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Thu, 30 Jan 2020 16:09:45 +0000 From: Johannes Thumshirn To: Christoph Hellwig CC: "dsterba@suse.cz" , David Sterba , Nikolay Borisov , "linux-btrfs @ vger . kernel . org" Subject: Re: [PATCH v3 0/5] btrfs: remove buffer heads form superblock handling Thread-Topic: [PATCH v3 0/5] btrfs: remove buffer heads form superblock handling Thread-Index: AQHV1SrKCplaciq1OEKao1x65qqI1g== Date: Thu, 30 Jan 2020 16:09:45 +0000 Message-ID: References: <20200127155931.10818-1-johannes.thumshirn@wdc.com> <20200129142526.GE3929@twin.jikos.cz> <20200130121530.GO3929@twin.jikos.cz> <20200130133921.GA21841@infradead.org> <20200130155616.GA14682@infradead.org> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: spf=none (sender IP is ) smtp.mailfrom=Johannes.Thumshirn@wdc.com; x-originating-ip: [129.253.240.72] x-ms-publictraffictype: Email x-ms-office365-filtering-ht: Tenant x-ms-office365-filtering-correlation-id: cc761374-fa17-4be1-4bec-08d7a59ed303 x-ms-traffictypediagnostic: DM5PR0401MB3622: x-microsoft-antispam-prvs: wdcipoutbound: EOP-TRUE x-ms-oob-tlc-oobclassifiers: OLM:10000; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: wdc.com X-MS-Exchange-CrossTenant-Network-Message-Id: cc761374-fa17-4be1-4bec-08d7a59ed303 X-MS-Exchange-CrossTenant-originalarrivaltime: 30 Jan 2020 16:09:45.8576 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: b61c8803-16f3-4c35-9b17-6f65f441df86 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: tGbkEuRjNZSZrNaoJDwAUT2ZUZhi+Jt4RGk/ccrbHOXNZHpttorDWYfBO7PGc6vudZYAY/B6VrTcp1eKV9ERq6hyEaWPctZnewRbKyGrL4Q= X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR0401MB3622 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 30/01/2020 16:56, Christoph Hellwig wrote:=0A= > On Thu, Jan 30, 2020 at 03:53:37PM +0000, Johannes Thumshirn wrote:=0A= >> On 30/01/2020 14:39, Christoph Hellwig wrote:=0A= >>> On Thu, Jan 30, 2020 at 01:15:30PM +0100, David Sterba wrote:=0A= >>>>> Sure but with hch's proposed change to using read_cache_page_gfp() th= is=0A= >>>>> doesn't make too much sense anymore at least for the read path.=0A= >>>>>=0A= >>>>> Maybe "use page cache for superblock reading"?=0A= >>>>=0A= >>>> That works too. We might need a new iteration that summarizes up all t= he=0A= >>>> feedback so far, so we have same code to refer to.=0A= >>>=0A= >>> Per my question on the second patch: why even use the page cache at=0A= >>> all. btrfs already caches the value outside the pagecache, so why=0A= >>> even bother with the page cache overhead?=0A= >>>=0A= >> This is what my first version did, alloc_page() and submit_bio()=0A= >> directly [1]. But reviewers told me to go the route via page cache.=0A= > =0A= > I only see your patch at the url, not any reply. What is the issue=0A= > of not using the page cache? Also you really shoudn't need a separate=0A= > alloc_page - you should be able to use the already cached superblock=0A= > as the destination and source of I/O, assuming they are properly aligned= =0A= > (and if not that could be fixed easily).=0A= > =0A= =0A= Care to elaborate? Who would have cached the superblock, when we haven't = =0A= mounted the FS yet.=0A= =0A= So here's the answer from that thread:=0A= =0A= "IIRC we had some funny bugs when mount and device scan (udev) raced =0A= just after mkfs, the page cache must be used so there's no way to read =0A= stale data."=0A= https://lore.kernel.org/linux-btrfs/20200117151352.GK3929@twin.jikos.cz/=0A= =0A=