From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754592AbaJMUgc (ORCPT ); Mon, 13 Oct 2014 16:36:32 -0400 Received: from mta-out1.inet.fi ([62.71.2.197]:58657 "EHLO kirsi1.inet.fi" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753581AbaJMUga (ORCPT ); Mon, 13 Oct 2014 16:36:30 -0400 Date: Mon, 13 Oct 2014 23:36:15 +0300 From: "Kirill A. Shutemov" To: Matthew Wilcox Cc: Matthew Wilcox , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v1 2/7] mm: Prepare for DAX huge pages Message-ID: <20141013203615.GA30140@node.dhcp.inet.fi> References: <1412774729-23956-1-git-send-email-matthew.r.wilcox@intel.com> <1412774729-23956-3-git-send-email-matthew.r.wilcox@intel.com> <20141008152124.GA7288@node.dhcp.inet.fi> <20141008155758.GK5098@wil.cx> <20141008194335.GA9232@node.dhcp.inet.fi> <20141009204026.GP5098@wil.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141009204026.GP5098@wil.cx> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 09, 2014 at 04:40:26PM -0400, Matthew Wilcox wrote: > On Wed, Oct 08, 2014 at 10:43:35PM +0300, Kirill A. Shutemov wrote: > > On Wed, Oct 08, 2014 at 11:57:58AM -0400, Matthew Wilcox wrote: > > > On Wed, Oct 08, 2014 at 06:21:24PM +0300, Kirill A. Shutemov wrote: > > > > On Wed, Oct 08, 2014 at 09:25:24AM -0400, Matthew Wilcox wrote: > > > > > From: Matthew Wilcox > > > > > > > > > > DAX wants to use the 'special' bit to mark PMD entries that are not backed > > > > > by struct page, just as for PTEs. > > > > > > > > Hm. I don't see where you use PMD without special set. > > > > > > Right ... I don't currently insert PMDs that point to huge pages of DRAM, > > > only to huge pages of PMEM. > > > > Looks like you don't need pmd_{mk,}special() then. It seems you have all > > inforamtion you need -- vma -- to find out what's going on. Right? > > That would prevent us from putting huge pages of DRAM into a VM_MIXEDMAP | > VM_HUGEPAGE vma. Is that acceptable to the wider peanut gallery? We didn't have huge pages on VM_MIXEDMAP | VM_HUGEPAGE before and we don't have them there after the patchset. Nothing changed. It probably worth adding VM_BUG_ON() in some code path to be able to catch this situation. > > > > No private THP pages with THP? Why? > > > > It should be trivial: we already have a code path for !page case for zero > > > > page and it shouldn't be too hard to modify do_dax_pmd_fault() to support > > > > COW. > > > > > > > > I remeber I've mentioned that you don't think it's reasonable to allocate > > > > 2M page on COW, but that's what we do for anon memory... > > > > > > I agree that it shouldn't be too hard, but I have no evidence that it'll > > > be a performance win to COW 2MB pages for MAP_PRIVATE. I'd rather be > > > cautious for now and we can explore COWing 2MB chunks in a future patch. > > > > I would rather make it other way around: use the same apporoach as for > > anon memory until data shows it's doesn't make any good. Then consider > > switching COW for *both* anon and file THP to fallback path. > > This way we will get consistent behaviour for both types of mappings. > > I'm not sure that we want consistent behaviour for both types of mappings. > My understanding is that they're used for different purposes, and having > different bahaviour is acceptable. This should be described in commit message along with other design solutions (split wrt. mlock, etc) with their pros and cons. -- Kirill A. Shutemov