From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752431AbdGGSKs (ORCPT ); Fri, 7 Jul 2017 14:10:48 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:17217 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750941AbdGGSKq (ORCPT ); Fri, 7 Jul 2017 14:10:46 -0400 Subject: Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality To: "Kirill A. Shutemov" References: <1499357846-7481-1-git-send-email-mike.kravetz@oracle.com> <1499357846-7481-2-git-send-email-mike.kravetz@oracle.com> <20170707102324.kfihkf72sjcrtn5b@node.shutemov.name> <20170707174534.wdfbciyfpovi52dy@node.shutemov.name> Cc: linux-mm@kvack.org, linux-api@vger.kernel.org, linux-kernel@vger.kernel.org, Andrew Morton , Andrea Arcangeli , Michal Hocko , Aaron Lu , "Kirill A . Shutemov" From: Mike Kravetz Message-ID: <79eca23d-9f1a-9713-3f6b-8f7598d53190@oracle.com> Date: Fri, 7 Jul 2017 11:09:26 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0 MIME-Version: 1.0 In-Reply-To: <20170707174534.wdfbciyfpovi52dy@node.shutemov.name> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Source-IP: aserv0021.oracle.com [141.146.126.233] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/07/2017 10:45 AM, Kirill A. Shutemov wrote: > On Fri, Jul 07, 2017 at 10:29:52AM -0700, Mike Kravetz wrote: >> On 07/07/2017 03:23 AM, Kirill A. Shutemov wrote: >>> On Thu, Jul 06, 2017 at 09:17:26AM -0700, 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. >>>> >>>> This patch simply adds a new flag to mremap which will make this >>>> functionality part of the API. It maintains backward compatibility with >>>> the existing way of requesting mirroring (old_size == 0). >>>> >>>> If this new MREMAP_MIRROR flag is specified, then new_size must equal >>>> old_size. In addition, the MREMAP_MAYMOVE flag must be specified. >>> >>> The patch breaks important invariant that anon page can be mapped into a >>> process only once. >> >> Actually, the patch does not add any new functionality. It only provides >> a new interface to existing functionality. >> >> Is it not possible to have an anon page mapped twice into the same process >> via system V shared memory? shmget(anon), shmat(), shmat. >> Of course, those are shared rather than private anon pages. > > By anon pages I mean, private anon or file pages. These are subject to CoW. > >>> What is going to happen to mirrored after CoW for instance? >>> >>> In my opinion, it shouldn't be allowed for anon/private mappings at least. >>> And with this limitation, I don't see much sense in the new interface -- >>> just create mirror by mmap()ing the file again. >> >> The code today works for anon shared mappings. See simple program below. >> >> You are correct in that it makes little or no sense for private mappings. >> When looking closer at existing code, mremap() creates a new private >> mapping in this case. This is most likely a bug. > > IIRC, existing code doesn't create mirrors of private pages as it requires > old_len to be zero. There's no way to get private pages mapped twice this > way. Correct. As mentioned above, mremap does 'something' for private anon pages when old_len == 0. However, this may be considered a bug. In this case, mremap creates a new private anon mapping of length new_size. Since old_len == 0, it does not unmap any of the old mapping. So, in this case mremap basically creates a new private mapping (unrealted to the original) and does not modify the old mapping. -- Mike Kravetz