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=-5.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, 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 D91A5C33CB2 for ; Tue, 28 Jan 2020 15:26:50 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 73F8A2468A for ; Tue, 28 Jan 2020 15:26:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="OHv6vMz+" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 73F8A2468A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id E7AB86B000C; Tue, 28 Jan 2020 10:26:49 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id E2B086B000D; Tue, 28 Jan 2020 10:26:49 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D41596B000E; Tue, 28 Jan 2020 10:26:49 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0114.hostedemail.com [216.40.44.114]) by kanga.kvack.org (Postfix) with ESMTP id BB2EB6B000C for ; Tue, 28 Jan 2020 10:26:49 -0500 (EST) Received: from smtpin08.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id 8B012181AC9B6 for ; Tue, 28 Jan 2020 15:26:49 +0000 (UTC) X-FDA: 76427420538.08.farm83_1e6d128398062 X-HE-Tag: farm83_1e6d128398062 X-Filterd-Recvd-Size: 4312 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf35.hostedemail.com (Postfix) with ESMTP for ; Tue, 28 Jan 2020 15:26:48 +0000 (UTC) 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 In-Reply-To: <20200127053056.213679-1-bgeffon@google.com> User-Agent: Mutt/1.10.1 (2018-07-13) Content-Transfer-Encoding: quoted-printable X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. =A0If a userfaultfd was watchi= ng > the source, it will continue to watch the new mapping. =A0For 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. > =A0 =A0 > 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 worr= y > about VMA permission changes. > =A0 =A0 > 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 physi= cal > 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 consum= ing > 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 anonymo= us memory live simultaneously, you run the risk of hitting D-cache aliasing issues on some architectures and losing coherency between them as a resul= t (even in a single-threaded scenario). Is userspace just supposed to deal with this, or should we be enforcing SHMLBA alignment? =A0 > 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