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=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 8C408C04FF3 for ; Mon, 24 May 2021 22:12:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6FEDC61429 for ; Mon, 24 May 2021 22:12:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234017AbhEXWNe (ORCPT ); Mon, 24 May 2021 18:13:34 -0400 Received: from mail.kernel.org ([198.145.29.99]:58176 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234024AbhEXWN2 (ORCPT ); Mon, 24 May 2021 18:13:28 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 57B986109F; Mon, 24 May 2021 22:11:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1621894318; bh=9PyMNewsGq6ERDLxr4di8vSjGgfZV7Y7rZTjz/iLdIw=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=PBIRE9lNbG85SmL9IJ3O9MVl8G5MDhwYM3GLmzbY1P926o7iNhKuihUVyN2KyNYHM K1OjEYugDHTXljqnHNK970F1XJyzKax8xTENy1gnYj2uMypI18IEJlW0M8V+cTyNd6 2JMaQS5YyV2rUuewMxhxtpeT8m45NqOWvmPrPIvg= Date: Mon, 24 May 2021 15:11:57 -0700 From: Andrew Morton To: Alistair Popple Cc: , , , , , , , , , , , , , , , Christoph Hellwig Subject: Re: [PATCH v9 07/10] mm: Device exclusive memory access Message-Id: <20210524151157.2dc5d2bb510ff86dc449bf0c@linux-foundation.org> In-Reply-To: <20210524132725.12697-8-apopple@nvidia.com> References: <20210524132725.12697-1-apopple@nvidia.com> <20210524132725.12697-8-apopple@nvidia.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Mon, 24 May 2021 23:27:22 +1000 Alistair Popple wrote: > Some devices require exclusive write access to shared virtual > memory (SVM) ranges to perform atomic operations on that memory. This > requires CPU page tables to be updated to deny access whilst atomic > operations are occurring. > > In order to do this introduce a new swap entry > type (SWP_DEVICE_EXCLUSIVE). When a SVM range needs to be marked for > exclusive access by a device all page table mappings for the particular > range are replaced with device exclusive swap entries. This causes any > CPU access to the page to result in a fault. > > Faults are resovled by replacing the faulting entry with the original > mapping. This results in MMU notifiers being called which a driver uses > to update access permissions such as revoking atomic access. After > notifiers have been called the device will no longer have exclusive > access to the region. > > Walking of the page tables to find the target pages is handled by > get_user_pages() rather than a direct page table walk. A direct page > table walk similar to what migrate_vma_collect()/unmap() does could also > have been utilised. However this resulted in more code similar in > functionality to what get_user_pages() provides as page faulting is > required to make the PTEs present and to break COW. > > ... > > Documentation/vm/hmm.rst | 17 ++++ > include/linux/mmu_notifier.h | 6 ++ > include/linux/rmap.h | 4 + > include/linux/swap.h | 7 +- > include/linux/swapops.h | 44 ++++++++- > mm/hmm.c | 5 + > mm/memory.c | 128 +++++++++++++++++++++++- > mm/mprotect.c | 8 ++ > mm/page_vma_mapped.c | 9 +- > mm/rmap.c | 186 +++++++++++++++++++++++++++++++++++ > 10 files changed, 405 insertions(+), 9 deletions(-) > This is quite a lot of code added to core MM for a single driver. Is there any expectation that other drivers will use this code? Is there a way of reducing the impact (code size, at least) for systems which don't need this code? How beneficial is this code to nouveau users? I see that it permits a part of OpenCL to be implemented, but how useful/important is this in the real world? Thanks.