From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f197.google.com (mail-qt0-f197.google.com [209.85.216.197]) by kanga.kvack.org (Postfix) with ESMTP id 1AA376B000C for ; Thu, 1 Mar 2018 09:47:38 -0500 (EST) Received: by mail-qt0-f197.google.com with SMTP id c26so5067860qtj.14 for ; Thu, 01 Mar 2018 06:47:38 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id k38si1109260qkh.198.2018.03.01.06.47.37 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 01 Mar 2018 06:47:37 -0800 (PST) Received: from pps.filterd (m0098420.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w21EjNDa054551 for ; Thu, 1 Mar 2018 09:47:36 -0500 Received: from e06smtp11.uk.ibm.com (e06smtp11.uk.ibm.com [195.75.94.107]) by mx0b-001b2d01.pphosted.com with ESMTP id 2gek14ss2g-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 01 Mar 2018 09:47:36 -0500 Received: from localhost by e06smtp11.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 1 Mar 2018 14:47:34 -0000 Date: Thu, 1 Mar 2018 15:47:30 +0100 From: Martin Schwidefsky 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> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-Id: <20180301154730.3bfeaf03@mschwideX1> Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: Matthew Wilcox , linux-mm@kvack.org, Matthew Wilcox , 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org