Re: [PATCH v4 3/4] mm/huge_memory: add a per-order counter for underused splits

[Usama Arif <usama.arif@linux.dev>]

On 03/11/2026 21:20, Joanne Koong wrote:
> thp_underused_split_page counts every folio that the deferred split
> shrinker splits for being underused. Today that is only ever a
> PMD-sized folio, but a later patch queues anonymous mTHP folios for
> underused splitting too, at which point this counter would start
> counting mTHP splits as well.
> 
> thp_underused_split_page could then end up larger than thp_split_page,
> and there would be no per-order count of underused splits. That doesn't
> match how the other THP split stats work. In /proc/vmstat,
> thp_split_page and thp_deferred_split_page only count PMD-sized folios.
> Splits of every size, PMD included, are counted per order in the split
> and split_deferred stats under hugepages-<size>/stats/.
> 
> Do the same for underused splits. Only count PMD-sized folios in
> thp_underused_split_page, and add a per-order split_underused stat that
> counts underused splits of every size. The folio's order has to be read
> before splitting it, since the folio is no longer large once the split
> succeeds.
> 
> There is no functional change to thp_underused_split_page yet, as only
> PMD-sized folios are split for being underused today.
> 
> Suggested-by: Kiryl Shutsemau <kas@kernel.org>
> Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst |  6 ++++++
>  include/linux/huge_mm.h                    |  1 +
>  mm/huge_memory.c                           | 10 ++++++++--
>  3 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index b187d618452f..6f2d92a2a556 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -719,6 +719,12 @@ split_deferred
>          it would free up some memory. Pages on split queue are going to
>          be split under memory pressure, if splitting is possible.
>  

In this same commit, would you mind also doing s/THP/PMD mapped THP/ in
thp_underused_split_page section in /proc/vmstat section?

Apart from that, LGTM

Acked-by: Usama Arif <usama.arif@linux.dev>

> +split_underused
> +	is incremented when a huge page on the split queue was split
> +	because it was underused. This happens when the number of zero-filled
> +	pages in the huge page is above a certain threshold scaled from
> +	/sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none.
> +
>  nr_anon
>         the number of anonymous THP we have in the whole system. These THPs
>         might be currently entirely mapped or have partially unmapped/unused
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index c745f7ad2298..421217aca4ed 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -142,6 +142,7 @@ enum mthp_stat_item {
>  	MTHP_STAT_SPLIT,
>  	MTHP_STAT_SPLIT_FAILED,
>  	MTHP_STAT_SPLIT_DEFERRED,
> +	MTHP_STAT_SPLIT_UNDERUSED,
>  	MTHP_STAT_NR_ANON,
>  	MTHP_STAT_NR_ANON_PARTIALLY_MAPPED,
>  	MTHP_STAT_COLLAPSE_EXCEED_SWAP,
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 903bbe972011..8f90fb609e57 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -822,6 +822,7 @@ DEFINE_MTHP_STAT_ATTR(shmem_fallback_charge, MTHP_STAT_SHMEM_FALLBACK_CHARGE);
>  DEFINE_MTHP_STAT_ATTR(split, MTHP_STAT_SPLIT);
>  DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
>  DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
> +DEFINE_MTHP_STAT_ATTR(split_underused, MTHP_STAT_SPLIT_UNDERUSED);
>  DEFINE_MTHP_STAT_ATTR(nr_anon, MTHP_STAT_NR_ANON);
>  DEFINE_MTHP_STAT_ATTR(nr_anon_partially_mapped, MTHP_STAT_NR_ANON_PARTIALLY_MAPPED);
>  DEFINE_MTHP_STAT_ATTR(collapse_exceed_swap_pte, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
> @@ -842,6 +843,7 @@ static struct attribute *anon_stats_attrs[] = {
>  	&swpout_fallback_attr.attr,
>  #endif
>  	&split_deferred_attr.attr,
> +	&split_underused_attr.attr,
>  	&nr_anon_attr.attr,
>  	&nr_anon_partially_mapped_attr.attr,
>  	&collapse_exceed_swap_pte_attr.attr,
> @@ -4703,6 +4705,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  					    deferred_split_isolate, &dispose);
>  
>  	list_for_each_entry_safe(folio, next, &dispose, _deferred_list) {
> +		const unsigned int order = folio_order(folio);
>  		bool did_split = false;
>  		bool underused = false;
>  
> @@ -4724,8 +4727,11 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
>  			goto requeue;
>  		if (!split_folio(folio)) {
>  			did_split = true;
> -			if (underused)
> -				count_vm_event(THP_UNDERUSED_SPLIT_PAGE);
> +			if (underused) {
> +				if (is_pmd_order(order))
> +					count_vm_event(THP_UNDERUSED_SPLIT_PAGE);
> +				count_mthp_stat(order, MTHP_STAT_SPLIT_UNDERUSED);
> +			}
>  			split++;
>  		}
>  		folio_unlock(folio);