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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 09505C433EF for ; Mon, 31 Jan 2022 14:06:09 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 751876B009C; Mon, 31 Jan 2022 09:06:08 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6D9D06B009D; Mon, 31 Jan 2022 09:06:08 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 57A4F6B009E; Mon, 31 Jan 2022 09:06:08 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0027.hostedemail.com [216.40.44.27]) by kanga.kvack.org (Postfix) with ESMTP id 44F106B009C for ; Mon, 31 Jan 2022 09:06:08 -0500 (EST) Received: from smtpin18.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 09DB9824C421 for ; Mon, 31 Jan 2022 14:06:08 +0000 (UTC) X-FDA: 79090756416.18.49334D7 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf12.hostedemail.com (Postfix) with ESMTP id 6985940006 for ; Mon, 31 Jan 2022 14:06:07 +0000 (UTC) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id B6F8CB829CA; Mon, 31 Jan 2022 14:06:05 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 16100C340E8; Mon, 31 Jan 2022 14:06:01 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1643637964; bh=So9Iv50mwaXcx1SzfcPfKkBZeUZtzsa0FE4U2ib7Gkk=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=pHpVoFjJNyhb0tC/G3WiONGnGm7f5SFz8Calk/AzF9tetWPH685joGn1KlcEpZHyv 7e59E56YTh2Hq1NmMo3xsrLgZhP2/KE02alN8z/SrwzkjPlzX9d1Z6BlQDE6EcCqLK jB6zH26It5kNp2cO/yY9DJ97z2UxxQfvm8dDyLEw3xKdZ1uByBpccdU/wnIu6UV9Ie wx+yUC4ODlEDrl2tRdQWdMZHP9nbp5A/PprJvLX+RSUIkIH3j4kluy8jBcB4r1n8Lr ikHZiX6T5arJawTyRalb8y2puNY+CpfDqP4JmAl7EgTAAbtlpo5p2J4P4hvHPnW38F Jg95evjFTMISw== Date: Mon, 31 Jan 2022 16:05:56 +0200 From: Mike Rapoport To: David Hildenbrand Cc: Nadav Amit , Mike Rapoport , Andrea Arcangeli , Peter Xu , Linux-MM Subject: Re: userfaultfd: usability issue due to lack of UFFD events ordering Message-ID: References: <11831b20-0b46-92df-885a-1220430f9257@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <11831b20-0b46-92df-885a-1220430f9257@redhat.com> X-Stat-Signature: 35zzczar5ydfap1cr4muws1on9swztqf X-Rspam-User: nil Authentication-Results: imf12.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=pHpVoFjJ; spf=pass (imf12.hostedemail.com: domain of rppt@kernel.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=rppt@kernel.org; dmarc=pass (policy=none) header.from=kernel.org X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 6985940006 X-HE-Tag: 1643637967-538255 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: On Mon, Jan 31, 2022 at 11:48:27AM +0100, David Hildenbrand wrote: > On 31.01.22 11:42, Mike Rapoport wrote: > > Hi Nadav, > > > > On Sat, Jan 29, 2022 at 10:23:55PM -0800, Nadav Amit wrote: > >> Using userfautlfd and looking at the kernel code, I encountered a usability > >> issue that complicates userspace UFFD-monitor implementation. I obviosuly > >> might be wrong, so I would appreciate a (polite?) feedback. I do have a > >> userspace workaround, but I thought it is worthy to share and to hear your > >> opinion, as well as feedback from other UFFD users. > >> > >> The issue I encountered regards the ordering of UFFD events tbat might not > >> reflect the actual order in which events took place. > >> > >> In more detail, UFFD events (e.g., unmap, fork) are not ordered against > >> themselves [*]. The mm-lock is dropped before notifying the userspace > >> UFFD-monitor, and therefore there is no guarantee as to whether the order of > >> the events actually reflects the order in which the events took place. > >> This can prevent a UFFD-monitor from using the events to track which > >> ranges are mapped. Specifically, UFFD_EVENT_FORK message and a > >> UFFD_EVENT_UNMAP message (which reflects unmap in the parent process) can > >> be reordered, if the events are triggered by two different threads. In > >> this case the UFFD-monitor cannot figure from the events whether the > >> child process has the unmapped memory range still mapped (because fork > >> happened first) or not. > > > > Yeah, it seems that something like this is possible: > > > > > > fork() munmap() > > mmap_write_unlock(); > > mmap_write_lock_killable(); > > do_things(); > > mmap_{read,write}_unlock(); > > userfaultfd_unmap_complete(); > > dup_userfaultfd_complete(); > > > > I was thinking about other possible races, e.g., MADV_DONTNEED/MADV_FREE > racing with UFFD_EVENT_PAGEFAULT -- where we only hold the mmap_lock in > read mode. But not sure if they apply. The userspace can live with these, at least for uffd missing page faults. If the monitor will try to resolve a page fault for a removed area, the errno from UFFDIO_COPY/ZERO can be used to detect such races. > The fork() vs. munmap() is somewhat "obviously problematic" :) Nothing funny about it ;-) > -- > Thanks, > > David / dhildenb > > -- Sincerely yours, Mike.