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 8D248C433EF for ; Mon, 31 Jan 2022 18:47:29 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 096308D001D; Mon, 31 Jan 2022 13:47:29 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 01EA08D0001; Mon, 31 Jan 2022 13:47:28 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DDA1D8D001D; Mon, 31 Jan 2022 13:47:28 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0192.hostedemail.com [216.40.44.192]) by kanga.kvack.org (Postfix) with ESMTP id C90598D0001 for ; Mon, 31 Jan 2022 13:47:28 -0500 (EST) Received: from smtpin27.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 87DD01822F674 for ; Mon, 31 Jan 2022 18:47:28 +0000 (UTC) X-FDA: 79091465376.27.2231AD8 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by imf06.hostedemail.com (Postfix) with ESMTP id F2C17180003 for ; Mon, 31 Jan 2022 18:47:27 +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 38247B82B9F; Mon, 31 Jan 2022 18:47:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A03EAC340E8; Mon, 31 Jan 2022 18:47:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1643654844; bh=DULSnU+1gMOpJ8qDJzsCUdXMWDyAxyyNm5+lS9MYR3g=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=d4atJBT6WfV7pu76+LHNtS6T6HipgBT/QvsZOegRJkIir0ZQX4bJ51Z4kVWSPNFVI qbXQRpsWDY9U9+LYaz5DDgkSowNxHtAqweWJyIGDSS7vS7b0rM3Gir1IJVB5FtGdV7 6uh5xtiwD0Rv8I3Jx/Fvw2nnMlTlwL3/S533mljlTXsEGCLMHvh/cxSQP1XSibQnrG Fd7dr42i4EUnfUC4WSrPDlNfPZ+fEI7oxBVHiTATqyk4HTOZG/KStaeTmthvKeqCe8 2UN9mmgfD5EF29XBHDGUmKr7ZDBPf1ol/tu4mvs/2YzflUKDD5hQZ8S5bPPAiwXl0l jgxDwnSV5B/MA== Date: Mon, 31 Jan 2022 20:47:16 +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> <63a8a665-4431-a13c-c320-1b46e5f62005@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: F2C17180003 X-Rspam-User: nil Authentication-Results: imf06.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=d4atJBT6; dmarc=pass (policy=none) header.from=kernel.org; spf=pass (imf06.hostedemail.com: domain of rppt@kernel.org designates 145.40.68.75 as permitted sender) smtp.mailfrom=rppt@kernel.org X-Stat-Signature: y6ab8e938ebet55wd7h6s8wx4x98o7ip X-HE-Tag: 1643654847-725814 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 03:41:05PM +0100, David Hildenbrand wrote: > On 31.01.22 15:28, Mike Rapoport wrote: > > On Mon, Jan 31, 2022 at 03:12:36PM +0100, David Hildenbrand wrote: > >> On 31.01.22 15:05, Mike Rapoport wrote: > >>> 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. > >> > >> I was wondering if the monitor could get confused if he just resolved a > >> page fault via UFFDIO_COPY/ZERO and then receives a REMOVE event. > > > > And why would it be confused? > > My thinking was that the monitor might use REMOVE events to track which > pages are actually populated. If you receive REMOVE after > UFFDIO_COPY/ZERO the monitor would conclude that the page is not > populated, just like if we'd get the MADV_DONTNEED/MADV_REMOVE > immediately after placing a page. I still don't follow your usecase. In CRIU we simply discard whatever content we had to fill when there is REMOVE event. If a page fault occurs in that region we use UFFDIO_ZEROPAGE, just as it would happen in "normal" page fault processing (note, CRIU does not support uffd with hugetlb or shmem) > Of course, it heavily depends on the target use case in the monitor or I > might just be wrong. > > -- > Thanks, > > David / dhildenb > -- Sincerely yours, Mike.