Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755985AbdGXP7S (ORCPT ); Mon, 24 Jul 2017 11:59:18 -0400 Received: from mx2.suse.de ([195.135.220.15]:53354 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751671AbdGXP7M (ORCPT ); Mon, 24 Jul 2017 11:59:12 -0400 Date: Mon, 24 Jul 2017 17:59:06 +0200 From: Jan Kara To: Ross Zwisler Cc: Jan Kara , Andrew Morton , linux-kernel@vger.kernel.org, "Darrick J. Wong" , "Theodore Ts'o" , Alexander Viro , Andreas Dilger , Christoph Hellwig , Dan Williams , Dave Chinner , David Airlie , Ingo Molnar , Inki Dae , Jonathan Corbet , Joonyoung Shim , Krzysztof Kozlowski , Kukjin Kim , Kyungmin Park , Matthew Wilcox , Patrik Jakobsson , Rob Clark , Seung-Woo Kim , Steven Rostedt , Tomi Valkeinen , dri-devel@lists.freedesktop.org, freedreno@lists.freedesktop.org, linux-arm-kernel@lists.infradead.org, linux-arm-msm@vger.kernel.org, linux-doc@vger.kernel.org, linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@lists.01.org, linux-samsung-soc@vger.kernel.org, linux-xfs@vger.kernel.org Subject: Re: [PATCH v4 1/5] mm: add mkwrite param to vm_insert_mixed() Message-ID: <20170724155906.GR652@quack2.suse.cz> References: <20170721223956.29485-1-ross.zwisler@linux.intel.com> <20170721223956.29485-2-ross.zwisler@linux.intel.com> <20170724112530.GI652@quack2.suse.cz> <20170724152357.GB1639@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170724152357.GB1639@linux.intel.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2223 Lines: 58 On Mon 24-07-17 09:23:57, Ross Zwisler wrote: > On Mon, Jul 24, 2017 at 01:25:30PM +0200, Jan Kara wrote: > > > @@ -1658,14 +1658,28 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr, > > > if (!pte) > > > goto out; > > > retval = -EBUSY; > > > - if (!pte_none(*pte)) > > > - goto out_unlock; > > > + if (!pte_none(*pte)) { > > > + if (mkwrite) { > > > + if (WARN_ON_ONCE(pte_pfn(*pte) != pfn_t_to_pfn(pfn))) > > > > Is the WARN_ON_ONCE() really appropriate here? Your testcase with private > > mappings has triggered this situation if I'm right... > > Yep, I think this WARN_ON_ONCE() is correct. The test with private mappings > had collisions between read-only DAX mappings which were being faulted in via > insert_pfn(), and read/write COW page cache mappings which were being faulted > in by wp_page_copy(). > > I was hitting a false-positive warning when I had the WARN_ON_ONCE() in > insert_pfn() outside of the mkwrite case, i.e.: > > if (!pte_none(*pte)) { > if (WARN_ON_ONCE(pte_pfn(*pte) != pfn_t_to_pfn(pfn))) > goto out_unlock; > if (mkwrite) { > entry = *pte; > goto out_mkwrite; > } else > goto out_unlock; > } > > This was triggering when one thread was faulting in a read-only DAX mapping > when another thread had already faulted in a read-write COW page cache page. > > The patches I sent out have the warning in the mkwrite case, which would mean > that we were getting a fault for a read/write PTE in insert_pfn() and the PFN > didn't match what was already in the PTE. > > This can't ever happen in the private mapping case because we will never > install a read/write PTE for normal storage, only for COW page cache pages. > Essentially I don't think we should ever be able to hit this warning, and if > we do I'd like to get the bug report so that I can track down how it was > happening and make sure that it's safe. It is in the mkwrite path of > insert_pfn() which is currently only used by the DAX code. > > Does that make sense to you, or would you recommend leaving it out? (If so, > why?) Ah, OK, makes sense. So feel free to add: Reviewed-by: Jan Kara Honza -- Jan Kara SUSE Labs, CR