Re: [PATCH mm-unstable v15 07/13] mm/khugepaged: add per-order mTHP collapse failure statistics

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

On 2/26/26 04:25, 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 swap
> 	PTEs
> 
> - 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 shared
>   	PTEs
> 
> 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 dont 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.
> 
> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 24 ++++++++++++++++++++++
>  include/linux/huge_mm.h                    |  3 +++
>  mm/huge_memory.c                           |  7 +++++++
>  mm/khugepaged.c                            | 16 ++++++++++++---
>  4 files changed, 47 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index c51932e6275d..eebb1f6bbc6c 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -714,6 +714,30 @@ 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. For mTHP collapse, Currently only max_ptes_none
> +       values of 0 and (HPAGE_PMD_NR - 1) are supported. Any other value will
> +       emit a warning and no mTHP collapse will be attempted. khugepaged will
> +       try to collapse to the largest enabled (m)THP size; if it fails, it will
> +       try the next lower enabled mTHP size. This counter records the number of
> +       times a collapse attempt was skipped for exceeding the max_ptes_none
> +       threshold, and khugepaged will move on to the next available mTHP size.
> +
> +collapse_exceed_swap_pte
> +       The number of anonymous mTHP PTE ranges which were unable to collapse due
> +       to containing at least one swap PTE. Currently khugepaged does not
> +       support collapsing mTHP regions that contain a swap PTE. This counter can
> +       be used to monitor the number of khugepaged mTHP collapses that failed
> +       due to the presence of a swap PTE.
> +
> +collapse_exceed_shared_pte
> +       The number of anonymous mTHP PTE ranges which were unable to collapse due
> +       to containing at least one shared PTE. Currently khugepaged does not
> +       support collapsing mTHP PTE ranges that contain a shared PTE. This
> +       counter can be used to monitor the number of khugepaged mTHP collapses
> +       that failed due to the presence of a 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 9941fc6d7bd8..e8777bb2347d 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 228f35e962b9..1049a207a257 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -642,6 +642,10 @@ 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);
> +
>  
>  static struct attribute *anon_stats_attrs[] = {
>  	&anon_fault_alloc_attr.attr,
> @@ -658,6 +662,9 @@ static struct attribute *anon_stats_attrs[] = {
>  	&split_deferred_attr.attr,
>  	&nr_anon_attr.attr,
>  	&nr_anon_partially_mapped_attr.attr,
> +	&collapse_exceed_swap_pte_attr.attr,
> +	&collapse_exceed_none_pte_attr.attr,
> +	&collapse_exceed_shared_pte_attr.attr,
>  	NULL,
>  };
>  
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index c739f26dd61e..a6cf90e09e4a 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -595,7 +595,9 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  				continue;
>  			} else {
>  				result = SCAN_EXCEED_NONE_PTE;
> -				count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> +				if (is_pmd_order(order))
> +					count_vm_event(THP_SCAN_EXCEED_NONE_PTE);
> +				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_NONE);
>  				goto out;
>  			}
>  		}
> @@ -631,10 +633,17 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma,
>  			 * shared may cause a future higher order collapse on a
>  			 * rescan of the same range.
>  			 */
> -			if (!is_pmd_order(order) || (cc->is_khugepaged &&
> -			    shared > khugepaged_max_ptes_shared)) {
> +			if (!is_pmd_order(order)) {
> +				result = SCAN_EXCEED_SHARED_PTE;
> +				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
> +				goto out;
> +			}
> +
> +			if (cc->is_khugepaged &&
> +			    shared > khugepaged_max_ptes_shared) {
>  				result = SCAN_EXCEED_SHARED_PTE;
>  				count_vm_event(THP_SCAN_EXCEED_SHARED_PTE);
> +				count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED);
>  				goto out;

With the suggested earlier rework, this should hopefully become simply

if (++shared > max_ptes_shared) {
	result = 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);
}

With that (no code duplication) LGTM.

-- 
Cheers,

David