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Wed, 4 Jan 2023 05:06:29 +0000 Received: from IA1PR11MB6097.namprd11.prod.outlook.com ([fe80::16c0:1ae3:13ee:c40e]) by IA1PR11MB6097.namprd11.prod.outlook.com ([fe80::16c0:1ae3:13ee:c40e%3]) with mapi id 15.20.5944.019; Wed, 4 Jan 2023 05:06:29 +0000 From: "Yu, Fenghua" To: Lorenzo Stoakes CC: Vinod Koul , "Jiang, Dave" , "dmaengine@vger.kernel.org" , "Zhu, Tony" , Andrew Morton , "linux-mm@kvack.org" , Christoph Hellwig , "Shankar, Ravi V" Subject: RE: [PATCH 09/17] mm: export access_remote_vm() symbol Thread-Topic: [PATCH 09/17] mm: export access_remote_vm() symbol Thread-Index: AQHZH5FkNhHM4hOH50a42EHT2Acr4K6M9zQAgAABTYCAAA0YkIAAGukAgACKFVA= Date: Wed, 4 Jan 2023 05:06:29 +0000 Message-ID: References: <20230103163505.1569356-1-fenghua.yu@intel.com> <20230103163505.1569356-10-fenghua.yu@intel.com> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-publictraffictype: Email x-ms-traffictypediagnostic: IA1PR11MB6097:EE_|PH0PR11MB5625:EE_ x-ms-office365-filtering-correlation-id: c8093891-b2f8-4e50-978d-08daee11707d 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: IA1PR11MB6097.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: c8093891-b2f8-4e50-978d-08daee11707d X-MS-Exchange-CrossTenant-originalarrivaltime: 04 Jan 2023 05:06:29.5167 (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: leHSWEyHVzJhLUcKcIFgmp3+bGgeec7NEJtMxFJdxJ3pCeimfWYpqmj6PQ90i4627QEPvtXjVRXEc7GdY6W/kg== X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH0PR11MB5625 X-OriginatorOrg: intel.com X-Rspam-User: X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 6B28E8000A X-Stat-Signature: yfzz33pc6qg566sdg656riyrof4uwnf1 X-HE-Tag: 1672808797-759122 X-HE-Meta: U2FsdGVkX18MuHNOUoPvy8HX1lr+o8a5wySIt1PYBUbkFXAb5+ruwA8+rIBJC4IPPFHgo4FHzhK4ihgHpONny8SavYoNrw2nMGTEVQXuWV9hyze1GpbiPqIIaIDiuw0bgihUHWNfr0NcWcgs6ezwBLoRkUnpRk4w2iLkbAwIH/TBCLtEJHqzAKiHEIOz0roEB0KXzhDep0F1Alp/WP1CdmPUsIC8BSW2aSzk1VWsWXhCEnjg6/KlGyVbhxvOvzPP52ROiSN03xingZAVuW2K2EtRXB7hcfSkapMja1sYJffnqtmyv2WzP9LCo2v7i7AwH9hsTkMzM35mqJXX9YTpbfauqNj1pFumjl4c/Se2Ky67vSoKhn+B0Kas84JIHkF8/s3RjfmuqSOCnyhNIqhFoyGVSI+nJY0xLpVXbd2xrqMvWjpHWZab++wNl1+OULdUfMRjOl13nGCpXy2cxT92MOsaHwNcXsph12BOu0Z8/sHuzPT+T07r2j6us8NuFEfuZ/76HNQuVT5eZc16oBDHfdpNuvkkIsmYWT0fVWooL+9ezV/i7JCynfFoP4B+CjAlazBUY9j18jiHFwy8oadrcU59LNcVlL6oThm1SmvcDIQr0rMZzjkV/IknYBv9/b72rpAig8Ax32JSZNMGHEUOwXcl/RsRl6ckoQDGeN++B6xiL05AxvUGnsEt7emZuMqtbXccJmsFFb91QKGKkGxbmZkZaFm+HfQPXVk54lSG7VOSbXRBGUG0FGNRfVObpNL6SF/A+I7gQgr2X3ZWGRkNGZMGeaybmyWQGoHDKKyHL7IprEybNQAnTYbT8kJKLaHAQKQCX5j6TWX+VMk4PtW4X76GYAKm5Y5wwVn7Iz0q6rRgLwthCb0N4vF86T8KLHNGN0DzSSFomtYgIsCcmLh3QX91ALnbY3r0X9DYtDeVStruNYwbaJWWZWfZIHi8Xslyjc+jqGnWJWy+rtNKAQ6 IwQt2tZ+ oeh5Nkc8AAYlu2RXDEVY8qNA5LNTnmc+wssti X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hi, Lorenzo, > Hey Fenghua :) >=20 > > access_remote_vm(mm) directly call __access_remote_vm(mm). > > access_process_vm(tsk) calls mm=3Dget_task_mm() then > __access_remote_vm(mm). > > > > So instead of access_remote_vm(mm), it's access_process_vm(tsk) that > > holds a reference count on the mm, right? >=20 > Indeed! >=20 > > > > > > > > > > Is there a reason you can't use access_process_vm() which is > > > > exported and additionally handles the refrencing? > > > > IDXD interrupt handler starts a work which needs to access remote vm. > > The remote mm is found by PASID which is saved in device event log. > > > > In the work, it's hard to get the remote mm from a task because > > mm->owner could be NULL but the mm is still existing. >=20 > That makes sense, however I do feel nervous about exporting something tha= t > that relies on this reference. >=20 > The issue is ensuring that the mm can't be taken from underneath you, the= only > user of access_remote_vm(), procfs, does a careful dance using get_task_m= m() > and > mm_access() to ensure this can't happen, if _sometimes_ the remote mm mig= ht > have an owner and _sometimes_ not it feels like any exported function nee= ds to > be equally careful? >=20 > I definitely don't feel as if simply exporting this is a safe option, and= you would in > that case need a new function that handles different scenarios of mm > ownership/not. >=20 > I may be missing something here and I will wait for others to chime in bu= t I think > we would definitely need something more than simply exporting this. I may define and export a new wrapper access_remote_vm_ref() which will hol= d mm's reference count before accessing it: int access_remote_vm_ref(mm) { int ret; if (mm =3D=3D &init_mm) return 0; mmget(mm); ret =3D access_remote_vm(mm); mmput(mm); return ret; } EXPORT_SYMBOL_GPL(access_remote_vm_ref); IDXD or any driver calls this and holds mm reference count while accesses t= he mm. This is useful for caller to directly access mm even if mm's owner could be= NULL. Do you think this is sufficient to take care of the mm reference and is a g= ood way to go? Thanks. -Fenghua