From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f173.google.com ([209.85.213.173]:38624 "EHLO mail-ig0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751916AbbGVV7s (ORCPT ); Wed, 22 Jul 2015 17:59:48 -0400 Received: by iggf3 with SMTP id f3so144073738igg.1 for ; Wed, 22 Jul 2015 14:59:47 -0700 (PDT) Message-ID: <55B01252.90004@kernel.dk> Date: Wed, 22 Jul 2015 15:59:46 -0600 From: Jens Axboe MIME-Version: 1.0 To: Christoph Hellwig CC: "Martin K. Petersen" , Neil Brown , Liu Bo , linux-raid@vger.kernel.org, dm-devel@redhat.com, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] block: add a bi_error field to struct bio References: <1437398977-8492-1-git-send-email-hch@lst.de> <1437398977-8492-2-git-send-email-hch@lst.de> <55AFE643.5000704@kernel.dk> In-Reply-To: <55AFE643.5000704@kernel.dk> Content-Type: text/plain; charset=windows-1252; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 07/22/2015 12:51 PM, Jens Axboe wrote: > On 07/20/2015 07:29 AM, Christoph Hellwig wrote: >> Currently we have two different ways to signal an I/O error on a BIO: >> >> (1) by clearing the BIO_UPTODATE flag >> (2) by returning a Linux errno value to the bi_end_io callback >> >> The first one has the drawback of only communicating a single possible >> error (-EIO), and the second one has the drawback of not beeing >> persistent >> when bios are queued up, and are not passed along from child to parent >> bio in the ever more popular chaining scenario. Having both mechanisms >> available has the additional drawback of utterly confusing driver authors >> and introducing bugs where various I/O submitters only deal with one of >> them, and the others have to add boilerplate code to deal with both kinds >> of error returns. >> >> So add a new bi_error field to store an errno value directly in struct >> bio and remove the existing mechanisms to clean all this up. > > I think this is a good change, the only part I _really_ dislike is that > this now bumps a struct bio from 2 cache lines to 3. Have you done any > perf testing? One possible solution would be to shrink bi_flags to an unsigned int, no problems fitting that in. Then we could stuff bi_error in that (new) hole, and we would end up having the same size again. -- Jens Axboe