Re: [PATCH v2] mm: Fix khugepaged activation policy

[Ryan Roberts <ryan.roberts@arm.com>]

On 04/07/2024 19:33, Andrew Morton wrote:
> On Thu,  4 Jul 2024 10:10:50 +0100 Ryan Roberts <ryan.roberts@arm.com> wrote:
> 
>> Since the introduction of mTHP, the docuementation has stated that
>> khugepaged would be enabled when any mTHP size is enabled, and disabled
>> when all mTHP sizes are disabled. There are 2 problems with this; 1.
>> this is not what was implemented by the code and 2. this is not the
>> desirable behavior.
>>
>> Desirable behavior is for khugepaged to be enabled when any PMD-sized
>> THP is enabled, anon or file. (Note that file THP is still controlled by
>> the top-level control so we must always consider that, as well as the
>> PMD-size mTHP control for anon). khugepaged only supports collapsing to
>> PMD-sized THP so there is no value in enabling it when PMD-sized THP is
>> disabled. So let's change the code and documentation to reflect this
>> policy.
>>
>> Further, per-size enabled control modification events were not
>> previously forwarded to khugepaged to give it an opportunity to start or
>> stop. Consequently the following was resulting in khugepaged eroneously
>> not being activated:
>>
>>   echo never > /sys/kernel/mm/transparent_hugepage/enabled
>>   echo always > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
>>
>> ...
>>
>> -static inline bool hugepage_flags_enabled(void)
>> +static inline bool hugepage_pmd_enabled(void)
>>  {
>>  	/*
>> -	 * We cover both the anon and the file-backed case here; we must return
>> -	 * true if globally enabled, even when all anon sizes are set to never.
>> -	 * So we don't need to look at huge_anon_orders_inherit.
>> +	 * We cover both the anon and the file-backed case here; file-backed
>> +	 * hugepages, when configured in, are determined by the global control.
>> +	 * Anon pmd-sized hugepages are determined by the pmd-size control.
>>  	 */
>> -	return hugepage_global_enabled() ||
>> -	       READ_ONCE(huge_anon_orders_always) ||
>> -	       READ_ONCE(huge_anon_orders_madvise);
>> +	return (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && hugepage_global_enabled()) ||
>> +	       test_bit(PMD_ORDER, &huge_anon_orders_always) ||
>> +	       test_bit(PMD_ORDER, &huge_anon_orders_madvise) ||
>> +	       (test_bit(PMD_ORDER, &huge_anon_orders_inherit) && hugepage_global_enabled());
>>  }
> 
> That's rather a mouthful.  Is this nicer?

Sure, I'll take your version into v3.

> 
> static inline bool hugepage_pmd_enabled(void)
> {
> 	/*
> 	 * We cover both the anon and the file-backed case here; file-backed
> 	 * hugepages, when configured in, are determined by the global control.
> 	 * Anon pmd-sized hugepages are determined by the pmd-size control.
> 	 */
> 	if (IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) &&
> 			hugepage_global_enabled())
> 		return true;
> 	if (test_bit(PMD_ORDER, &huge_anon_orders_always))
> 		return true;
> 	if (test_bit(PMD_ORDER, &huge_anon_orders_madvise))
> 		return true;
> 	if (test_bit(PMD_ORDER, &huge_anon_orders_inherit) &&
> 			hugepage_global_enabled())
> 		return true;
> 	return false;
> }
> 
> Also, that's a pretty large function to be inlined.  It could be a
> non-inline function static to khugepaged.c.  But I suppose that's a
> separate patch.

Yeah fair point. I'll respin it now as a static in khugepaged.c.