From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933503AbdGSPdW (ORCPT ); Wed, 19 Jul 2017 11:33:22 -0400 Received: from mx2.suse.de ([195.135.220.15]:40277 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S933364AbdGSPdS (ORCPT ); Wed, 19 Jul 2017 11:33:18 -0400 Date: Wed, 19 Jul 2017 17:33:14 +0200 From: Jan Kara To: Ross Zwisler Cc: Andrew Morton , linux-kernel@vger.kernel.org, "Darrick J. Wong" , "Theodore Ts'o" , Alexander Viro , Andreas Dilger , Christoph Hellwig , Dan Williams , Dave Hansen , Ingo Molnar , Jan Kara , Jonathan Corbet , Matthew Wilcox , Steven Rostedt , 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-xfs@vger.kernel.org Subject: Re: [PATCH v3 3/5] dax: use common 4k zero page for dax mmap reads Message-ID: <20170719153314.GC15908@quack2.suse.cz> References: <20170628220152.28161-1-ross.zwisler@linux.intel.com> <20170628220152.28161-4-ross.zwisler@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170628220152.28161-4-ross.zwisler@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 On Wed 28-06-17 16:01:50, Ross Zwisler wrote: > Another major change is that we remove dax_pfn_mkwrite() from our fault > flow, and instead rely on the page fault itself to make the PTE dirty and > writeable. The following description from the patch adding the > vm_insert_mixed_mkwrite() call explains this a little more: > > *** > To be able to use the common 4k zero page in DAX we need to have our PTE > fault path look more like our PMD fault path where a PTE entry can be > marked as dirty and writeable as it is first inserted, rather than > waiting for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call. > > Right now we can rely on having a dax_pfn_mkwrite() call because we can > distinguish between these two cases in do_wp_page(): > > case 1: 4k zero page => writable DAX storage > case 2: read-only DAX storage => writeable DAX storage > > This distinction is made by via vm_normal_page(). vm_normal_page() > returns false for the common 4k zero page, though, just as it does for > DAX ptes. Instead of special casing the DAX + 4k zero page case, we will > simplify our DAX PTE page fault sequence so that it matches our DAX PMD > sequence, and get rid of dax_pfn_mkwrite() completely. > > This means that insert_pfn() needs to follow the lead of insert_pfn_pmd() > and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set > insert_pfn() will do the work that was previously done by wp_page_reuse() > as part of the dax_pfn_mkwrite() call path. > *** Hum, thinking about this in context of this patch... So what if we have allocated storage, a process faults it read-only, we map it to page tables writeprotected. Then the process writes through mmap to the area - the code in handle_pte_fault() ends up in do_wp_page() if I'm reading it right. Then, since we are missing ->pfn_mkwrite() handlers, the PTE will be marked writeable but radix tree entry stays clean - bug. Am I missing something? Honza -- Jan Kara SUSE Labs, CR