From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 07988C2D0DB for ; Tue, 28 Jan 2020 15:26:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CC04B2467E for ; Tue, 28 Jan 2020 15:26:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580225209; bh=dj+J/+PtbTGjVK/oHj202CDxS9GCGCuwxkfK9zJalX8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=2UvsKjvKq5fzR+M+JrEqz7PtAUMl9mWrHsdQU8eCc51gdHB3nYw6xLQJgwM6F6ofc 0Do2Hb7RudHrKZ2TnnAH/CZVMAmr9Gvvc5E0P203hrm5pltM5wBhj68zFdPmVscdIO XNSwE+A0HxXZTMGyViRU59Oezz3NPwPrWjQoHUF4= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726582AbgA1P0t (ORCPT ); Tue, 28 Jan 2020 10:26:49 -0500 Received: from mail.kernel.org ([198.145.29.99]:34362 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726141AbgA1P0t (ORCPT ); Tue, 28 Jan 2020 10:26:49 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B4087207FD; Tue, 28 Jan 2020 15:26:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580225208; bh=dj+J/+PtbTGjVK/oHj202CDxS9GCGCuwxkfK9zJalX8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=OHv6vMz+yD5U8x6jcim8pbmwQnkrN4uZpX7DzpHQsMWV60vUZW+5b6hLCopr+YzlV m58G8lzyFU8S/neJVArtj7ee0qKQp3rxEnBv8B767ntyNTp9k+KoplSvW6qGdVbECq lE7l0SpLW/1wIyvzaUMpqwNlQSy8Oc2/OTqCeXKc= Date: Tue, 28 Jan 2020 15:26:42 +0000 From: Will Deacon To: Brian Geffon Cc: Andrew Morton , "Michael S . Tsirkin" , Arnd Bergmann , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Andy Lutomirski , Andrea Arcangeli , Sonny Rao , Minchan Kim , Joel Fernandes , Yu Zhao , Jesse Barnes , Nathan Chancellor Subject: Re: [PATCH v3] mm: Add MREMAP_DONTUNMAP to mremap(). Message-ID: <20200128152641.GA29776@willie-the-truck> References: <20200123014627.71720-1-bgeffon@google.com> <20200127053056.213679-1-bgeffon@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200127053056.213679-1-bgeffon@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Brian, On Sun, Jan 26, 2020 at 09:30:56PM -0800, Brian Geffon wrote: > When remapping an anonymous, private mapping, if MREMAP_DONTUNMAP is > set, the source mapping will not be removed. Instead it will be > cleared as if a brand new anonymous, private mapping had been created > atomically as part of the mremap() call.  If a userfaultfd was watching > the source, it will continue to watch the new mapping.  For a mapping > that is shared or not anonymous, MREMAP_DONTUNMAP will cause the > mremap() call to fail. MREMAP_DONTUNMAP requires that MREMAP_FIXED is > also used. The final result is two equally sized VMAs where the > destination contains the PTEs of the source. >     > We hope to use this in Chrome OS where with userfaultfd we could write > an anonymous mapping to disk without having to STOP the process or worry > about VMA permission changes. >     > This feature also has a use case in Android, Lokesh Gidra has said > that "As part of using userfaultfd for GC, We'll have to move the physical > pages of the java heap to a separate location. For this purpose mremap > will be used. Without the MREMAP_DONTUNMAP flag, when I mremap the java > heap, its virtual mapping will be removed as well. Therefore, we'll > require performing mmap immediately after. This is not only time consuming > but also opens a time window where a native thread may call mmap and > reserve the java heap's address range for its own usage. This flag > solves the problem." Hmm, this sounds like you're dealing with a multi-threaded environment, yet your change only supports private mappings. How does that work? It's also worrying because, with two private mappings of the same anonymous memory live simultaneously, you run the risk of hitting D-cache aliasing issues on some architectures and losing coherency between them as a result (even in a single-threaded scenario). Is userspace just supposed to deal with this, or should we be enforcing SHMLBA alignment?   > Signed-off-by: Brian Geffon > --- > include/uapi/linux/mman.h | 5 +++-- > mm/mremap.c | 38 +++++++++++++++++++++++++++++++------- > 2 files changed, 34 insertions(+), 9 deletions(-) Could you also a include a patch to update the mremap man page, please? https://www.kernel.org/doc/man-pages/patches.html Cheers, Will