Re: [PATCH mm-unstable v18 08/14] mm/khugepaged: add per-order mTHP collapse failure statistics

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/22/26 17:00, Nico Pache wrote:
> Add three new mTHP statistics to track collapse failures for different
> orders when encountering swap PTEs, excessive none PTEs, and shared PTEs:
> 
> - collapse_exceed_swap_pte: Increment when mTHP collapse fails due to
> 	encountering a swap PTE.
> 
> - collapse_exceed_none_pte: Counts when mTHP collapse fails due to
>   	exceeding the none PTE threshold for the given order
> 
> - collapse_exceed_shared_pte: Counts when mTHP collapse fails due to
> 	encountering a shared PTE.
> 
> These statistics complement the existing THP_SCAN_EXCEED_* events by
> providing per-order granularity for mTHP collapse attempts. The stats are
> exposed via sysfs under
> `/sys/kernel/mm/transparent_hugepage/hugepages-*/stats/` for each
> supported hugepage size.
> 
> As we currently do not support collapsing mTHPs that contain a swap or
> shared entry, those statistics keep track of how often we are
> encountering failed mTHP collapses due to these restrictions.
> 
> We will add support for mTHP collapse for anonymous pages next; lets also
> track when this happens at the PMD level within the per-mTHP stats.
> 
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 14 ++++++++++++++
>  include/linux/huge_mm.h                    |  3 +++
>  mm/huge_memory.c                           |  7 +++++++
>  mm/khugepaged.c                            | 21 +++++++++++++++++++--
>  4 files changed, 43 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index c51932e6275d..80a4d0bed70b 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -714,6 +714,20 @@ nr_anon_partially_mapped
>         an anonymous THP as "partially mapped" and count it here, even though it
>         is not actually partially mapped anymore.
>  
> +collapse_exceed_none_pte
> +       The number of collapse attempts that failed due to exceeding the
> +       max_ptes_none threshold.
> +
> +collapse_exceed_swap_pte
> +       The number of collapse attempts that failed due to exceeding the
> +       max_ptes_swap threshold. For non-PMD orders this occurs if a mTHP range
> +       contains at least one swap PTE.
> +
> +collapse_exceed_shared_pte
> +       The number of collapse attempts that failed due to exceeding the
> +       max_ptes_shared threshold. For non-PMD orders this occurs if a mTHP range
> +       contains at least one shared PTE.
> +
>  As the system ages, allocating huge pages may be expensive as the
>  system uses memory compaction to copy data around memory to free a
>  huge page for use. There are some counters in ``/proc/vmstat`` to help
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index ba7ae6808544..48496f09909b 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -144,6 +144,9 @@ enum mthp_stat_item {
>  	MTHP_STAT_SPLIT_DEFERRED,
>  	MTHP_STAT_NR_ANON,
>  	MTHP_STAT_NR_ANON_PARTIALLY_MAPPED,
> +	MTHP_STAT_COLLAPSE_EXCEED_SWAP,
> +	MTHP_STAT_COLLAPSE_EXCEED_NONE,
> +	MTHP_STAT_COLLAPSE_EXCEED_SHARED,
>  	__MTHP_STAT_COUNT
>  };
>  
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 345c54133c83..5c128cdec810 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -703,6 +703,10 @@ DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
>  DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
>  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);
> +DEFINE_MTHP_STAT_ATTR(collapse_exceed_none_pte, MTHP_STAT_COLLAPSE_EXCEED_NONE);
> +DEFINE_MTHP_STAT_ATTR(collapse_exceed_shared_pte, MTHP_STAT_COLLAPSE_EXCEED_SHARED)

[...]

>  		/* See collapse_scan_pmd(). */
>  		if (folio_maybe_mapped_shared(folio)) {
> +			/*
> +			 * TODO: Support shared pages without leading to further
> +			 * mTHP collapses. Currently bringing in new pages via
> +			 * shared may cause a future higher order collapse on a
> +			 * rescan of the same range.
> +			 */

This comment actually belongs into an earlier patch, no?

>  			if (++shared > max_ptes_shared) {
>  				result = SCAN_EXCEED_SHARED_PTE;
> -				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> +				if (is_pmd_order(order))
> +					count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> +				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
>  				goto out;
>  			}
>  		}
> @@ -1138,6 +1148,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm,
>  		 * range.
>  		 */
>  		if (!is_pmd_order(order)) {
> +			count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP);
>  			pte_unmap(pte);
>  			mmap_read_unlock(mm);
>  			result = SCAN_EXCEED_SWAP_PTE;
> @@ -1433,6 +1444,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  			if (++none_or_zero > max_ptes_none) {
>  				result = SCAN_EXCEED_NONE_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> +				count_mthp_stat(HPAGE_PMD_ORDER,
> +						MTHP_STAT_COLLAPSE_EXCEED_NONE);
>  				goto out_unmap;
>  			}
>  			continue;
> @@ -1441,6 +1454,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  			if (++unmapped > max_ptes_swap) {
>  				result = SCAN_EXCEED_SWAP_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_SWAP_PTE);
> +				count_mthp_stat(HPAGE_PMD_ORDER,
> +						MTHP_STAT_COLLAPSE_EXCEED_SWAP);
>  				goto out_unmap;
>  			}
>  			/*
> @@ -1498,6 +1513,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
>  			if (++shared > max_ptes_shared) {
>  				result = SCAN_EXCEED_SHARED_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> +				count_mthp_stat(HPAGE_PMD_ORDER,
> +						MTHP_STAT_COLLAPSE_EXCEED_SHARED);

Can be done as a later cleanup, but having a single function that obtains an
order and knows which stats to update would be cleaner (and a good preparation
for shmem mTHP collapse support).

Nothing jumped at me, so

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David