Re: Request for Feedback on Releasing Reserved Memory Back to the Buddy Allocator

[Harry Yoo <harry.yoo@oracle.com>]

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?

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