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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 33780C169C4 for ; Thu, 7 Feb 2019 02:39:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F31B5218D9 for ; Thu, 7 Feb 2019 02:39:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726792AbfBGCjJ (ORCPT ); Wed, 6 Feb 2019 21:39:09 -0500 Received: from mx2.suse.de ([195.135.220.15]:45290 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726245AbfBGCjI (ORCPT ); Wed, 6 Feb 2019 21:39:08 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 0407BAF0C; Thu, 7 Feb 2019 02:39:07 +0000 (UTC) Subject: Re: bcache on XFS: metadata I/O (dirent I/O?) not getting cached at all? To: Dave Chinner , Andre Noll Cc: Nix , linux-bcache@vger.kernel.org, linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, Christoph Hellwig , axboe@kernel.dk References: <87h8dgefee.fsf@esperi.org.uk> <20190206234328.GH14116@dastard> <20190207002425.GX24140@tuebingen.mpg.de> <20190207022657.GI14116@dastard> From: Coly Li Openpgp: preference=signencrypt Organization: SUSE Labs Message-ID: <438851ef-ef77-b5f2-d46d-05762b6330b2@suse.de> Date: Thu, 7 Feb 2019 10:38:58 +0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <20190207022657.GI14116@dastard> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2019/2/7 10:26 上午, Dave Chinner wrote: > On Thu, Feb 07, 2019 at 01:24:25AM +0100, Andre Noll wrote: >> On Thu, Feb 07, 10:43, Dave Chinner wrote >>> File data readahead: REQ_RAHEAD >>> Metadata readahead: REQ_META | REQ_RAHEAD >>> >>> drivers/md/bcache/request.c::check_should_bypass(): >>> >>> /* >>> * Flag for bypass if the IO is for read-ahead or background, >>> * unless the read-ahead request is for metadata (eg, for gfs2). >>> */ >>> if (bio->bi_opf & (REQ_RAHEAD|REQ_BACKGROUND) && >>> !(bio->bi_opf & REQ_PRIO)) >>> goto skip; >>> >>> bcache needs fixing - it thinks REQ_PRIO means metadata IO. That's >>> wrong - REQ_META means it's metadata IO, and so this is a bcache >>> bug. >> >> Do you think 752f66a75abad is bad (ha!) and should be reverted? > > Yes, that change is just broken. From include/linux/blk_types.h: > > __REQ_META, /* metadata io request */ > __REQ_PRIO, /* boost priority in cfq */ > > Hi Dave, > i.e. REQ_META means that it is a metadata request, REQ_PRIO means it > is a "high priority" request. Two completely different things, often > combined, but not interchangeable. I found in file system metadata IO, most of time REQ_META and REQ_PRIO are tagged together for bio, but XFS seems not use REQ_PRIO. Is there any basic principle for when should these tags to be used or not ? e.g. If REQ_META is enough for meta data I/O, why REQ_PRIO is used too. And if REQ_PRIO is necessary, why it is not used in fs/xfs/ code ? > > So, yeah, that needs to be reverted if you want bcache to function > properly for metadata caching. Sure, I will fix this, once I make it clear to me. Thanks for the hint. -- Coly Li