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?
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.
