Re: [RFC 02/12] mm/thp: add mTHP stats infrastructure for PUD THP

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Sun, Feb 01, 2026 at 04:50:19PM -0800, Usama Arif wrote:
> Extend the mTHP (multi-size THP) statistics infrastructure to support
> PUD-sized transparent huge pages.
>
> The mTHP framework tracks statistics for each supported THP size through
> per-order counters exposed via sysfs. To add PUD THP support, PUD_ORDER
> must be included in the set of tracked orders.
>
> With this change, PUD THP events (allocations, faults, splits, swaps)
> are tracked and exposed through the existing sysfs interface at
> /sys/kernel/mm/transparent_hugepage/hugepages-1048576kB/stats/. This
> provides visibility into PUD THP behavior for debugging and performance
> analysis.
>
> Signed-off-by: Usama Arif <usamaarif642@gmail.com>

Yeah we really need to be basing this on mm-unstable once Nico's series is
landed.

I think it's quite important as well for you to check that khugepaged mTHP works
with all of this.

> ---
>  include/linux/huge_mm.h | 42 +++++++++++++++++++++++++++++++++++++----
>  mm/huge_memory.c        |  3 ++-
>  2 files changed, 40 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e672e45bb9cc7..5509ba8555b6e 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -76,7 +76,13 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
>   * and including PMD_ORDER, except order-0 (which is not "huge") and order-1
>   * (which is a limitation of the THP implementation).
>   */
> -#define THP_ORDERS_ALL_ANON	((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1)))
> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> +#define THP_ORDERS_ALL_ANON_PUD		BIT(PUD_ORDER)
> +#else
> +#define THP_ORDERS_ALL_ANON_PUD		0
> +#endif
> +#define THP_ORDERS_ALL_ANON	(((BIT(PMD_ORDER + 1) - 1) & ~(BIT(0) | BIT(1))) | \
> +				 THP_ORDERS_ALL_ANON_PUD)

Err what is this change doing in a 'stats' change? This quietly updates
__thp_vma_allowable_orders() to also support PUD order for anon memory... Can we
put this in the right place?

>
>  /*
>   * Mask of all large folio orders supported for file THP. Folios in a DAX
> @@ -146,18 +152,46 @@ enum mthp_stat_item {
>  };
>
>  #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_SYSFS)
> +
> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD

By the way I'm not a fan of us treating an 'arch has' as a 'will use'.

> +#define MTHP_STAT_COUNT		(PMD_ORDER + 2)

Yeah I hate this. This is just 'one more thing to remember'.

> +#define MTHP_STAT_PUD_INDEX	(PMD_ORDER + 1)  /* PUD uses last index */
> +#else
> +#define MTHP_STAT_COUNT		(PMD_ORDER + 1)
> +#endif
> +
>  struct mthp_stat {
> -	unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
> +	unsigned long stats[MTHP_STAT_COUNT][__MTHP_STAT_COUNT];
>  };
>
>  DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
>
> +static inline int mthp_stat_order_to_index(int order)
> +{
> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> +	if (order == PUD_ORDER)
> +		return MTHP_STAT_PUD_INDEX;

This seems like a hack again.

> +#endif
> +	return order;
> +}
> +
>  static inline void mod_mthp_stat(int order, enum mthp_stat_item item, int delta)
>  {
> -	if (order <= 0 || order > PMD_ORDER)
> +	int index;
> +
> +	if (order <= 0)
> +		return;
> +
> +#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> +	if (order != PUD_ORDER && order > PMD_ORDER)
>  		return;
> +#else
> +	if (order > PMD_ORDER)
> +		return;
> +#endif

Or we could actually define a max order... except now the hack contorts this
code.

Is it really that bad to just take up memory for the order between PMD_ORDER and
PUD_ORDER? ~72 bytes * cores and we avoid having to do this silly dance.

>
> -	this_cpu_add(mthp_stats.stats[order][item], delta);
> +	index = mthp_stat_order_to_index(order);
> +	this_cpu_add(mthp_stats.stats[index][item], delta);
>  }
>
>  static inline void count_mthp_stat(int order, enum mthp_stat_item item)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 3128b3beedb0a..d033624d7e1f2 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -598,11 +598,12 @@ static unsigned long sum_mthp_stat(int order, enum mthp_stat_item item)
>  {
>  	unsigned long sum = 0;
>  	int cpu;
> +	int index = mthp_stat_order_to_index(order);
>
>  	for_each_possible_cpu(cpu) {
>  		struct mthp_stat *this = &per_cpu(mthp_stats, cpu);
>
> -		sum += this->stats[order][item];
> +		sum += this->stats[index][item];
>  	}
>
>  	return sum;
> --
> 2.47.3
>