Re: [RFC 2/2] mm: thp: add THP_SPLIT_PMD_PTE_ALLOC_FAILED counter

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

On 2/11/26 13:49, Usama Arif wrote:
> Add a vmstat counter to track PTE allocation failures during PMD split.
> This enables monitoring of split failures due to memory pressure after
> the lazy PTE page table allocation change.
> 
> The counter is incremented in three places:
> - __split_huge_pmd(): Main entry point for splitting a PMD
> - try_to_unmap_one(): When reclaim needs to split a PMD-mapped THP
> - try_to_migrate_one(): When migration needs to split a PMD-mapped THP
> 
> Visible via /proc/vmstat as thp_split_pmd_pte_alloc_failed.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>   include/linux/vm_event_item.h | 1 +
>   mm/huge_memory.c              | 1 +
>   mm/rmap.c                     | 3 +++
>   mm/vmstat.c                   | 1 +
>   4 files changed, 6 insertions(+)
> 
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 22a139f82d75f..827c9a8c251de 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -111,6 +111,7 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
>   		THP_DEFERRED_SPLIT_PAGE,
>   		THP_UNDERUSED_SPLIT_PAGE,
>   		THP_SPLIT_PMD,
> +		THP_SPLIT_PMD_PTE_ALLOC_FAILED,

Probably sufficient to call this THP_SPLIT_PMD_FAILED and count any 
(future) failures (if any) as well.

It's a shame that we called a remapping a "split" and keep causing 
confusion.

-- 
Cheers,

David