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[148.163.156.1]) by mx.google.com with ESMTPS id d34si2784656pld.416.2017.07.09.00.32.14 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Sun, 09 Jul 2017 00:32:14 -0700 (PDT) Received: from pps.filterd (m0098409.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v697TFMH131208 for ; Sun, 9 Jul 2017 03:32:14 -0400 Received: from e23smtp05.au.ibm.com (e23smtp05.au.ibm.com [202.81.31.147]) by mx0a-001b2d01.pphosted.com with ESMTP id 2bjtpt4ar7-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Sun, 09 Jul 2017 03:32:14 -0400 Received: from localhost by e23smtp05.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Sun, 9 Jul 2017 17:32:11 +1000 Received: from d23av01.au.ibm.com (d23av01.au.ibm.com [9.190.234.96]) by d23relay08.au.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id v697W9Vm14680250 for ; Sun, 9 Jul 2017 17:32:09 +1000 Received: from d23av01.au.ibm.com (localhost [127.0.0.1]) by d23av01.au.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id v697W8hS012667 for ; Sun, 9 Jul 2017 17:32:08 +1000 Subject: Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality 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> <79eca23d-9f1a-9713-3f6b-8f7598d53190@oracle.com> From: Anshuman Khandual Date: Sun, 9 Jul 2017 13:02:02 +0530 MIME-Version: 1.0 In-Reply-To: <79eca23d-9f1a-9713-3f6b-8f7598d53190@oracle.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Message-Id: <662d372a-5737-5f0b-8ac1-c997f3a935eb@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Mike Kravetz , "Kirill A. Shutemov" 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" On 07/07/2017 11:39 PM, Mike Kravetz wrote: > 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. > Yeah, in my experiment, after the mremap() exists we have two different VMAs which can contain two different set of data. No page sharing is happening. -- 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