Re: [Bug 221804] New: WARNING at mm/memblock.c:904 __free_reserved_area(): "Cannot free reserved memory because of deferred initialization of the memory map" during housekeeping_init() at boot

[Andrew Morton <akpm@linux-foundation.org>]

(switched to email.  Please respond via emailed reply-to-all, not via the
bugzilla web interface).

On Tue, 28 Jul 2026 15:52:54 +0000 bugzilla-daemon@kernel.org wrote:

> https://bugzilla.kernel.org/show_bug.cgi?id=221804
> 
>             Bug ID: 221804
>            Summary: WARNING at mm/memblock.c:904 __free_reserved_area():
>                     "Cannot free reserved memory because of deferred
>                     initialization of the memory map" during
>                     housekeeping_init() at boot
>            Product: Memory Management
>            Version: 2.5
>           Hardware: All
>                 OS: Linux
>             Status: NEW
>           Severity: normal
>           Priority: P3
>          Component: Other
>           Assignee: akpm@linux-foundation.org
>           Reporter: ionut.nechita@windriver.com
>         Regression: No
> 
> Hello Kernel Team,
> 
> I notice this today:
> 
> ...
> [    0.075447] Dynamic Preempt: lazy
> [    0.075489] ------------[ cut here ]------------
> [    0.075490] Cannot free reserved memory because of deferred initialization
> of the memory map

Thanks.  I asked Gemini "what happened here" and the response was
illuminating.

The bottom lines:


The Core Issue: Initialization Order Conflict

  The kernel hit an order-of-operations conflict between memory setup
  and kernel parameter parsing:

  CONFIG_DEFERRED_STRUCT_PAGE_INIT=y delays initializing struct page
  tracking structures for large pools of RAM until secondary CPUs come
  online (to speed up boot times on large systems).

  housekeeping_init() runs very early during start_kernel().  It
  parses CPU isolation/housekeeping flags (isolcpus=, nohz_full=,
  rcu_nocbs=) and allocates temporary cpumask buffers in memblock. 
  Once parsed, it tries to free those temporary buffers back to the
  memory pool using memblock_phys_free().

  The Collision: memblock_phys_free() calls __free_reserved_area(),
  which requires valid struct page mappings.  Because page
  initialization was deferred, __free_reserved_area() detects that the
  target memory's struct page maps aren't ready yet (at time 0.075s,
  whereas deferred init only finished around 0.325s).

As a safeguard, __free_reserved_area() issues a WARN() and refuses to
free the pages.

Fix/Workaround:

  Fix the nohz_full syntax on the command line (e.g.,
  nohz_full=8-55,64-111).

  To eliminate the warning entirely until fixed upstream, boot with
  deferred_init=off as a kernel parameter or disable
  CONFIG_DEFERRED_STRUCT_PAGE_INIT in the kernel build.  Upstream,
  housekeeping_init() should either avoid early memblock freeing or use
  allocations that don't depend on fully initialized page maps.


I don't think this is very high priority.