From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753546AbaESLo2 (ORCPT ); Mon, 19 May 2014 07:44:28 -0400 Received: from mail-ie0-f171.google.com ([209.85.223.171]:55890 "EHLO mail-ie0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750869AbaESLo0 (ORCPT ); Mon, 19 May 2014 07:44:26 -0400 MIME-Version: 1.0 In-Reply-To: References: <1397587118-1214-1-git-send-email-dh.herrmann@gmail.com> <537396A2.9090609@cybernetics.com> Date: Mon, 19 May 2014 13:44:25 +0200 Message-ID: Subject: Re: [PATCH v2 0/3] File Sealing & memfd_create() From: David Herrmann To: Hugh Dickins Cc: Tony Battersby , Al Viro , Jan Kara , Michael Kerrisk , Ryan Lortie , Linus Torvalds , Andrew Morton , linux-mm , linux-fsdevel , linux-kernel , Johannes Weiner , Tejun Heo , Greg Kroah-Hartman , John Stultz , Kristian Hogsberg , Lennart Poettering , Daniel Mack , Kay Sievers Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi On Thu, May 15, 2014 at 12:35 AM, Hugh Dickins wrote: > The aspect which really worries me is this: the maintenance burden. > This approach would add some peculiar new code, introducing a rare > special case: which we might get right today, but will very easily > forget tomorrow when making some other changes to mm. If we compile > a list of danger areas in mm, this would surely belong on that list. I tried doing the page-replacement in the last 4 days, but honestly, it's far more complex than I thought. So if no-one more experienced with mm/ comes up with a simple implementation, I'll have to delay this for some more weeks. However, I still wonder why we try to fix this as part of this patchset. Using FUSE, a DIRECT-IO call can be delayed for an arbitrary amount of time. Same is true for network block-devices, NFS, iscsi, maybe loop-devices, ... This means, _any_ once mapped page can be written to after an arbitrary delay. This can break any feature that makes FS objects read-only (remounting read-only, setting S_IMMUTABLE, sealing, ..). Shouldn't we try to fix the _cause_ of this? Isn't there a simple way to lock/mark/.. affected vmas in get_user_pages(_fast)() and release them once done? We could increase i_mmap_writable on all affected address_space and decrease it on release. This would at least prevent sealing and could be check on other operations, too (like setting S_IMMUTABLE). This should be as easy as checking page_mapping(page) != NULL and then adjusting ->i_mmap_writable in get_writable_user_pages/put_writable_user_pages, right? Thanks David