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 creyxXWN 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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? Or does manual probing work for your memory? [1] https://docs.kernel.org/admin-guide/mm/memory-hotplug.html#memory-hotplug-notifications > - 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? > > I would be very grateful for your input and guidance. > > Best regards, > Moatasem Zaaroura > OS Team – Mobileye > -- Cheers, Harry / Hyeonggon