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[2003:d8:2f02:2900:f54f:bad7:c5f4:9404]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43f2338db0dsm214123655e9.7.2025.04.15.06.19.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Apr 2025 06:19:25 -0700 (PDT) Message-ID: <4e138b07-a4e3-4ac1-a274-6d9b54da2efe@redhat.com> Date: Tue, 15 Apr 2025 15:19:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Request for Feedback on Releasing Reserved Memory Back to the Buddy Allocator To: Harry Yoo , moatasem zaaroura Cc: linux-mm@kvack.org References: 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: 2WOwxPJ_SNYJo1QV5Oy6Jr4WjSq5YQGCsVQWKrnvA5Q_1744723167 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: C2C49180008 X-Stat-Signature: 79a1ee3om9mkfzwwtj7djijq4yohpae9 X-Rspam-User: X-Rspamd-Server: rspam12 X-HE-Tag: 1744723169-676763 X-HE-Meta: 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 QitYfrIg HJkJU2W8nrHr0ITa03l5DSDduhPauO4NbDQSKgsWm3JYt851woXFeUookfk+6xQsT+m1RvyPs2fyfJ69ID2JuBbaUAy4BmPIFD3q/d4x//xNEWzM7s7IVdvNi3ivghkF9DD+scGi8X5tKG6zVe4+0F5kz4JGGZsrlXGfVtwBBbTD3ghpuQsAfMG+ambS61hXi6jn/FNWlFMedfw9v81v2eZvupdSiUI/II9qsLgZ6hUlrHOBVucjUFkWqblQ2gpb1YxgNvo9sw16RDLUp97uwA9qfcjcIDUzDMNQimTgnxrJzkmIHgTrcbHQBTBM0DBtWoBpwQRiBZG4IMo0OFeN/1PyaPWhiO8UP6YpyIbD0lIv9DX+Xph9IHjd2KT4Blo474ZIMjeTHtXmvSPkaN3aOHXb0xI5LQ2qBc7KSjgEqR5bAd60GavaI0MJMUzzQwlxJpHV94o8pl/q2pCk= 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 14.04.25 22:25, Harry Yoo wrote: > On Wed, Apr 09, 2025 at 05:28:00PM +0300, moatasem zaaroura wrote: >> Dear Linux MM Community, >> >> I am working on a system that requires reserving a known physical >> memory region during the early boot phase and later releasing it back >> to the kernel for general use. I would highly appreciate your feedback >> on the approach I’ve taken, including any concerns, possible pitfalls, >> or alternative recommendations. >> >> == Problem Context == >> >> In my use case, the boot manager must copy data from flash to a known >> DRAM location while the Linux kernel is still booting. This data is >> then used in user space. After the user-space component finishes using >> the data, I want to release this memory back to the system so it can >> be utilized by the buddy allocator. >> >> == My Solution == >> >> 1. I reserved the memory region using a "reserved-memory" node in the >> device tree with a fixed physical address and size. >> >> 2. This address is shared with the boot manager, which copies the >> required data there before the kernel accesses it. >> >> 3. After the data is no longer needed (in user space), I expose a >> sysfs interface to manually trigger the release of this reserved >> memory back to the kernel. >> >> == Freeing Logic == >> >> In the release function: >> - I validate that the physical address (cache_addr) is page-aligned. >> - I calculate the PFN using: pfn = PFN_DOWN(cache_addr); >> - Then I loop over the pages: >> >> size_t i; >> struct page *page; >> unsigned long pfn; >> unsigned long number_of_pages = cache_size >> PAGE_SHIFT; >> >> if (cache_addr & (PAGE_SIZE - 1)) { >> pr_err("Physical address is not page-aligned\n"); >> return; >> } >> >> pfn = PFN_DOWN(cache_addr); >> for (i = 0; i < number_of_pages; i++) { >> page = pfn_to_page(pfn); >> >> // Ensure the page is not part of the reserved pages > > You mean "Ensure the page is the part of the reserved pages" ? > >> if (PageReserved(page)) >> free_reserved_page(page); >> pfn += 1; >> } >> >> - I verified that the memory is successfully returned to the buddy >> allocator by observing the increased number of free pages at >> /proc/buddyinfo. >> >> == What I'm Asking For == >> >> - Is this approach correct and safe under the current kernel memory >> management design? >> - Are there any problems I may have missed? >> - Is there a better or more canonical way to achieve this? > > I think memory hotplug [1] is an existing Linux approach to achieve this. > > Does your reserved memory show up as a memory device in > /sys/devices/system/memory/memoryXXX? It's a memblock allocation, so that memory would already be added to the system and hotplug does not apply. -- Cheers, David / dhildenb