From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752849AbdK3SRp (ORCPT ); Thu, 30 Nov 2017 13:17:45 -0500 Received: from mx2.suse.de ([195.135.220.15]:39009 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751936AbdK3SRo (ORCPT ); Thu, 30 Nov 2017 13:17:44 -0500 Date: Thu, 30 Nov 2017 19:17:41 +0100 From: Michal Hocko To: Dan Williams Cc: Andrew Morton , Linux MM , "linux-kernel@vger.kernel.org" , Christoph Hellwig , "stable@vger.kernel.org" , "linux-nvdimm@lists.01.org" Subject: Re: [PATCH v3 1/4] mm: introduce get_user_pages_longterm Message-ID: <20171130181741.2y5nyflyhqxg6y5p@dhcp22.suse.cz> References: <151197872943.26211.6551382719053304996.stgit@dwillia2-desk3.amr.corp.intel.com> <151197873499.26211.11687422577653326365.stgit@dwillia2-desk3.amr.corp.intel.com> <20171130095323.ovrq2nenb6ztiapy@dhcp22.suse.cz> <20171130174201.stbpuye4gu5rxwkm@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 30-11-17 10:03:26, Dan Williams wrote: > On Thu, Nov 30, 2017 at 9:42 AM, Michal Hocko wrote: > > > > On Thu 30-11-17 08:39:51, Dan Williams wrote: > > > On Thu, Nov 30, 2017 at 1:53 AM, Michal Hocko wrote: > > > > On Wed 29-11-17 10:05:35, Dan Williams wrote: > > > >> Until there is a solution to the dma-to-dax vs truncate problem it is > > > >> not safe to allow long standing memory registrations against > > > >> filesytem-dax vmas. Device-dax vmas do not have this problem and are > > > >> explicitly allowed. > > > >> > > > >> This is temporary until a "memory registration with layout-lease" > > > >> mechanism can be implemented for the affected sub-systems (RDMA and > > > >> V4L2). > > > > > > > > One thing is not clear to me. Who is allowed to pin pages for ever? > > > > Is it possible to pin LRU pages that way as well? If yes then there > > > > absolutely has to be a limit for that. Sorry I could have studied the > > > > code much more but from a quick glance it seems to me that this is not > > > > limited to dax (or non-LRU in general) pages. > > > > > > I would turn this question around. "who can not tolerate a page being > > > pinned forever?". > > > > Any struct page on the movable zone or anything that is living on the > > LRU list because such a memory is unreclaimable. > > > > > In the case of filesytem-dax a page is > > > one-in-the-same object as a filesystem-block, and a filesystem expects > > > that its operations will not be blocked indefinitely. LRU pages can > > > continue to be pinned indefinitely because operations can continue > > > around the pinned page, i.e. every agent, save for the dma agent, > > > drops their reference to the page and its tolerable that the final > > > put_page() never arrives. > > > > I do not understand. Are you saying that a user triggered IO can pin LRU > > pages indefinitely. This would be _really_ wrong. It would be basically > > an mlock without any limit. So I must be misreading you here > > You're not misreading. See ib_umem_get() for example, it pins pages in > response to the userspace library call ibv_reg_mr() (memory > registration), and will not release those pages unless/until a call to > ibv_dereg_mr() is made. Who and how many LRU pages can pin that way and how do you prevent nasty users to DoS systems this way? I remember PeterZ wanted to address a similar issue by vmpin syscall that would be a subject of a rlimit control. Sorry but I cannot find a reference here but if this is at g-u-p level without any accounting then it smells quite broken to me. -- Michal Hocko SUSE Labs