From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f69.google.com (mail-pg0-f69.google.com [74.125.83.69]) by kanga.kvack.org (Postfix) with ESMTP id B43E16B0313 for ; Mon, 24 Jul 2017 11:23:59 -0400 (EDT) Received: by mail-pg0-f69.google.com with SMTP id c14so153593826pgn.11 for ; Mon, 24 Jul 2017 08:23:59 -0700 (PDT) Received: from mga05.intel.com (mga05.intel.com. [192.55.52.43]) by mx.google.com with ESMTPS id h63si4082077pgc.820.2017.07.24.08.23.58 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 24 Jul 2017 08:23:58 -0700 (PDT) Date: Mon, 24 Jul 2017 09:23:57 -0600 From: Ross Zwisler Subject: Re: [PATCH v4 1/5] mm: add mkwrite param to vm_insert_mixed() Message-ID: <20170724152357.GB1639@linux.intel.com> References: <20170721223956.29485-1-ross.zwisler@linux.intel.com> <20170721223956.29485-2-ross.zwisler@linux.intel.com> <20170724112530.GI652@quack2.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170724112530.GI652@quack2.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: Ross Zwisler , 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 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?) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org