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Tue, 3 Jun 2025 17:50:25 +0000 Received: from SN6PR02MB4157.namprd02.prod.outlook.com ([fe80::cedd:1e64:8f61:b9df]) by SN6PR02MB4157.namprd02.prod.outlook.com ([fe80::cedd:1e64:8f61:b9df%4]) with mapi id 15.20.8813.016; Tue, 3 Jun 2025 17:50:25 +0000 From: Michael Kelley To: Thomas Zimmermann , David Hildenbrand , "simona@ffwll.ch" , "deller@gmx.de" , "haiyangz@microsoft.com" , "kys@microsoft.com" , "wei.liu@kernel.org" , "decui@microsoft.com" , "akpm@linux-foundation.org" CC: "weh@microsoft.com" , "hch@lst.de" , "dri-devel@lists.freedesktop.org" , "linux-fbdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-hyperv@vger.kernel.org" , "linux-mm@kvack.org" Subject: RE: [PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel memory framebuffers Thread-Topic: [PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel memory framebuffers Thread-Index: AQHby/4dA8oNLnhKakm3U+ZOuvHFDrPvrtEAgAEJQeCAAFCFAIAAuDlw Date: Tue, 3 Jun 2025 17:50:24 +0000 Message-ID: References: <20250523161522.409504-1-mhklinux@outlook.com> <20250523161522.409504-4-mhklinux@outlook.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: SN6PR02MB4157:EE_|SA0PR02MB7353:EE_ x-ms-office365-filtering-correlation-id: c3527edc-b362-4a95-5f14-08dda2c71e7f x-ms-exchange-slblob-mailprops: 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 x-microsoft-antispam: BCL:0;ARA:14566002|15080799009|8060799009|8062599006|19110799006|461199028|3412199025|440099028|12091999003|102099032|56899033; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: SN6PR02MB4157.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-CrossTenant-Network-Message-Id: c3527edc-b362-4a95-5f14-08dda2c71e7f X-MS-Exchange-CrossTenant-originalarrivaltime: 03 Jun 2025 17:50:24.9884 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa X-MS-Exchange-CrossTenant-rms-persistedconsumerorg: 00000000-0000-0000-0000-000000000000 X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR02MB7353 X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: D3FDF40014 X-Stat-Signature: 6xzn1os9nbipnrtx186fzu3qw471qozd X-Rspam-User: X-HE-Tag: 1748973027-963456 X-HE-Meta: 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 mrsXsz9D U0NqgpescfRsAS5f034Z+yxNdNengEotxYlxca1f6S2DTw/JnAItNw5XLMGNY1cKnMLEqAo8PqfRsoj3YGiGtvViC6ki6XZQOwsx/ 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: List-Subscribe: List-Unsubscribe: From: Thomas Zimmermann Sent: Monday, June 2, 2025 11= :25 PM >=20 > Hi >=20 > Am 03.06.25 um 03:49 schrieb Michael Kelley: > [...] > >> Will the VMA have VM_PFNMAP or VM_MIXEDMAP set? PFN_SPECIAL is a > >> horrible hack. > >> > >> In another thread, you mention that you use PFN_SPECIAL to bypass the > >> check in vm_mixed_ok(), so VM_MIXEDMAP is likely not set? > > The VMA has VM_PFNMAP set, not VM_MIXEDMAP. It seemed like > > VM_MIXEDMAP is somewhat of a superset of VM_PFNMAP, but maybe that's > > a wrong impression. vm_mixed_ok() does a thorough job of validating the > > use of __vm_insert_mixed(), and since what I did was allowed, I thought > > perhaps it was OK. Your feedback has set me straight, and that's what I > > needed. :-) > > > > But the whole approach is moot with Alistair Popple's patch set that > > eliminates pfn_t. Is there an existing mm API that will do mkwrite on a > > special PTE in a VM_PFNMAP VMA? I didn't see one, but maybe I missed > > it. If there's not one, I'll take a crack at adding it in the next vers= ion of my > > patch set. >=20 > What is the motivation behind this work? The driver or fbdev as a whole > does not have much of a future anyway. >=20 > I'd like to suggest removing hyperv_fb entirely in favor of hypervdrm? >=20 Yes, I think that's the longer term direction. A couple months ago I had an email conversation with Saurabh Sengar from the Microsoft Linux team where he raised this idea. I think the Microsoft folks will need to drive the dep= recation process, as they need to coordinate with the distro vendors who publish images for running on local Hyper-V and in the Azure cloud. And my understanding is that the Linux kernel process would want the driver to be available but marked "deprecated" for a year or so before it actually goes away. I do have some concerns about the maturity of the hyperv_drm driver "around the edges". For example, somebody just recently submitted a patch to flush output on panic. I have less familiarity hyperv_drm vs. hyperv_fb, so some of my concern is probably due to that. We might need to do review of hyperv_drm and see if there's anything else to deal with before hyperv_fb goes away. This all got started when I was looking at a problem with hyperv_fb, and I found several other related problems, some of which also existed in hyperv_drm. You've seen several small'ish fixes from me and Saurabh as a result, and this issue with mmap()'ing /dev/fb0 is the last one of tha= t set. This fix is definitely a bit bigger, but it's the right fix. On the fl= ip side,=20 if we really get on a path to deprecate hyperv_fb, there are hack fixes for the mmap problem that are smaller and contained to hyperv_fb. I would be OK with a hack fix in that case. Michael