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[2003:d8:2f30:ec00:8f7e:58a4:ebf0:6a36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4eac7d8d9sm2083360f8f.27.2025.05.28.11.03.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 28 May 2025 11:03:04 -0700 (PDT) Message-ID: <7c2198e8-01e4-41ac-a69b-00f06767e5f9@redhat.com> Date: Wed, 28 May 2025 20:03:03 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: fix COW mapping handing in generic_access_phys To: Jason Gunthorpe Cc: Peter Xu , Jinjiang Tu , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, wangkefeng.wang@huawei.com References: <0d4f0180-52e6-47c9-b141-54e7e7c86880@redhat.com> <5b9f5952-9979-426f-857a-dffa9b7963af@redhat.com> <20250528162915.GR61950@nvidia.com> <8b307f83-6a16-46c1-b71b-64af9ca0f592@redhat.com> <20250528174754.GA61950@nvidia.com> <20250528175915.GB61950@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: zen55c9rqprb4kx7bjwexp1oafyyijjb X-Rspamd-Queue-Id: 620F11C0013 X-Rspamd-Server: rspam11 X-HE-Tag: 1748455390-987240 X-HE-Meta: U2FsdGVkX18mdzsse28vxMUMJdITe4qFAUvTUT3pENQUV7kQ5lKqk1ybNPsuvGxcGu5KcoiftFXxbTmMc1MSZMT5Ps3acJUGP+quK229+vw+UFNXm915HGLURMGFqQCUI8GddMfDd8CxyD25ceancb1Jdsb2eY85YXDlJpN8vFYVrOIhyRILRtciXARGtw/5ChGmGLBS5uLeBNxG8NfueeLyjUXgtRd4FuiCEBTNMkGmNnu0KQuI36gouZI76t/T88HnxyR+67ftVyK445W4WmW80gLxwzZgc/yJXbNa32MdIXXobjRed2B0EZo2BHMjTssweMGKiijrJi6fFbY5p0PSCtJNMiBAtE5oBi2cNOzsxFyBdQ2DK85GpUtxS3hOi4RMsP2BYIa4I69eb19lrIJUyrvIW5AJ4UQlL9MZrpTPyHX2xViIkaT4ggfDPUYVZ6UhoWUHhkZZ8SWWaCEIXicaMba80dqE3R9h9v83EbPicPWumtd7ZtPvKAl4ZtrI4J9VQH1NMf+85kkSygO2iF5+jIKL+MSdSXtxwVxwpZPDgZ3H/nKFZvHxTWFYKdtrKyFmThiswVZ3BP+a0vety7V21dllN2YJB4XAVav5YHHxJlzJlO8fePqbzFmRSWPRJtLI8rj5p0V0tVwGpsJcDSqt1CXqdYhA2aQ6EB5I9NKNRAn4o75DHadlghE+MxjlGgFxvLaslK9+tRHeKQ0OYE8UqC7Sn6+un+OG+BCITz/STzegkJwsIzqa8XTUOVhTi/b7H33rDOUGUS7V7pSE3khwMWXv/ZF+oLQipzmgybvdOTVNd1J/Y/XH8IWZHn2yO2srr6RKn2UfX5qSGuA4zZgs6FQZg/8ZSUFIw+qvubmDK4xquO9Mz7JK7v/nWc+9V4Uf+MExn9TXqM4fyERbxcuwycuDeYkwVcisT4krvRUe9s4Vo+gLpsBFau6qmHL4OJu28smOdBfKz1IH8LT Purceieu 3w3F15VDravxSap0BmYj9A5oArxtmEwYr3YsKsb2xzZioXmc7mtc4EMWSqvUIf9+WFPcGQNSNxTpEKdSMWY7+SOyj6XpsvKlfRWRG2HjCwa1G7TamEH3NIm/h7myi39oZKob51NDHWMkOFRLQKrFRibLlKzi0U6vit9/3jzBZwvUsZd6/UjVheHc4W0xAA3CIOZGZXPiIMAcD7soxO0LEUxBaEBQG/ESCKKofaKQy1ejwsOmZlt97Oup/m3nh9zjQovMo332GAIHhWvQu7/N51cX+54KsbOo0bqfBGW6VARO2khdcm+VboyWC8bdMAHLxU4V9fx73gYx6yLVBg+iw4+Slx0Z1NoAvUJxFzML/AON3KmgoVsUzCgxmJEblET60g4sdHOafVdFbsK4pazInu12Nwje5VogwJPTnfNyBiKC5JH7ZMqxGDYYbhJxq+ZTv3BKJuaDZrAiqaHc= 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: On 28.05.25 19:59, Jason Gunthorpe wrote: > On Wed, May 28, 2025 at 02:47:54PM -0300, Jason Gunthorpe wrote: >> On Wed, May 28, 2025 at 07:32:59PM +0200, David Hildenbrand wrote: >>> On 28.05.25 18:29, Jason Gunthorpe wrote: >>>> On Wed, May 28, 2025 at 12:06:07PM -0400, Peter Xu wrote: >>>>> #define VM_PFNMAP 0x00000400 /* Page-ranges managed without "struct page", just pure PFN */ >>>>> >>>>> I'm not confident to blame any driver yet to have those special cases for >>>>> VM_PFNMAP, because it only says "managed without struct page", it didn't >>>>> say "it must not contain struct page".. Hence it hints the core mm "please >>>>> do not manage these mappings with struct page at all". Still sounds fair >>>>> contract, even if not ideal. >>>> >>>> I think it is pretty clear, if a VMA has VM_PFNMAP then nothing must >>>> ever try to obtain a struct page from any PTEs in it, for any reason, >>>> even if things in it might have a struct page. In practice it means >>>> nothing can call vm_normal_page() on a VM_PFNMAP. >>> >>> No, not until we remove any COW mappings of VM_PFNMAP. >> >> I stand by the statement, the COW mapping thing is broken. Just >> because it is wrong doesn't mean it is allowed to call >> vm_normal_page() on VM_PFNMAP. > > I guess I got this backwards in my memory, the vm_normal_page() thing > is specifically to fix the COW nastyness on VM_PFNMAP. > > So it is OK to call vm_normal_page() on both kinds of VMAs and it will > return NULL if you are not allowed to touch the struct page. > > This is still what GUP should be doing, vm_normal_page() and if no > struct page comes back then failure. It should never try to get a > struct page some other way. Exactly. And we can even do that for VM_PFNMAP and just have COW VM_PFNMAP supported in GUP, likely resolving the issue here. Should be fairly easy ... > > Yes, GUP fast will be inconsistent with GUP slow on non-special > arches, but that is unfixable without taking a performance hit. IIRC, GUP-fast requires PTE-special. So it should then all work as expected ... -- Cheers, David / dhildenb