From: Ross Zwisler Subject: Re: [PATCH v3 1/5] mm: add vm_insert_mixed_mkwrite() Date: Wed, 19 Jul 2017 11:51:12 -0600 Message-ID: <20170719175112.GA24588@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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Theodore Ts'o , Matthew Wilcox , linux-doc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, "Darrick J. Wong" , Jonathan Corbet , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Steven Rostedt , Christoph Hellwig , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, Dave Hansen , Ingo Molnar , Andreas Dilger , Alexander Viro , linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andrew Morton , linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org To: Jan Kara Return-path: Content-Disposition: inline In-Reply-To: <20170719141659.GB15908-4I4JzKEfoa/jFM9bn6wA6Q@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 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. > and if I understand the code right, we need to > invalidate all zero page mappings at given file offset (via > unmap_mapping_range()) before mapping an allocated block there and thus the > case of filling the hole won't be affected by this? Correct. Here's the call tree if we already have a zero page mapped and are now faulting in an allocated block: dax_iomap_pte_fault() dax_insert_mapping() dax_insert_mapping_entry() unmap_mapping_range() for our zero page vm_insert_mixed_mkwrite() installs the new PTE. We have pte_none(), so we skip the new mkwrite goto in insert_pfn().