From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yw1-f170.google.com (mail-yw1-f170.google.com [209.85.128.170]) by mail19.linbit.com (LINBIT Mail Daemon) with ESMTP id F12CB4208EC for ; Wed, 12 Jun 2024 10:01:23 +0200 (CEST) Received: by mail-yw1-f170.google.com with SMTP id 00721157ae682-62ce53782f4so24562037b3.0 for ; Wed, 12 Jun 2024 01:01:23 -0700 (PDT) Date: Wed, 12 Jun 2024 10:01:18 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= To: Christoph Hellwig Subject: Re: [PATCH 10/26] xen-blkfront: don't disable cache flushes when they fail Message-ID: References: <20240611051929.513387-1-hch@lst.de> <20240611051929.513387-11-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20240611051929.513387-11-hch@lst.de> Cc: nvdimm@lists.linux.dev, "Michael S. Tsirkin" , Jason Wang , linux-nvme@lists.infradead.org, Song Liu , linux-mtd@lists.infradead.org, Vineeth Vijayan , linux-bcache@vger.kernel.org, Alasdair Kergon , drbd-dev@lists.linbit.com, linux-s390@vger.kernel.org, linux-scsi@vger.kernel.org, Richard Weinberger , Geert Uytterhoeven , Yu Kuai , dm-devel@lists.linux.dev, linux-um@lists.infradead.org, Mike Snitzer , Josef Bacik , Ming Lei , linux-raid@vger.kernel.org, linux-m68k@lists.linux-m68k.org, Mikulas Patocka , xen-devel@lists.xenproject.org, ceph-devel@vger.kernel.org, nbd@other.debian.org, Jens Axboe , linux-block@vger.kernel.org, "Martin K. Petersen" , linux-mmc@vger.kernel.org, Philipp Reisner , virtualization@lists.linux.dev, Lars Ellenberg , linuxppc-dev@lists.ozlabs.org List-Id: "*Coordination* of development, patches, contributions -- *Questions* \(even to developers\) go to drbd-user, please." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, Jun 11, 2024 at 07:19:10AM +0200, Christoph Hellwig wrote: > blkfront always had a robust negotiation protocol for detecting a write > cache. Stop simply disabling cache flushes when they fail as that is > a grave error. It's my understanding the current code attempts to cover up for the lack of guarantees the feature itself provides: * feature-barrier * Values: 0/1 (boolean) * Default Value: 0 * * A value of "1" indicates that the backend can process requests * containing the BLKIF_OP_WRITE_BARRIER request opcode. Requests * of this type may still be returned at any time with the * BLKIF_RSP_EOPNOTSUPP result code. * * feature-flush-cache * Values: 0/1 (boolean) * Default Value: 0 * * A value of "1" indicates that the backend can process requests * containing the BLKIF_OP_FLUSH_DISKCACHE request opcode. Requests * of this type may still be returned at any time with the * BLKIF_RSP_EOPNOTSUPP result code. So even when the feature is exposed, the backend might return EOPNOTSUPP for the flush/barrier operations. Such failure is tied on whether the underlying blkback storage supports REQ_OP_WRITE with REQ_PREFLUSH operation. blkback will expose "feature-barrier" and/or "feature-flush-cache" without knowing whether the underlying backend supports those operations, hence the weird fallback in blkfront. I'm unsure whether lack of REQ_PREFLUSH support is not something that we should worry about, it seems like it was when the code was introduced, but that's > 10y ago. Overall blkback should ensure that REQ_PREFLUSH is supported before exposing "feature-barrier" or "feature-flush-cache", as then the exposed features would really match what the underlying backend supports (rather than the commands blkback knows about). Thanks, Roger.