From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759370AbbLBTHD (ORCPT ); Wed, 2 Dec 2015 14:07:03 -0500 Received: from g4t3426.houston.hp.com ([15.201.208.54]:30429 "EHLO g4t3426.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756260AbbLBTHA (ORCPT ); Wed, 2 Dec 2015 14:07:00 -0500 Message-ID: <1449086521.31589.39.camel@hpe.com> Subject: Re: [PATCH] mm: Fix mmap MAP_POPULATE for DAX pmd mapping From: Toshi Kani To: Dan Williams Cc: Andrew Morton , "Kirill A. Shutemov" , Matthew Wilcox , Ross Zwisler , mauricio.porto@hpe.com, Linux MM , linux-fsdevel , "linux-nvdimm@lists.01.org" , "linux-kernel@vger.kernel.org" Date: Wed, 02 Dec 2015 13:02:01 -0700 In-Reply-To: References: <1448309082-20851-1-git-send-email-toshi.kani@hpe.com> <1449022764.31589.24.camel@hpe.com> <1449078237.31589.30.camel@hpe.com> <1449084362.31589.37.camel@hpe.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 (3.16.5-3.fc22) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2015-12-02 at 11:00 -0800, Dan Williams wrote: > On Wed, Dec 2, 2015 at 11:26 AM, Toshi Kani wrote: > > On Wed, 2015-12-02 at 10:06 -0800, Dan Williams wrote: > > > On Wed, Dec 2, 2015 at 9:01 AM, Dan Williams > > > wrote: > > > > On Wed, Dec 2, 2015 at 9:43 AM, Toshi Kani wrote: > > > > > Oh, I see. I will setup the memmap array and run the tests again. > > > > > > > > > > But, why does the PMD mapping depend on the memmap array? We have > > > > > observed major performance improvement with PMD. This feature should > > > > > always be enabled with DAX regardless of the option to allocate the > > > > > memmap > > > > > array. > > > > > > > > > > > > > Several factors drove this decision, I'm open to considering > > > > alternatives but here's the reasoning: > > > > > > > > 1/ DAX pmd mappings caused crashes in the get_user_pages path leading > > > > to commit e82c9ed41e8 "dax: disable pmd mappings". The reason pte > > > > mappings don't crash and instead trigger -EFAULT is due to the > > > > _PAGE_SPECIAL pte bit. > > > > > > > > 2/ To enable get_user_pages for DAX, in both the page and huge-page > > > > case, we need a new pte bit _PAGE_DEVMAP. > > > > > > > > 3/ Given the pte bits are hard to come I'm assuming we won't get two, > > > > i.e. both _PAGE_DEVMAP and a new _PAGE_SPECIAL for pmds. Even if we > > > > could get a _PAGE_SPECIAL for pmds I'm not in favor of pursuing it. > > > > > > Actually, Dave says they aren't that hard to come by for pmds, so we > > > could go add _PMD_SPECIAL if we really wanted to support the limited > > > page-less DAX-pmd case. > > > > > > But I'm still of the opinion that we run away from the page-less case > > > until it can be made a full class citizen with O_DIRECT for pfn > > > support. > > > > I may be missing something, but per vm_normal_page(), I think _PAGE_SPECIAL > > can > > be substituted by the following check when we do not have the memmap. > > > > if ((vma->vm_flags & VM_PFNMAP) || > > ((vma->vm_flags & VM_MIXEDMAP) && (!pfn_valid(pfn)))) { > > > > This is what I did in this patch for follow_trans_huge_pmd(), although I > > missed > > the pfn_valid() check. > > That works for __get_user_pages but not __get_user_pages_fast where we > don't have access to the vma. __get_user_page_fast already refers current->mm, so we should be able to get the vma, and pass it down to gup_pud_range(). Thanks, -Toshi