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

[Usama Arif <usamaarif642@gmail.com>]

On 02/02/2026 03:56, Lorenzo Stoakes wrote:
> 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?
> 

Yeah I didnt place it in the right place. Thanks!

>>
>>  /*
>>   * 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.


So up until a few hours before I sent the series. What you are saying is exactly what
I was doing, i.e. allocating up until PUD order. Its not a lot of memory wastage,
but it is there, and I saw this patch as an easy solution to it. For a server
with 512 cores, this is 36KB. Its not a lot because a server with 512 cores will
also have several 100GBs or TBs of memory.

I know its not elegant, but I do like the approach in this patch more. If there is a
very strong preference to switch to having all order to PUD as it would the make the
code more elegant, than I can switch to it.

> 
>>
>> -	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
>>