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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 31D93C433E2 for ; Wed, 9 Sep 2020 10:51:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E9A2C21D7D for ; Wed, 9 Sep 2020 10:51:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729507AbgIIKvh (ORCPT ); Wed, 9 Sep 2020 06:51:37 -0400 Received: from mx2.suse.de ([195.135.220.15]:52316 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726426AbgIIKvg (ORCPT ); Wed, 9 Sep 2020 06:51:36 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 6FD05ABD2; Wed, 9 Sep 2020 10:51:34 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 79C291E12E1; Wed, 9 Sep 2020 12:51:33 +0200 (CEST) Date: Wed, 9 Sep 2020 12:51:33 +0200 From: Jan Kara To: "Darrick J. Wong" Cc: Matthew Wilcox , Jan Kara , linux-fsdevel , xfs , hch@lst.de Subject: Re: [PATCH v3] quota: widen timestamps for the fs_disk_quota structure Message-ID: <20200909105133.GC24207@quack2.suse.cz> References: <20200909013251.GG7955@magnolia> <20200909014933.GC6583@casper.infradead.org> <20200909022909.GI7955@magnolia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200909022909.GI7955@magnolia> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Tue 08-09-20 19:29:09, Darrick J. Wong wrote: > On Wed, Sep 09, 2020 at 02:49:33AM +0100, Matthew Wilcox wrote: > > On Tue, Sep 08, 2020 at 06:32:51PM -0700, Darrick J. Wong wrote: > > > +static inline void copy_to_xfs_dqblk_ts(const struct fs_disk_quota *d, > > > + __s32 *timer_lo, __s8 *timer_hi, s64 timer) > > > +{ > > > + *timer_lo = timer; > > > + if (d->d_fieldmask & FS_DQ_BIGTIME) > > > + *timer_hi = timer >> 32; > > > + else > > > + *timer_hi = 0; > > > +} > > > > I'm still confused by this. What breaks if you just do: > > > > *timer_lo = timer; > > *timer_hi = timer >> 32; > > "I don't know." > > The manpage for quotactl doesn't actually specify the behavior of the > padding fields. The /implementation/ is careful enough to zero > everything, but the interface specification doesn't explicitly require > software to do so. > > Because the contents of the padding fields aren't defined by the > documentation, the kernel cannot simply start using the d_padding2 field > because there could be an old kernel that doesn't zero the padding, > which would lead to confusion if the new userspace were mated to such a > kernel. > > Therefore, we have to add a flag that states explicitly that we are > using the timer_hi fields. This is also the only way that an old > program can detect that it's being fed a structure that it might not > recognise. Well, this is in the direction from kernel to userspace and what Matthew suggests would just make kernel posssibly store non-zero value in *timer_hi without setting FS_DQ_BIGTIME flag (for negative values of timer). I don't think it would break anything but I agree the complication isn't big so let's be careful and only set *timer_hi to non-zero if FS_DQ_BIGTIME is set. Honza -- Jan Kara SUSE Labs, CR