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Tue, 14 Jan 2020 01:07:37 +0000 Received: from [172.30.120.61] (2620:10d:c091:480::1025) by MN2PR18CA0021.namprd18.prod.outlook.com (2603:10b6:208:23c::26) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2623.13 via Frontend Transport; Tue, 14 Jan 2020 01:07:36 +0000 From: Chris Mason To: Matthew Wilcox CC: Jens Axboe , "linux-xfs@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "jlayton@kernel.org" , "hch@infradead.org" Subject: Re: [RFC 0/8] Replacing the readpages a_op Thread-Topic: [RFC 0/8] Replacing the readpages a_op Thread-Index: AQHVyieJJr3DU4NMDUiHtIM35/TqzKfozHmAgAAQNgCAAAXGAIAAQlOAgAAAsgCAAALTgIAAAQUAgAADiACAAAIAgIAAKS0AgAABqwA= Date: Tue, 14 Jan 2020 01:07:37 +0000 Message-ID: <510518B2-4E9F-4F84-B546-102B387334D6@fb.com> References: <20200113153746.26654-1-willy@infradead.org> <6CA4CD96-0812-4261-8FF9-CD28AA2EC38A@fb.com> <20200113174008.GB332@bombadil.infradead.org> <15C84CC9-3196-441D-94DE-F3FD7AC364F0@fb.com> <20200113215811.GA18216@bombadil.infradead.org> <910af281-4e2b-3e5d-5533-b5ceafd59665@kernel.dk> <20200113221047.GB18216@bombadil.infradead.org> <1b94e6b6-29dc-2e90-d1ca-982accd3758c@kernel.dk> <20200113222704.GC18216@bombadil.infradead.org> <20200114010136.GA25922@bombadil.infradead.org> In-Reply-To: <20200114010136.GA25922@bombadil.infradead.org> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: MailMate (1.13.1r5671) x-clientproxiedby: MN2PR18CA0021.namprd18.prod.outlook.com (2603:10b6:208:23c::26) To SN6PR15MB2446.namprd15.prod.outlook.com (2603:10b6:805:22::25) x-ms-exchange-messagesentrepresentingtype: 1 x-originating-ip: [2620:10d:c091:480::1025] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: bcc0296e-6699-4283-7241-08d7988e252f x-ms-traffictypediagnostic: SN6PR15MB2397: x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:10000; 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charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: bcc0296e-6699-4283-7241-08d7988e252f X-MS-Exchange-CrossTenant-originalarrivaltime: 14 Jan 2020 01:07:37.4288 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: j8G4/T7eGep7ek4qxeMsiJSXXTDAlAP4sUMHKFGBa7Gu4EUfd0py30tt+HPcXEmJ X-MS-Exchange-Transport-CrossTenantHeadersStamped: SN6PR15MB2397 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138,18.0.572 definitions=2020-01-13_08:2020-01-13,2020-01-13 signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 phishscore=0 spamscore=0 priorityscore=1501 malwarescore=0 clxscore=1015 suspectscore=0 bulkscore=0 lowpriorityscore=0 impostorscore=0 adultscore=0 mlxscore=0 mlxlogscore=594 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-1910280000 definitions=main-2001140007 X-FB-Internal: deliver Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On 13 Jan 2020, at 20:01, Matthew Wilcox wrote: > On Mon, Jan 13, 2020 at 10:34:16PM +0000, Chris Mason wrote: >>> I think what I want is a bio I can reach from current, somehow. And >>> the >>> plug feels like a natural place to keep it because it's basically >>> saying >>> "I want to do lots of little IOs and have them combined". The fact >>> that >>> the iomap code has a bio that it precombines fragments into suggests >>> to >>> me that the existing antifragmentation code in the plugging=20 >>> mechanism >>> isn't good enough. So let's make it better by storing a bio in the >>> plug >>> and then we can get rid of the bio in the iomap code. >> >> Both btrfs and xfs do this, we have a bio that we pass around and=20 >> build >> and submit. We both also do some gymnastics in writepages to avoid >> waiting for the bios we've been building to finish while we're=20 >> building >> them. >> >> I love the idea of the plug api having a way to hold that for us, but >> sometimes we really are building the bios, and we don't want the plug=20 >> to >> let it go if we happen to schedule. > > The plug wouldn't have to let the bio go. I appreciate the plug does=20 > let > requests go on context switch, but it wouldn't have to let the bio go. > This bio is being stored on the stack, just as now, so it's still=20 > there. Plugging is great because it's building up state for the block layer=20 across a wide variety of MM and FS code. btrfs and xfs are building state for themselves across a tiny cross=20 section of their own code. I'd rather not complicate magic state in=20 current in places we can easily pass the bio down. -chris