From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754021AbaDKB2B (ORCPT ); Thu, 10 Apr 2014 21:28:01 -0400 Received: from mail-pd0-f182.google.com ([209.85.192.182]:55802 "EHLO mail-pd0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753327AbaDKB17 (ORCPT ); Thu, 10 Apr 2014 21:27:59 -0400 Message-ID: <5347451C.4060106@amacapital.net> Date: Thu, 10 Apr 2014 18:27:56 -0700 From: Andy Lutomirski User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0 MIME-Version: 1.0 To: David Herrmann , Tony Battersby CC: linux-fsdevel , linux-mm , linux-kernel , "dri-devel@lists.freedesktop.org" Subject: Re: [PATCH 2/6] shm: add sealing API References: <53470E26.2030306@cybernetics.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/10/2014 05:22 PM, David Herrmann wrote: > Hi > > On Thu, Apr 10, 2014 at 11:33 PM, Tony Battersby wrote: >> For O_DIRECT the kernel pins the submitted pages in memory for DMA by >> incrementing the page reference counts when the I/O is submitted, >> allowing the pages to be modified by DMA even if they are no longer >> mapped in the address space of the process. This is different from a >> regular read(), which uses the CPU to copy the data and will fail if the >> pages are not mapped. > > Can you please provide an example code-path? For instance, > file_read_actor() does not pin any pages but only keeps the user-space > address and resolves it once it has data to write. This may be an issue for anything in the kernel that calls get_user_pages and holds onto the result at any time that mmap_sem isn't held. I don't know exactly what does that, but RDMA comes to mind. So does (ugh!) vmsplice, although I suspect that vmsplice doesn't write. --Andy