From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1031566AbeCAOrj (ORCPT ); Thu, 1 Mar 2018 09:47:39 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:45628 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1031450AbeCAOrh (ORCPT ); Thu, 1 Mar 2018 09:47:37 -0500 Date: Thu, 1 Mar 2018 15:47:30 +0100 From: Martin Schwidefsky To: "Kirill A. Shutemov" Cc: Matthew Wilcox , linux-mm@kvack.org, Matthew Wilcox , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 0/4] Split page_type out from mapcount In-Reply-To: <20180301124412.gm6jxwzyfskzxspa@node.shutemov.name> References: <20180228223157.9281-1-willy@infradead.org> <20180301081750.42b135c3@mschwideX1> <20180301124412.gm6jxwzyfskzxspa@node.shutemov.name> X-Mailer: Claws Mail 3.13.2 (GTK+ 2.24.30; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 x-cbid: 18030114-0040-0000-0000-000004397E56 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18030114-0041-0000-0000-000020DC82F8 Message-Id: <20180301154730.3bfeaf03@mschwideX1> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10432:,, definitions=2018-03-01_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1803010186 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 1 Mar 2018 15:44:12 +0300 "Kirill A. Shutemov" wrote: > On Thu, Mar 01, 2018 at 08:17:50AM +0100, Martin Schwidefsky wrote: > > On Wed, 28 Feb 2018 14:31:53 -0800 > > Matthew Wilcox wrote: > > > > > From: Matthew Wilcox > > > > > > I want to use the _mapcount field to record what a page is in use as. > > > This can help with debugging and we can also expose that information to > > > userspace through /proc/kpageflags to help diagnose memory usage (not > > > included as part of this patch set). > > > > > > First, we need s390 to stop using _mapcount for its own purposes; > > > Martin, I hope you have time to look at this patch. I must confess I > > > don't quite understand what the different bits are used for in the upper > > > nybble of the _mapcount, but I tried to replicate what you were doing > > > faithfully. > > > > Yeah, that is a nasty bit of code. On s390 we have 2K page tables (pte) > > but 4K pages. If we use full pages for the pte tables we waste 2K of > > memory for each of the tables. So we allocate 4K and split it into two > > 2K pieces. Now we have to keep track of the pieces to be able to free > > them again. > > Have you considered to use slab for page table allocation instead? > IIRC some architectures practice this already. Well there is a complication with KVM and the page table management for gmaps. If mm_alloc_pgste(mm) == true then a 4K page page table has to be allocated. For the gmap I need a place to store an 8 byte value, currently we use page->index. But the slab/slub code uses page->index for its own purpose. This creates a conflict, but maybe doing a get_free_page for mm_alloc_pgste(mm) == true and using a slab cache for normal page tables might work. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.