From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 202C06B0279 for ; Fri, 7 Jul 2017 13:45:38 -0400 (EDT) Received: by mail-wr0-f198.google.com with SMTP id z81so9634190wrc.2 for ; Fri, 07 Jul 2017 10:45:38 -0700 (PDT) Received: from mail-wr0-x243.google.com (mail-wr0-x243.google.com. [2a00:1450:400c:c0c::243]) by mx.google.com with ESMTPS id b8si2496520wrb.130.2017.07.07.10.45.36 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 07 Jul 2017 10:45:36 -0700 (PDT) Received: by mail-wr0-x243.google.com with SMTP id z45so9185398wrb.2 for ; Fri, 07 Jul 2017 10:45:36 -0700 (PDT) Date: Fri, 7 Jul 2017 20:45:34 +0300 From: "Kirill A. Shutemov" Subject: Re: [RFC PATCH 1/1] mm/mremap: add MREMAP_MIRROR flag for existing mirroring functionality Message-ID: <20170707174534.wdfbciyfpovi52dy@node.shutemov.name> 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Mike Kravetz 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 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. -- Kirill A. Shutemov -- 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