Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751407AbbBEVjo (ORCPT ); Thu, 5 Feb 2015 16:39:44 -0500 Received: from mga02.intel.com ([134.134.136.20]:17382 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750961AbbBEVjm (ORCPT ); Thu, 5 Feb 2015 16:39:42 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.97,862,1389772800"; d="scan'208";a="450439127" Date: Thu, 5 Feb 2015 16:39:39 -0500 From: Matthew Wilcox To: Yigal Korman Cc: Matthew Wilcox , linux-fsdevel@vger.kernel.org, "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , willy@linux.intel.com, Andrew Morton Subject: Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW Message-ID: <20150205213939.GA3364@wil.cx> References: <1414185652-28663-1-git-send-email-matthew.r.wilcox@intel.com> <1414185652-28663-5-git-send-email-matthew.r.wilcox@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2039 Lines: 49 On Thu, Feb 05, 2015 at 11:16:53AM +0200, Yigal Korman wrote: > I have a question on a related issue (I think). > I've noticed that for pfn-only mappings (VM_FAULT_NOPAGE) > do_shared_fault only maps the pfn with r/o permissions. > So if I use DAX to write the mmap()-ed pfn I get two faults - first > handled by do_shared_fault and then again for making it r/w in > do_wp_page. > Is this simply a missing optimization like was done here with the > cow_page? or am I missing something? I have also noticed this behaviour. I tracked down why it's happening: DAX calls: error = vm_insert_mixed(vma, vaddr, pfn); which calls: return insert_pfn(vma, addr, pfn, vma->vm_page_prot); If you insert some debugging, you'll notice here that vm_page_prot does not include PROT_WRITE. That got cleared during mmap_region() where it does: if (vma_wants_writenotify(vma)) { pgprot_t pprot = vma->vm_page_prot; ... vma->vm_page_prot = vm_get_page_prot(vm_flags & ~VM_SHARED); And why do we want writenotify (according to the VM)? Because we have: /* The backer wishes to know when pages are first written to? */ if (vma->vm_ops && vma->vm_ops->page_mkwrite) return 1; We don't really want to be notified on a first write; we want the page to be inserted write-enabled. But in the case where we've covered a hole with a read-only zero page, we need to be notified so we can allocate a page of storage. So, how to fix? We could adjust vm_page_prot to include PROT_WRITE. I think that should work, since we'll only insert zeroed pages for read faults, and so the maybe_mkwrite() won't be called in do_set_pte(). I'm just not entirely sure where to set it. Perhaps a MM person could make a helpful suggestion? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/