been installed at vmf->pmd, we will bail and retry the fault. This is correct, but there is nothing preventing the PMD from being installed after this check but before we actually get to the DAX PTE fault handlers. In my testing these races result in the following types of errors: BUG: Bad rss-counter state mm:ffff8800a817d280 idx:1 val:1 BUG: non-zero nr_ptes on freeing mm: 15 Fix this issue by having the DAX fault handlers verify that it is safe to continue their fault after they have taken an entry lock to block other racing faults. Signed-off-by: Ross Zwisler Reported-by: Pawel Lebioda Cc: stable@vger.kernel.org --- I've written a new xfstest for this race, which I will send in response to this patch series. This series has also survived an xfstest run without any new issues. --- fs/dax.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/fs/dax.c b/fs/dax.c index c22eaf1..3cc02d1 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -1155,6 +1155,15 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf, } /* + * It is possible, particularly with mixed reads & writes to private + * mappings, that we have raced with a PMD fault that overlaps with + * the PTE we need to set up. Now that we have a locked mapping entry + * we can safely unmap the huge PMD so that we can install our PTE in + * our page tables. + */ + split_huge_pmd(vmf->vma, vmf->pmd, vmf->address); + + /* * Note that we don't bother to use iomap_apply here: DAX required * the file system block size to be equal the page size, which means * that we never have to deal with more than a single extent here. @@ -1398,6 +1407,15 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf, goto fallback; /* + * It is possible, particularly with mixed reads & writes to private + * mappings, that we have raced with a PTE fault that overlaps with + * the PMD we need to set up. If so we just fall back to a PTE fault + * ourselves. + */ + if (!pmd_none(*vmf->pmd)) + goto unlock_entry; + + /* * Note that we don't use iomap_apply here. We aren't doing I/O, only * setting up a mapping, so really we're using iomap_begin() as a way * to look up our filesystem block.