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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 8BB5CC433E1 for ; Tue, 25 Aug 2020 04:27:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 76BA120715 for ; Tue, 25 Aug 2020 04:27:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725970AbgHYE1S (ORCPT ); Tue, 25 Aug 2020 00:27:18 -0400 Received: from mail106.syd.optusnet.com.au ([211.29.132.42]:51151 "EHLO mail106.syd.optusnet.com.au" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725900AbgHYE1Q (ORCPT ); Tue, 25 Aug 2020 00:27:16 -0400 Received: from dread.disaster.area (pa49-181-146-199.pa.nsw.optusnet.com.au [49.181.146.199]) by mail106.syd.optusnet.com.au (Postfix) with ESMTPS id 349066AC401; Tue, 25 Aug 2020 14:27:12 +1000 (AEST) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1kAQYB-0006L5-Ky; Tue, 25 Aug 2020 14:27:11 +1000 Date: Tue, 25 Aug 2020 14:27:11 +1000 From: Dave Chinner To: Andreas Dilger Cc: Matthew Wilcox , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Darrick J . Wong" , linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 9/9] iomap: Change calling convention for zeroing Message-ID: <20200825042711.GL12131@dread.disaster.area> References: <20200824145511.10500-1-willy@infradead.org> <20200824145511.10500-10-willy@infradead.org> <20200825002735.GI12131@dread.disaster.area> <20200825032603.GL17456@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Optus-CM-Score: 0 X-Optus-CM-Analysis: v=2.3 cv=QKgWuTDL c=1 sm=1 tr=0 cx=a_idp_d a=GorAHYkI+xOargNMzM6qxQ==:117 a=GorAHYkI+xOargNMzM6qxQ==:17 a=kj9zAlcOel0A:10 a=y4yBn9ojGxQA:10 a=JfrnYn6hAAAA:8 a=7-415B0cAAAA:8 a=Avm1FSHoLamJ-oWrJvIA:9 a=fEKI0cE5RSuP_qZy:21 a=LEqab9mPDQQxAqv4:21 a=CjuIK1q_8ugA:10 a=1CNFftbPRP8L7MoqJWF3:22 a=biEYGPWJfzWAr4FL6Ov7:22 Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Mon, Aug 24, 2020 at 09:35:59PM -0600, Andreas Dilger wrote: > On Aug 24, 2020, at 9:26 PM, Matthew Wilcox wrote: > > > > On Tue, Aug 25, 2020 at 10:27:35AM +1000, Dave Chinner wrote: > >>> do { > >>> - unsigned offset, bytes; > >>> - > >>> - offset = offset_in_page(pos); > >>> - bytes = min_t(loff_t, PAGE_SIZE - offset, count); > >>> + loff_t bytes; > >>> > >>> if (IS_DAX(inode)) > >>> - status = dax_iomap_zero(pos, offset, bytes, iomap); > >>> + bytes = dax_iomap_zero(pos, length, iomap); > >> > >> Hmmm. everything is loff_t here, but the callers are defining length > >> as u64, not loff_t. Is there a potential sign conversion problem > >> here? (sure 64 bit is way beyond anything we'll pass here, but...) > > > > I've gone back and forth on the correct type for 'length' a few times. > > size_t is too small (not for zeroing, but for seek()). An unsigned type > > seems right -- a length can't be negative, and we don't want to give > > the impression that it can. But the return value from these functions > > definitely needs to be signed so we can represent an error. So a u64 > > length with an loff_t return type feels like the best solution. And > > the upper layers have to promise not to pass in a length that's more > > than 2^63-1. > > The problem with allowing a u64 as the length is that it leads to the > possibility of an argument value that cannot be returned. Checking > length < 0 is not worse than checking length > 0x7ffffffffffffff, > and has the benefit of consistency with the other argument types and > signs... I think the problem here is that we have no guaranteed 64 bit size type. when that was the case with off_t, we created loff_t to always represent a 64 bit offset value. However, we never created one for the count/size that is passed alongside loff_t in many places - it was said that "syscalls are limited to 32 bit sizes" and "size_t is 64 bit on 64 bit platforms" and so on and so we still don't have a clean way to pass 64 bit sizes through the IO path. We've been living with this shitty situation for a long time now, so perhaps it's time for us to define lsize_t for 64 bit lengths and start using that everywhere that needs a 64 bit clean path through the code, regardless of whether the arch is 32 or 64 bit... Thoughts? -Dave. -- Dave Chinner david@fromorbit.com