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Mon, 3 Aug 2026 14:19:40 +0000 (UTC) Received: from [10.44.50.44] (unknown [10.44.50.44]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id EF83441C; Mon, 3 Aug 2026 14:19:35 +0000 (UTC) Message-ID: <58fc5ce1-b6f5-48df-bd3b-054443c60cdf@redhat.com> Date: Mon, 3 Aug 2026 16:19:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFT 0/3] mm, drm: ensure .fault() does not have to be followed by .pfn_mkwrite() for write faults To: "David Hildenbrand (Arm)" , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Boris Brezillon , Thomas Zimmermann , Michal Hocko , Sergio Lopez , Christian Koenig , Huang Rui , bcm-kernel-feedback-list@broadcom.com, dri-devel@lists.freedesktop.org, linux-mm@kvack.org References: <20260731164341.1109827-1-pbonzini@redhat.com> <8d94897f-0f56-4ecf-bcc8-50bffd6750b2@kernel.org> From: Paolo Bonzini Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: <8d94897f-0f56-4ecf-bcc8-50bffd6750b2@kernel.org> X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 X-Mimecast-MFC-PROC-ID: oWXs-7iky58kThdRQ96Pi2pKkmfbE5E4Md9fw-E3FyA_1785766781 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: CB04D40002 X-Rspam-User: X-Stat-Signature: pg9crdfa4uisiep5ow6wrgo7kaxtccdg X-Rspamd-Server: rspam04 X-HE-Tag: 1785766789-64719 X-HE-Meta: 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 SL778nIp gbXwMoIHEq4EKufSnLH9oE5EQt4ikgZ6Iebws7i9LCaI4DO4T/xJOo81mta6CXz72wDw1Qib7h+GIyosez/pRUNZ+/IBQ6ByK5xVlmnTdqv36PgZe7gN5sK5WyDd1AVPbmjZDszb1agAPR+Bw5J6FDmm56A4CAfTZ7cQQSgjW5M8pKm/p+5jIOhMK3yhSxR5t21kmawZXxGNGEzSvE9USVrwvrb3mUqhYcesWeEyxBHhqe7ui26HjoIE0S2DwV9rOPoigvyqpUQ82U47yjJ6S+n66xAR2wI5nYpGe/X6u3oufaUEXgblstaZ56sXZ+cVKOv0yA9qyt0ECyxjCz9lfuPvJ7KX4ZnJ0oRBBpZ6wUJbpsMGHYMHFKfdTcg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/3/26 13:54, David Hildenbrand (Arm) wrote: > On 7/31/26 18:43, Paolo Bonzini wrote: >> Warning - DRM parts (i.e. most of the patches) untested; I have Cc'd >> the reporter to help with testing these patches. >> >> Right now, users of .pfn_mkwrite() have no way to create a PTE >> that has gone through maybe_mkwrite(). Because vma_set_page_prot() >> will have cleared the writable PTE bit, users of fixup_user_fault() >> will see a read-only PTE and have no clue that the page needs >> a *second* fault to reach its final status. >> >> Handling this in fixup_user_fault() is problematic: the information >> about the presence of *_mkwrite is only recorded in vma->vm_page_prot, >> which is an opaque pgprot_t, therefore only follow_pfnmap_start() >> knows how to retrieve it. > > How is mprotect() supposed to work in that case? Hi David, not sure what you are worried about specifically, but fixup_user_fault() catches !VM_WRITE VMAs and returns early (see vma_permits_fault()). Also, do_wp_page() has the comment: /* * Shared mapping: we are guaranteed to have VM_WRITE and * FAULT_FLAG_WRITE set at this point. */ before the call to wp_pfn_shared() which is where .pfn_mkwrite() is called. Let me know if this was not what you were asking. Thanks for the review of patch 1---I mentioned here in the cover letter that the name was temporary and I'll take your suggestion. I can either use EXPORT_SYMBOL_GPL or switch to inlines, but not both because the existing functions like vmf_insert_pfn_prot() need to stay non-GPL-only. Anyhow, now that the series has a Tested-by I'll clean up everything, and repost later this week. Paolo