From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id 65D806B0005 for ; Mon, 18 Jun 2018 15:32:06 -0400 (EDT) Received: by mail-wr0-f200.google.com with SMTP id r2-v6so12557452wrm.15 for ; Mon, 18 Jun 2018 12:32:06 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id z12-v6sor7531574wrn.56.2018.06.18.12.32.04 for (Google Transport Security); Mon, 18 Jun 2018 12:32:04 -0700 (PDT) Date: Mon, 18 Jun 2018 13:31:58 -0600 From: Jason Gunthorpe Subject: Re: [PATCH 2/2] mm: set PG_dma_pinned on get_user_pages*() Message-ID: <20180618193158.GE6805@ziepe.ca> References: <20180617012510.20139-3-jhubbard@nvidia.com> <20180617200432.krw36wrcwidb25cj@ziepe.ca> <311eba48-60f1-b6cc-d001-5cc3ed4d76a9@nvidia.com> <20180618081258.GB16991@lst.de> <3898ef6b-2fa0-e852-a9ac-d904b47320d5@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Dan Williams Cc: John Hubbard , Christoph Hellwig , John Hubbard , Matthew Wilcox , Michal Hocko , Christopher Lameter , Jan Kara , Linux MM , LKML , linux-rdma On Mon, Jun 18, 2018 at 12:21:46PM -0700, Dan Williams wrote: > On Mon, Jun 18, 2018 at 11:14 AM, John Hubbard wrote: > > On 06/18/2018 10:56 AM, Dan Williams wrote: > >> On Mon, Jun 18, 2018 at 10:50 AM, John Hubbard wrote: > >>> On 06/18/2018 01:12 AM, Christoph Hellwig wrote: > >>>> On Sun, Jun 17, 2018 at 01:28:18PM -0700, John Hubbard wrote: > >>>>> Yes. However, my thinking was: get_user_pages() can become a way to indicate that > >>>>> these pages are going to be treated specially. In particular, the caller > >>>>> does not really want or need to support certain file operations, while the > >>>>> page is flagged this way. > >>>>> > >>>>> If necessary, we could add a new API call. > >>>> > >>>> That API call is called get_user_pages_longterm. > >>> > >>> OK...I had the impression that this was just semi-temporary API for dax, but > >>> given that it's an exported symbol, I guess it really is here to stay. > >> > >> The plan is to go back and provide api changes that bypass > >> get_user_page_longterm() for RDMA. However, for VFIO and others, it's > >> not clear what we could do. In the VFIO case the guest would need to > >> be prepared handle the revocation. > > > > OK, let's see if I understand that plan correctly: > > > > 1. Change RDMA users (this could be done entirely in the various device drivers' > > code, unless I'm overlooking something) to use mmu notifiers, and to do their > > DMA to/from non-pinned pages. > > The problem with this approach is surprising the RDMA drivers with > notifications of teardowns. It's the RDMA userspace applications that > need the notification, and it likely needs to be explicit opt-in, at > least for the non-ODP drivers. Well, more than that, we have no real plan on how to accomplish this, or any idea if it can even really work.. Most userspace give up control of the memory lifetime to the remote side of the connection and have no way to recover it other than a full teardown. Given that John is trying to fix a kernel oops, I don't think we should tie progress on it to the RDMA notification idea. .. and given that John is trying to fix a kernel oops, maybe the weird/bad/ugly behavior of ftruncte is a better bug to have than for unprivileged users to be able to oops the kernel??? Jason