Re: [PATCH] sched/isolation: Don't free memblock allocated cpumasks

[Mike Rapoport]

Hi Waiman,

On Tue, May 05, 2026 at 01:18:21AM -0400, Waiman Long wrote:
> When testing a v7.1 kernel with commit 59bd1d914bb5 ("memblock: warn when
> freeing reserved memory before memory map is initialized"), the following
> warning was hit when there was a "nohz_full" kernel boot parameter.
> 
> [    0.080911] Cannot free reserved memory because of deferred initialization of the memory map
> [    0.080911] WARNING: mm/memblock.c:904 at __free_reserved_area+0xde/0xf0, CPU#0: swapper/0/0
>   :
> [    0.080945] Call Trace:
> [    0.080947]  <TASK>
> [    0.080949]  memblock_phys_free+0xcb/0x100
> [    0.080953]  housekeeping_init+0x14c/0x170
> [    0.080957]  start_kernel+0x207/0x450
> [    0.080961]  x86_64_start_reservations+0x24/0x30
> [    0.080965]  x86_64_start_kernel+0xda/0xe0
> [    0.080967]  common_startup_64+0x13e/0x141
> [    0.080972]  </TASK>
> 
> The commit states that freeing of reserved memory before the memory
> map is fully initialized in deferred_init_memmap() would cause access
> to uninitialized struct pages and may crash when accessing spurious
> list pointers. However, if the memblock_free() call is deferred to
> the start of initcall processing in the bootup process, for instance,
> the following KASAN warning can appear.
> 
> [    8.514775] BUG: KASAN: use-after-free in memblock_isolate_range+0x4ac/0x650
> [    8.514775] Read of size 8 at addr ffff88a07fe6a000 by task swapper/0/1
>   :
> [    8.514775] Call Trace:
> [    8.514775]  <TASK>
> [    8.514775]  kasan_report+0xb2/0x1b0
> [    8.514775]  memblock_isolate_range+0x4ac/0x650
> [    8.514775]  memblock_phys_free+0xc4/0x190
> [    8.514775]  housekeeping_late_init+0x257/0x280
> [    8.514775]  do_one_initcall+0xaa/0x470
> [    8.514775]  do_initcalls+0x1b4/0x1f0
> [    8.514775]  kernel_init_freeable+0x4b5/0x550
> [    8.514775]  kernel_init+0x1c/0x150
> [    8.514775]  ret_from_fork+0x5dc/0x8e0
> [    8.514775]  ret_from_fork_asm+0x1a/0x30
> [    8.514775]  </TASK>
> 
> It is likely that memblock_discard() may discard memblock data needed
> for memblock_free(). One workaround for now to avoid these warning/bug
> messages is to keep the memblock allocated cpumasks even if they are
> no longer needed until the memblock subsystem is properly updated to
> handle memblock_free().
> 
> On most systems, memory occuipied by a cpumask is pretty small. So not
> much memory will be wasted if the memblock cpumasks are not freed.
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  kernel/sched/isolation.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index ef152d401fe2..ad9b1a1104e3 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -189,7 +189,13 @@ void __init housekeeping_init(void)
>  		WARN_ON_ONCE(cpumask_empty(omask));
>  		cpumask_copy(nmask, omask);
>  		RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask);
> -		memblock_free(omask, cpumask_size());
> +
> +		/*
> +		 * TODO: Don't free memblock allocated cpumasks until the
> +		 * memblock subystem is able to handle the memblock_free()
> +		 * properly.
> +		 */
> +		// memblock_free(omask, cpumask_size());

Before 59bd1d914bb5 it was a silent leak. housekeeping_init() is called
after memblock moves all the memory to buddy, so this would only update
memblock.reserved.

The comment a few lines above says that we reallocate to be able to kfree()
later. Is it possible to delay reallocation until an initcall?

>  	}
>  }
>  
> -- 
> 2.53.0
> 

-- 
Sincerely yours,
Mike.