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[2003:d8:2f02:2900:f54f:bad7:c5f4:9404]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43f2338d757sm208914365e9.5.2025.04.15.06.18.45 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Apr 2025 06:18:46 -0700 (PDT) Message-ID: Date: Tue, 15 Apr 2025 15:18:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Request for Feedback on Releasing Reserved Memory Back to the Buddy Allocator To: moatasem zaaroura , 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: M5GF6mcH1N4aWHzTX2ZKSo8vEkM4kahySxjNH9Yhwpw_1744723128 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 74F55C000B X-Rspamd-Server: rspam04 X-Stat-Signature: udzemhnakdgafkq16dpct15763bzucqg X-HE-Tag: 1744723131-43326 X-HE-Meta: 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 XWaecKAw scWyi+NgW5wM4OB62tRFrL4aOZgIWvw0SYJswkYuneoIu4vEwCi94nOT6pbi6eXj2uYMuaR7AmM6Z+I0lw/xArdakX93wjP0ZB7UfgJoRN6zsFZ59P22tyfQJS9ET1ytLL4ZqcAKbZwtpHdGOuKdxl1OjYz1Rgc+96BE25befAKMbiTb9OqBLvUd5SXLgVkqsoDD/KKlG06QwhNRzwEmRFKfd5cOgaFuIbVcLUAEf0qZOK3Po2/T3h3O7H5idc9+qQm4FtYUOKQYjKhLaCuYtQSyvdPlSLcemDmWQg/R2mqhdKiCD2YEXnkFQ+wDqsRbMwGbLwM/EI0MsRxc76jaDqx1CxcCorRhCxjY/BbMvasbzsSPH3mAHI7JImOqnvwf8Aq091NGbIdd6HGX5if2KdMyTN2FaUMqZJVPvEYONVns7bN1OBBob0Y5bfXyKETZAZ90PYEpNKtwqzz4= 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 09.04.25 16:28, 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 > if (PageReserved(page)) > free_reserved_page(page); > pfn += 1; Yeah, this is pretty basic freeing of memblock allocations. reserve_mem_release_by_name does something close to that. Which makes me wonder if the "reserved-memory" node you mention above is exactly what reserve_mem_release_by_name() is used for? "Release reserved memory region with a given name" > > == What I'm Asking For == > > - Is this approach correct and safe under the current kernel memory > management design? Yes, that's what free_reserved_area/free_reserved_page is for: to free memblock allocations to the buddy. > - Are there any problems I may have missed? Not that I am aware. > - Is there a better or more canonical way to achieve this? See above. I would either expect that we already have a helper for this, or we should add one. > - If the approach is sound, I believe this pattern may be useful for > others, especially in embedded systems. Would it make sense to > document or upstream a helper for this purpose? > Yeah. I'm really curious if reserve_mem_release_by_name() is what you are looking for, or if it's dealing with "different reserved memory". -- Cheers, David / dhildenb