From: Ross Zwisler Subject: Re: [PATCH v3 1/5] mm: add vm_insert_mixed_mkwrite() Date: Wed, 19 Jul 2017 15:58:31 -0600 Message-ID: <20170719215831.GC10923@linux.intel.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Ross Zwisler , Jan Kara , Andrew Morton , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.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-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org, linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org Return-path: Content-Disposition: inline In-Reply-To: <20170719175112.GA24588-VuQAYsv1563Yd54FQh9/CA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org 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.