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

[Andrew Morton]

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.