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

[David Hildenbrand <david@redhat.com>]

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