On 04/07/2024 19:33, Andrew Morton wrote:
> On Thu, 4 Jul 2024 10:10:50 +0100 Ryan Roberts <ryan.roberts@xxxxxxx> 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.
