Re: [PATCH 6/6] mm: change to return bool for the MMU notifier's young flag check

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 3/19/26 7:39 PM, Lorenzo Stoakes (Oracle) wrote:
> On Thu, Mar 19, 2026 at 11:24:05AM +0800, Baolin Wang wrote:
>> The MMU notifier young flag check related functions only return whether
>> the young flag was set. Change the return type to bool to make the
>> intention clearer.
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> I can see KVM is the only user for the mmu_notifier_ops clear_flush_young,
> clear_young and test_young hooks, which map to
> kvm_mmu_notifier_[clear_flush,clear,test]_young() functions, and you have
> updated them all.

Yes.

> So this LGTM with nits below, and so (with nits addressed as per the other R-b
> tags :):
> 
> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> 
> Thanks for doing this! Int as bool is a pet peeve of mine :))

Thanks for reviewing.

>>   include/linux/mmu_notifier.h | 76 +++++++++++++++++-------------------
>>   mm/internal.h                | 16 ++++----
>>   mm/mmu_notifier.c            | 20 +++++-----
>>   virt/kvm/kvm_main.c          | 40 +++++++++----------
>>   4 files changed, 72 insertions(+), 80 deletions(-)
>>
>> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
>> index 3705d350c863..17f2cdc77dd5 100644
>> --- a/include/linux/mmu_notifier.h
>> +++ b/include/linux/mmu_notifier.h
>> @@ -97,20 +97,20 @@ struct mmu_notifier_ops {
>>   	 * Start-end is necessary in case the secondary MMU is mapping the page
>>   	 * at a smaller granularity than the primary MMU.
>>   	 */
>> -	int (*clear_flush_young)(struct mmu_notifier *subscription,
>> -				 struct mm_struct *mm,
>> -				 unsigned long start,
>> -				 unsigned long end);
>> +	bool (*clear_flush_young)(struct mmu_notifier *subscription,
>> +				  struct mm_struct *mm,
>> +				  unsigned long start,
>> +				  unsigned long end);
>>
>>   	/*
>>   	 * clear_young is a lightweight version of clear_flush_young. Like the
>>   	 * latter, it is supposed to test-and-clear the young/accessed bitflag
>>   	 * in the secondary pte, but it may omit flushing the secondary tlb.
>>   	 */
>> -	int (*clear_young)(struct mmu_notifier *subscription,
>> -			   struct mm_struct *mm,
>> -			   unsigned long start,
>> -			   unsigned long end);
>> +	bool (*clear_young)(struct mmu_notifier *subscription,
>> +			    struct mm_struct *mm,
>> +			    unsigned long start,
>> +			    unsigned long end);
>>
>>   	/*
>>   	 * test_young is called to check the young/accessed bitflag in
>> @@ -118,9 +118,9 @@ struct mmu_notifier_ops {
>>   	 * frequently used without actually clearing the flag or tearing
>>   	 * down the secondary mapping on the page.
>>   	 */
>> -	int (*test_young)(struct mmu_notifier *subscription,
>> -			  struct mm_struct *mm,
>> -			  unsigned long address);
>> +	bool (*test_young)(struct mmu_notifier *subscription,
>> +			   struct mm_struct *mm,
>> +			   unsigned long address);
>>
>>   	/*
>>   	 * invalidate_range_start() and invalidate_range_end() must be
>> @@ -376,14 +376,12 @@ mmu_interval_check_retry(struct mmu_interval_notifier *interval_sub,
>>
>>   extern void __mmu_notifier_subscriptions_destroy(struct mm_struct *mm);
>>   extern void __mmu_notifier_release(struct mm_struct *mm);
>> -extern int __mmu_notifier_clear_flush_young(struct mm_struct *mm,
>> -					  unsigned long start,
>> -					  unsigned long end);
>> -extern int __mmu_notifier_clear_young(struct mm_struct *mm,
>> -				      unsigned long start,
>> -				      unsigned long end);
>> -extern int __mmu_notifier_test_young(struct mm_struct *mm,
>> -				     unsigned long address);
>> +bool __mmu_notifier_clear_flush_young(struct mm_struct *mm,
>> +		unsigned long start, unsigned long end);
>> +bool __mmu_notifier_clear_young(struct mm_struct *mm,
>> +		unsigned long start, unsigned long end);
>> +bool __mmu_notifier_test_young(struct mm_struct *mm,
>> +		unsigned long address);
>>   extern int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *r);
>>   extern void __mmu_notifier_invalidate_range_end(struct mmu_notifier_range *r);
>>   extern void __mmu_notifier_arch_invalidate_secondary_tlbs(struct mm_struct *mm,
> 
> I mean damn, at this point maybe it's legit to drop the surrounding externs here
> too? But maybe not :))

I prefer to leave the others as is:)

[snip]

>>
>> -static __always_inline int kvm_age_hva_range(struct mmu_notifier *mn,
>> -						unsigned long start,
>> -						unsigned long end,
>> -						gfn_handler_t handler,
>> -						bool flush_on_ret)
>> +static __always_inline bool kvm_age_hva_range(struct mmu_notifier *mn,
>> +					      unsigned long start,
>> +					      unsigned long end,
>> +					      gfn_handler_t handler,
>> +					      bool flush_on_ret)
> 
> Can we please fix this terrrible indentation while we're here :)?
> 
> static __always_inline bool kvm_age_hva_range(struct mmu_notifier *mn,
> 		unsigned long start, unsigned long end, gfn_handler_t handler,
> 		bool flush_on_ret)
> 
> Would be nicer, thanks!

Will do if KVM maintainers are also happy with this.