From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mga14.intel.com (mga14.intel.com [192.55.52.115]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id AA6FC21951C94 for ; Tue, 25 Apr 2017 15:59:38 -0700 (PDT) Date: Tue, 25 Apr 2017 16:59:36 -0600 From: Ross Zwisler Subject: Re: [PATCH 2/2] dax: fix data corruption due to stale mmap reads Message-ID: <20170425225936.GA29655@linux.intel.com> References: <20170420191446.GA21694@linux.intel.com> <20170421034437.4359-1-ross.zwisler@linux.intel.com> <20170421034437.4359-2-ross.zwisler@linux.intel.com> <20170425111043.GH2793@quack2.suse.cz> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20170425111043.GH2793@quack2.suse.cz> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-nvdimm-bounces@lists.01.org Sender: "Linux-nvdimm" To: Jan Kara Cc: Latchesar Ionkov , Trond Myklebust , linux-mm@kvack.org, Christoph Hellwig , linux-cifs@vger.kernel.org, Matthew Wilcox , Andrey Ryabinin , Eric Van Hensbergen , linux-nvdimm@lists.01.org, Alexander Viro , v9fs-developer@lists.sourceforge.net, Jens Axboe , linux-nfs@vger.kernel.org, "Darrick J. Wong" , samba-technical@lists.samba.org, linux-kernel@vger.kernel.org, Steve French , Alexey Kuznetsov , Johannes Weiner , linux-fsdevel@vger.kernel.org, Ron Minnich , Andrew Morton , Anna Schumaker List-ID: On Tue, Apr 25, 2017 at 01:10:43PM +0200, Jan Kara wrote: <> > Hum, but now thinking more about it I have hard time figuring out why write > vs fault cannot actually still race: > > CPU1 - write(2) CPU2 - read fault > > dax_iomap_pte_fault() > ->iomap_begin() - sees hole > dax_iomap_rw() > iomap_apply() > ->iomap_begin - allocates blocks > dax_iomap_actor() > invalidate_inode_pages2_range() > - there's nothing to invalidate > grab_mapping_entry() > - we add zero page in the radix > tree & map it to page tables > > Similarly read vs write fault may end up racing in a wrong way and try to > replace already existing exceptional entry with a hole page? Yep, this race seems real to me, too. This seems very much like the issues that exist when a thread is doing direct I/O. One thread is doing I/O to an intermediate buffer (page cache for direct I/O case, zero page for us), and the other is going around it directly to media, and they can get out of sync. IIRC the direct I/O code looked something like: 1/ invalidate existing mappings 2/ do direct I/O to media 3/ invalidate mappings again, just in case. Should be cheap if there weren't any conflicting faults. This makes sure any new allocations we made are faulted in. I guess one option would be to replicate that logic in the DAX I/O path, or we could try and enhance our locking so page faults can't race with I/O since both can allocate blocks. I'm not sure, but will think on it. _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm