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[2003:d8:2f27:ec00:4f4d:d38:ba97:9aa2]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-451fb178895sm10101345e9.14.2025.06.05.01.10.32 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Jun 2025 01:10:32 -0700 (PDT) Message-ID: Date: Thu, 5 Jun 2025 10:10:31 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 3/4] fbdev/deferred-io: Support contiguous kernel memory framebuffers To: Michael Kelley , "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" , "tzimmermann@suse.de" , "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" References: <20250523161522.409504-1-mhklinux@outlook.com> <20250523161522.409504-4-mhklinux@outlook.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: XwXbvoUuKK3jDsSzseVTpBa5fx1xWgCYQvTtpQHUS5A_1749111034 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 048514000E X-Stat-Signature: 8jdg9o8555osctn9wg8nu5hewzfwuhe8 X-Rspam-User: X-HE-Tag: 1749111036-422713 X-HE-Meta: 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 07RO9Zw/ m0GETBTW43XiRTrs8rFBmpNz9eQP3gn46T3OT2TpW6CEW62n7iykihtEWYbgJZxJQjVaMjrcayjV2OeuMyRmN1jHl7NC92Aa4pEk812qXTxcYkBMBeHaQ+9DfAw== 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 04.06.25 23:58, Michael Kelley wrote: > From: Michael Kelley Sent: Tuesday, June 3, 2025 10:25 AM >> >> From: David Hildenbrand Sent: Tuesday, June 3, 2025 12:55 AM >>> >>> On 03.06.25 03:49, Michael Kelley wrote: >>>> From: David Hildenbrand Sent: Monday, June 2, 2025 2:48 AM >>>>> > > [snip] > >>>>>> @@ -182,20 +221,34 @@ static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long >>>>>> } >>>>>> >>>>>> /* >>>>>> - * We want the page to remain locked from ->page_mkwrite until >>>>>> - * the PTE is marked dirty to avoid mapping_wrprotect_range() >>>>>> - * being called before the PTE is updated, which would leave >>>>>> - * the page ignored by defio. >>>>>> - * Do this by locking the page here and informing the caller >>>>>> - * about it with VM_FAULT_LOCKED. >>>>>> + * The PTE must be marked writable before the defio deferred work runs >>>>>> + * again and potentially marks the PTE write-protected. If the order >>>>>> + * should be switched, the PTE would become writable without defio >>>>>> + * tracking the page, leaving the page forever ignored by defio. >>>>>> + * >>>>>> + * For vmalloc() framebuffers, the associated struct page is locked >>>>>> + * before releasing the defio lock. mm will later mark the PTE writaable >>>>>> + * and release the struct page lock. The struct page lock prevents >>>>>> + * the page from being prematurely being marked write-protected. >>>>>> + * >>>>>> + * For FBINFO_KMEMFB framebuffers, mm assumes there is no struct page, >>>>>> + * so the PTE must be marked writable while the defio lock is held. >>>>>> */ >>>>>> - lock_page(pageref->page); >>>>>> + if (info->flags & FBINFO_KMEMFB) { >>>>>> + unsigned long pfn = page_to_pfn(pageref->page); >>>>>> + >>>>>> + ret = vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, >>>>>> + __pfn_to_pfn_t(pfn, PFN_SPECIAL)); >>>>> >>>>> 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_PFNMAP: nothing is refcounted except anon pages >>> >>> VM_MIXEDMAP: anything with a "struct page" (pfn_valid()) is refcounted >>> >>> pte_special() is a way for GUP-fast to distinguish these refcounted (can >>> GUP) from non-refcounted (camnnot GUP) pages mapped by PTEs without any >>> locks or the VMA being available. >>> >>> Setting pte_special() in VM_MIXEDMAP on ptes that have a "struct page" >>> (pfn_valid()) is likely very bogus. >> >> OK, good to know. >> >>> >>>> 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. :-) >>> >>> What exactly are you trying to achieve? :) >>> >>> If it's mapping a page with a "struct page" and *not* refcounting it, >>> then vmf_insert_pfn() is the current way to achieve that in a VM_PFNMAP >>> mapping. It will set pte_special() automatically for you. >>> >> >> Yes, that's what I'm using to initially create the special PTE in the >> .fault callback. >> >>>> >>>> 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 version of my >>>> patch set. >>> >>> I assume you'd want vmf_insert_pfn_mkwrite(), correct? Probably >>> vmf_insert_pfn_prot() can be used by adding PAGE_WRITE to pgprot. (maybe >>> :) ) >> >> Ok, I'll look at that more closely. The sequence is that the special >> PTE gets created with vmf_insert_pfn(). Then when the page is first >> written to, the .pfn_mkwrite callback is invoked by mm. The question >> is the best way for that callback to mark the existing PTE as writable. >> > > FWIW, vmf_insert_pfn_prot() won't work. It calls insert_pfn() with > the "mkwrite" parameter set to 'false', in which case insert_pfn() > does nothing if the PTE already exists. Ah, you are worried about the "already exists but is R/O case". > > So I would need to create a new API that does appropriate validation > for a VM_PFNMAP VMA, and then calls insert_pfn() with the "mkwrite" > parameter set to 'true'. IMHO, nothing would speak against vmf_insert_pfn_mkwrite(). Much better than using that "mixed" ... beauty of a function. -- Cheers, David / dhildenb