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

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

[Waiman Long]

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

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

[Frederic Weisbecker]

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

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

[Waiman Long]

On 6/4/26 11:24 AM, Frederic Weisbecker wrote:
> 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()...

You are right. I should have just deferred the memblock_free() call. 
Will send a v3 to fix that.

Thanks,
Longman