From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753410AbdGXLUl (ORCPT ); Mon, 24 Jul 2017 07:20:41 -0400 Received: from mx2.suse.de ([195.135.220.15]:49105 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751558AbdGXLUe (ORCPT ); Mon, 24 Jul 2017 07:20:34 -0400 Date: Mon, 24 Jul 2017 13:20:30 +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 Hansen , Ingo Molnar , 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 1/5] mm: add vm_insert_mixed_mkwrite() Message-ID: <20170724112030.GH652@quack2.suse.cz> References: <20170628220152.28161-1-ross.zwisler@linux.intel.com> <20170628220152.28161-2-ross.zwisler@linux.intel.com> <20170719141659.GB15908@quack2.suse.cz> <20170719175112.GA24588@linux.intel.com> <20170719215831.GC10923@linux.intel.com> <20170721174405.GA18697@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170721174405.GA18697@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 Fri 21-07-17 11:44:05, Ross Zwisler wrote: > On Wed, Jul 19, 2017 at 03:58:31PM -0600, Ross Zwisler wrote: > > On Wed, Jul 19, 2017 at 11:51:12AM -0600, Ross Zwisler wrote: > > > On Wed, Jul 19, 2017 at 04:16:59PM +0200, Jan Kara wrote: > > > > On Wed 28-06-17 16:01:48, Ross Zwisler wrote: > > > > > 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. > > > > > > > > > > Signed-off-by: Ross Zwisler > > > > > > > > Just one small comment below. > > > > > > > > > @@ -1658,14 +1658,26 @@ 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) { > > > > > + entry = *pte; > > > > > + goto out_mkwrite; > > > > > > > > Can we maybe check here that (pte_pfn(*pte) == pfn_t_to_pfn(pfn)) and > > > > return -EBUSY otherwise? That way we are sure insert_pfn() isn't doing > > > > anything we don't expect > > > > > > Sure, that's fine. I'll add it as a WARN_ON_ONCE() so it's a very loud > > > failure. If the pfns don't match I think we're insane (and would have been > > > insane prior to this patch series as well) because we are getting a page fault > > > and somehow have a different PFN already mapped at that location. > > > > Umm...well, I added the warning, and during my regression testing hit a case > > where the PFNs didn't match. (generic/437 with both ext4 & XFS) > > > > I've verified that this behavior happens with vanilla v4.12, so it's not a new > > condition introduced by my patch. > > > > I'm off tracking that down - there's a bug lurking somewhere, I think. > > Actually, I think we're fine. What was happening was that two faults were > racing for a private mapping. One was installing a RW PTE for the COW page > cache page via wp_page_copy(), and the second was trying to install a > read-only PTE in insert_pfn(). The PFNs don't match because the two faults > are trying to map very different PTEs - one for DAX storage, one for a page > cache page. OK, so two threads (sharing page tables) were doing read and write fault at the same offset of a private mapping. OK, makes sense. > This collision is handled by insert_pfn() by just returning -EBUSY, which will > bail out of the fault and either re-fault if necessary, or use the PTE that > the other thread installed. For the case I described above I think both > faults will just happily use the page cache page, and the RO DAX fault won't > be retried. > > I think this is fine, and I'll preserve this behavior as you suggest in the > mkwrite case by validating that the PTE is what we think it should be after we > grab the PTL. Yeah, that seems to essential for the races of faults in private mappings to work as they should. Thanks for analysing this! Honza -- Jan Kara SUSE Labs, CR