Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755402AbbBHLsS (ORCPT ); Sun, 8 Feb 2015 06:48:18 -0500 Received: from mail-wi0-f177.google.com ([209.85.212.177]:47375 "EHLO mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753911AbbBHLsQ (ORCPT ); Sun, 8 Feb 2015 06:48:16 -0500 MIME-Version: 1.0 In-Reply-To: <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> <20150205213939.GA3364@wil.cx> Date: Sun, 8 Feb 2015 13:48:14 +0200 Message-ID: Subject: Re: [PATCH v12 04/20] mm: Allow page fault handlers to perform the COW From: Yigal Korman To: Matthew Wilcox Cc: Matthew Wilcox , linux-fsdevel@vger.kernel.org, "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , Andrew Morton Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2489 Lines: 62 On Thu, Feb 5, 2015 at 11:39 PM, Matthew Wilcox wrote: > > 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? I was thinking that do_shared_fault should simply call maybe_mkwrite() in case of VM_FAULT_NOPAGE. I think it's what do_wp_page does afterwards anyway: entry = maybe_mkwrite(pte_mkdirty(entry), vma); But I'm sure it's not the whole picture... Help from MM would indeed be appreciated. Y -- 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/