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Tue, 25 Jul 2023 22:24:22 +0000 Received: from CH3PR11MB7177.namprd11.prod.outlook.com ([fe80::9cba:c1de:ea9f:fba7]) by CH3PR11MB7177.namprd11.prod.outlook.com ([fe80::9cba:c1de:ea9f:fba7%5]) with mapi id 15.20.6609.032; Tue, 25 Jul 2023 22:24:21 +0000 From: "Kasireddy, Vivek" To: Hugh Dickins CC: Jason Gunthorpe , Alistair Popple , Gerd Hoffmann , "Kim, Dongwon" , David Hildenbrand , "Chang, Junxiao" , Peter Xu , "linux-mm@kvack.org" , "dri-devel@lists.freedesktop.org" , Mike Kravetz Subject: RE: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages) Thread-Topic: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages) Thread-Index: AQHZuVToVshy99YkkE2dGyNcPWpnjq/AWUSAgAGkBBCAAGFsAIAF9M/ggAChIwCAADi24IAAwXOAgADrBVA= Date: Tue, 25 Jul 2023 22:24:21 +0000 Message-ID: References: <20230718082858.1570809-1-vivek.kasireddy@intel.com> <20230718082858.1570809-2-vivek.kasireddy@intel.com> <87jzuwlkae.fsf@nvdebian.thelocal> <87pm4nj6s5.fsf@nvdebian.thelocal> <75e3a74a-68f5-df-9a49-a0553c04320@google.com> In-Reply-To: <75e3a74a-68f5-df-9a49-a0553c04320@google.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-publictraffictype: Email x-ms-traffictypediagnostic: CH3PR11MB7177:EE_|IA0PR11MB7956:EE_ x-ms-office365-filtering-correlation-id: 6932a0c5-21d0-4eae-b6dd-08db8d5de51e x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: CH3PR11MB7177.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 6932a0c5-21d0-4eae-b6dd-08db8d5de51e X-MS-Exchange-CrossTenant-originalarrivaltime: 25 Jul 2023 22:24:21.8455 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: fSveNeVCihLO4uUwNGUSYO/Uj9JQl9Y6TqVdNaHhdjveCKbe6ofdDH+y+5sx+2FdqB8wjOqkVmiuOi3R8dod+1YiBT4C/HkHv3rTP/fom58= X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA0PR11MB7956 X-OriginatorOrg: intel.com X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 769EF12000E X-Stat-Signature: 8mtroooh44i5qn9kyr77fwu4craffzmy X-Rspam-User: X-HE-Tag: 1690323914-243584 X-HE-Meta: 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 c1P3dJcd cIUDmoN2H1OTVMVoHBqhJYYzqKmGdsl4sIQqs9+4H6BLBIaf+Q3Hup6rIlLnx0rQOgpELE90/NEP46z69SdPfIxUzY+yUmp42laZVwRJHc3WXCKVe6OnQgIwkDY+ZW1gvetVHP0aJ2dkVhnHFj6bWnVxzO3YsidVFeWXQxwVZrLgW5oF6Plc/UUXHMIQONENESDvfaYeLhmNSEB6I8Gl0mlrqiTIllVSNax64wwvHVqXpJWkZu0CWVM1RyX0x+M7SQP1VeuNHIyx3x8HKH48W9aIwmRJHqd/P1mjbNajideoI1MJcPjBnYkDJkLKTg7hhA91vCdfb/18aaydWnqIw8BTdXJdis6CJ4NeTlurQ0rc3l/HlMS/goqVZFpy7i4Fc/Vo1 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000433, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hi Hugh, >=20 > On Mon, 24 Jul 2023, Kasireddy, Vivek wrote: > > Hi Jason, > > > On Mon, Jul 24, 2023 at 07:54:38AM +0000, Kasireddy, Vivek wrote: > > > > > > > > I'm not at all familiar with the udmabuf use case but that sounds > > > > > brittle and effectively makes this notifier udmabuf specific righ= t? > > > > Oh, Qemu uses the udmabuf driver to provide Host Graphics > components > > > > (such as Spice, Gstreamer, UI, etc) zero-copy access to Guest creat= ed > > > > buffers. In other words, from a core mm standpoint, udmabuf just > > > > collects a bunch of pages (associated with buffers) scattered insid= e > > > > the memfd (Guest ram backed by shmem or hugetlbfs) and wraps > > > > them in a dmabuf fd. And, since we provide zero-copy access, we > > > > use DMA fences to ensure that the components on the Host and > > > > Guest do not access the buffer simultaneously. > > > > > > So why do you need to track updates proactively like this? > > As David noted in the earlier series, if Qemu punches a hole in its mem= fd > > that goes through pages that are registered against a udmabuf fd, then > > udmabuf needs to update its list with new pages when the hole gets > > filled after (guest) writes. Otherwise, we'd run into the coherency > > problem (between udmabuf and memfd) as demonstrated in the selftest > > (patch #3 in this series). >=20 > Wouldn't this all be very much better if Qemu stopped punching holes ther= e? I think holes can be punched anywhere in the memfd for various reasons. Som= e of the use-cases where this would be done were identified by David. Here is= what he said in an earlier discussion: "There are *probably* more issues on the QEMU side when udmabuf is paired=20 with things like MADV_DONTNEED/FALLOC_FL_PUNCH_HOLE used for=20 virtio-balloon, virtio-mem, postcopy live migration, ... for example, in" Thanks, Vivek >=20 > Hugh