From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f199.google.com (mail-wr0-f199.google.com [209.85.128.199]) by kanga.kvack.org (Postfix) with ESMTP id EE9CA6B0279 for ; Fri, 7 Jul 2017 04:20:03 -0400 (EDT) Received: by mail-wr0-f199.google.com with SMTP id v88so6281953wrb.1 for ; Fri, 07 Jul 2017 01:20:03 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id v193si2322061wme.197.2017.07.07.01.20.02 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 07 Jul 2017 01:20:02 -0700 (PDT) Received: from pps.filterd (m0098420.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v678JDml026586 for ; Fri, 7 Jul 2017 04:20:01 -0400 Received: from e23smtp05.au.ibm.com (e23smtp05.au.ibm.com [202.81.31.147]) by mx0b-001b2d01.pphosted.com with ESMTP id 2bhwbyrrjj-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Fri, 07 Jul 2017 04:20:00 -0400 Received: from localhost by e23smtp05.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 7 Jul 2017 18:19:58 +1000 Received: from d23av06.au.ibm.com (d23av06.au.ibm.com [9.190.235.151]) by d23relay09.au.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id v678JspO7143758 for ; Fri, 7 Jul 2017 18:19:54 +1000 Received: from d23av06.au.ibm.com (localhost [127.0.0.1]) by d23av06.au.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id v678Js61008806 for ; Fri, 7 Jul 2017 18:19:54 +1000 Subject: Re: [RFC PATCH 0/1] mm/mremap: add MREMAP_MIRROR flag References: <1499357846-7481-1-git-send-email-mike.kravetz@oracle.com> From: Anshuman Khandual Date: Fri, 7 Jul 2017 13:49:46 +0530 MIME-Version: 1.0 In-Reply-To: <1499357846-7481-1-git-send-email-mike.kravetz@oracle.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: <6f1460ef-a896-aef4-c0dc-66227232e025@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Mike Kravetz , linux-mm@kvack.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Andrew Morton , Andrea Arcangeli , Michal Hocko , Aaron Lu , "Kirill A . Shutemov" On 07/06/2017 09:47 PM, Mike Kravetz wrote: > The mremap system call has the ability to 'mirror' parts of an existing > mapping. To do so, it creates a new mapping that maps the same pages as > the original mapping, just at a different virtual address. This > functionality has existed since at least the 2.6 kernel [1]. A comment > was added to the code to help preserve this feature. Is this the comment ? If yes, then its not very clear. /* * We allow a zero old-len as a special case * for DOS-emu "duplicate shm area" thing. But * a zero new-len is nonsensical. */ > > The Oracle JVM team has discovered this feature and used it while > prototyping a new garbage collection model. This new model shows promise, > and they are considering its use in a future release. However, since > the only mention of this functionality is a single comment in the kernel, > they are concerned about its future. > > I propose the addition of a new MREMAP_MIRROR flag to explicitly request > this functionality. The flag simply provides the same functionality as > the existing undocumented 'old_size == 0' interface. As an alternative, > we could simply document the 'old_size == 0' interface in the man page. > In either case, man page modifications would be needed. Right. Adding MREMAP_MIRROR sounds cleaner from application programming point of view. But it extends the interface. > > Future Direction > > After more formally adding this to the API (either new flag or documenting > existing interface), the mremap code could be enhanced to optimize this > case. Currently, 'mirroring' only sets up the new mapping. It does not > create page table entries for new mapping. This could be added as an > enhancement. Then how it achieves mirroring, both the pointers should see the same data, that can happen with page table entries pointing to same pages, right ? -- 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