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[2003:d8:2f0d:f000:eec9:2b8d:4913:f32a]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a4efe5b7b0sm17496378f8f.10.2025.06.03.00.55.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 03 Jun 2025 00:55:03 -0700 (PDT) Message-ID: Date: Tue, 3 Jun 2025 09:55:02 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 Content-Language: en-US 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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 03.06.25 03:49, Michael Kelley wrote: > From: David Hildenbrand Sent: Monday, June 2, 2025 2:48 AM >> >> On 23.05.25 18:15, mhkelley58@gmail.com wrote: >>> From: Michael Kelley >>> >>> Current defio code works only for framebuffer memory that is allocated >>> with vmalloc(). The code assumes that the underlying page refcount can >>> be used by the mm subsystem to manage each framebuffer page's lifecycle, >>> including freeing the page if the refcount goes to 0. This approach is >>> consistent with vmalloc'ed memory, but not with contiguous kernel memory >>> allocated via alloc_pages() or similar. The latter such memory pages >>> usually have a refcount of 0 when allocated, and would be incorrectly >>> freed page-by-page if used with defio. That free'ing corrupts the memory >>> free lists and Linux eventually panics. Simply bumping the refcount after >>> allocation doesn’t work because when the framebuffer memory is freed, >>> __free_pages() complains about non-zero refcounts. >>> >>> Commit 37b4837959cb ("video: deferred io with physically contiguous >>> memory") from the year 2008 purported to add support for contiguous >>> kernel memory framebuffers. The motivating device, sh_mobile_lcdcfb, uses >>> dma_alloc_coherent() to allocate framebuffer memory, which is likely to >>> use alloc_pages(). It's unclear to me how this commit actually worked at >>> the time, unless dma_alloc_coherent() was pulling from a CMA pool instead >>> of alloc_pages(). Or perhaps alloc_pages() worked differently or on the >>> arm32 architecture on which sh_mobile_lcdcfb is used. >>> >>> In any case, for x86 and arm64 today, commit 37b4837959cb9 is not >>> sufficient to support contiguous kernel memory framebuffers. The problem >>> can be seen with the hyperv_fb driver, which may allocate the framebuffer >>> memory using vmalloc() or alloc_pages(), depending on the configuration >>> of the Hyper-V guest VM (Gen 1 vs. Gen 2) and the size of the framebuffer. >>> >>> Fix this limitation by adding defio support for contiguous kernel memory >>> framebuffers. A driver with a framebuffer allocated from contiguous >>> kernel memory must set the FBINFO_KMEMFB flag to indicate such. >>> >>> Tested with the hyperv_fb driver in both configurations -- with a vmalloc() >>> framebuffer and with an alloc_pages() framebuffer on x86. Also verified a >>> vmalloc() framebuffer on arm64. Hardware is not available to me to verify >>> that the older arm32 devices still work correctly, but the path for >>> vmalloc() framebuffers is essentially unchanged. >>> >>> Even with these changes, defio does not support framebuffers in MMIO >>> space, as defio code depends on framebuffer memory pages having >>> corresponding 'struct page's. >>> >>> Fixes: 3a6fb6c4255c ("video: hyperv: hyperv_fb: Use physical memory for fb on HyperV Gen 1 VMs.") >>> Signed-off-by: Michael Kelley >>> --- >>> Changes in v3: >>> * Moved definition of FBINFO_KMEMFB flag to a separate patch >>> preceeding this one in the patch set [Helge Deller] >>> Changes in v2: >>> * Tweaked code comments regarding framebuffers allocated with >>> dma_alloc_coherent() [Christoph Hellwig] >>> >>> drivers/video/fbdev/core/fb_defio.c | 128 +++++++++++++++++++++++----- >>> 1 file changed, 108 insertions(+), 20 deletions(-) >>> >>> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c >>> index 4fc93f253e06..f8ae91a1c4df 100644 >>> --- a/drivers/video/fbdev/core/fb_defio.c >>> +++ b/drivers/video/fbdev/core/fb_defio.c >>> @@ -8,11 +8,40 @@ >>> * for more details. >>> */ >>> >>> +/* >>> + * Deferred I/O ("defio") allows framebuffers that are mmap()'ed to user space >>> + * to batch user space writes into periodic updates to the underlying >>> + * framebuffer hardware or other implementation (such as with a virtualized >>> + * framebuffer in a VM). At each batch interval, a callback is invoked in the >>> + * framebuffer's kernel driver, and the callback is supplied with a list of >>> + * pages that have been modified in the preceding interval. The callback can >>> + * use this information to update the framebuffer hardware as necessary. The >>> + * batching can improve performance and reduce the overhead of updating the >>> + * hardware. >>> + * >>> + * Defio is supported on framebuffers allocated using vmalloc() and allocated >>> + * as contiguous kernel memory using alloc_pages() or kmalloc(). These >>> + * memory allocations all have corresponding "struct page"s. Framebuffers >>> + * allocated using dma_alloc_coherent() should not be used with defio. >>> + * Such allocations should be treated as a black box owned by the DMA >>> + * layer, and should not be deconstructed into individual pages as defio >>> + * does. Framebuffers in MMIO space are *not* supported because MMIO space >>> + * does not have corrresponding "struct page"s. >>> + * >>> + * For framebuffers allocated using vmalloc(), struct fb_info must have >>> + * "screen_buffer" set to the vmalloc address of the framebuffer. For >>> + * framebuffers allocated from contiguous kernel memory, FBINFO_KMEMFB must >>> + * be set, and "fix.smem_start" must be set to the physical address of the >>> + * frame buffer. In both cases, "fix.smem_len" must be set to the framebuffer >>> + * size in bytes. >>> + */ >>> + >>> #include >>> #include >>> #include >>> #include >>> #include >>> +#include >>> #include >>> #include >>> #include >>> @@ -37,7 +66,7 @@ static struct page *fb_deferred_io_get_page(struct fb_info *info, unsigned long >>> else if (info->fix.smem_start) >>> page = pfn_to_page((info->fix.smem_start + offs) >> PAGE_SHIFT); >>> >>> - if (page) >>> + if (page && !(info->flags & FBINFO_KMEMFB)) >>> get_page(page); >>> >>> return page; >>> @@ -137,6 +166,15 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf) >>> >>> BUG_ON(!info->fbdefio->mapping); >>> >>> + if (info->flags & FBINFO_KMEMFB) >>> + /* >>> + * In this path, the VMA is marked VM_PFNMAP, so mm assumes >>> + * there is no struct page associated with the page. The >>> + * PFN must be directly inserted and the created PTE will be >>> + * marked "special". >>> + */ >>> + return vmf_insert_pfn(vmf->vma, vmf->address, page_to_pfn(page)); >>> + >>> vmf->page = page; >>> return 0; >>> } >>> @@ -163,13 +201,14 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_fsync); >>> >>> /* >>> * Adds a page to the dirty list. Call this from struct >>> - * vm_operations_struct.page_mkwrite. >>> + * vm_operations_struct.page_mkwrite or .pfn_mkwrite. >>> */ >>> -static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long offset, >>> +static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, struct vm_fault *vmf, >>> struct page *page) >>> { >>> struct fb_deferred_io *fbdefio = info->fbdefio; >>> struct fb_deferred_io_pageref *pageref; >>> + unsigned long offset = vmf->pgoff << PAGE_SHIFT; >>> vm_fault_t ret; >>> >>> /* protect against the workqueue changing the page list */ >>> @@ -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. > 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. > > 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 :) ) -- Cheers, David / dhildenb