Re: [PATCH v2] sched/isolation: Postpone re-allocation of housekeeping cpumask until initcall

[Frederic Weisbecker <frederic@kernel.org>]

Le Tue, Jun 02, 2026 at 10:39:51AM -0400, Waiman Long a écrit :
> When testing a linux-next 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.
> 
>   Cannot free reserved memory because of deferred initialization of the memory map
>   WARNING: mm/memblock.c:904 at __free_reserved_area+0xde/0xf0, CPU#0: swapper/0/0
>     :
>   Call Trace:
>    <TASK>
>    memblock_phys_free+0xcb/0x100
>    housekeeping_init+0x14c/0x170
>    start_kernel+0x207/0x450
>    x86_64_start_reservations+0x24/0x30
>    x86_64_start_kernel+0xda/0xe0
>    common_startup_64+0x13e/0x141
>    </TASK>
> 
> IOW, we shouldn't free memblock allocated memory so early
> in the boot process when memory map isn't fully initialized in
> deferred_init_memmap(). Fix it by adding a new housekeeping_late_init()
> helper to defer the re-allocation of the housekeeping cpumasks to
> when initcall's are being processed. Also change rcu_reference() by
> rcu_reference_check() to prevent incorrect RCU lockdep splat as RCU
> will be active in this later boot stage.
> 
> This commit also depends on the presence of commit 7c2eee9c1367
> ("memblock: don't touch memblock arrays when memblock_free() is called
> late") to prevent a KASAN UAF bug report [1].
> 
>  [1] https://lore.kernel.org/lkml/20260505051821.1107133-1-longman@redhat.com/
> 
> Fixes: 27c3a5967f05 ("sched/isolation: Convert housekeeping cpumasks to rcu pointers")
> Signed-off-by: Waiman Long <longman@redhat.com>

Thanks for fixing it. Just some points below:

> ---
>  kernel/sched/isolation.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index ef152d401fe2..a947d75b43f1 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -164,8 +164,6 @@ int housekeeping_update(struct cpumask *isol_mask)
>  
>  void __init housekeeping_init(void)
>  {
> -	enum hk_type type;
> -
>  	if (!housekeeping.flags)
>  		return;
>  
> @@ -173,17 +171,27 @@ void __init housekeeping_init(void)
>  
>  	if (housekeeping.flags & HK_FLAG_KERNEL_NOISE)
>  		sched_tick_offload_init();
> +}
> +
> +static int __init housekeeping_late_init(void)
> +{
> +	enum hk_type type;
> +
> +	if (!housekeeping.flags)
> +		return 0;
> +
>  	/*
>  	 * Realloc with a proper allocator so that any cpumask update
> -	 * can indifferently free the old version with kfree().
> +	 * can indifferently free the old version with kfree(). This
> +	 * should be done after the completion of deferred_init_memmap().
>  	 */
>  	for_each_set_bit(type, &housekeeping.flags, HK_TYPE_MAX) {
>  		struct cpumask *omask, *nmask = kmalloc(cpumask_size(), GFP_KERNEL);
>  
>  		if (WARN_ON_ONCE(!nmask))
> -			return;
> +			return 0;
>  
> -		omask = rcu_dereference(housekeeping.cpumasks[type]);
> +		omask = rcu_dereference_check(housekeeping.cpumasks[type], 1);
>  
>  		/* We need at least one CPU to handle housekeeping work */
>  		WARN_ON_ONCE(cpumask_empty(omask));
> @@ -191,7 +199,9 @@ void __init housekeeping_init(void)
>  		RCU_INIT_POINTER(housekeeping.cpumasks[type], nmask);
>  		memblock_free(omask, cpumask_size());
>  	}
> +	return 0;
>  }
> +pure_initcall(housekeeping_late_init);

Now that it has become an initcall, what prevents other CPUs from accessing
the housekeeping cpumasks concurrently and then dereference the free'ed memory?

And even as a pre-smp initcall, nothing prevents interrupts or preempting tasks
from accessing them either.

From which point on can memblock_free() be called? I see very early calls such
as print_unknown_bootoptions()...

Thanks.

>  
>  static void __init housekeeping_setup_type(enum hk_type type,
>  					   cpumask_var_t housekeeping_staging)
> -- 
> 2.54.0
> 

-- 
Frederic Weisbecker
SUSE Labs