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

[Usama Arif]

On 11/02/2026 13:27, David Hildenbrand (Arm) wrote:
> 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@xxxxxxxxx>
>> ---
>>   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.
> 

Makes sense. This was just a patch I was using for testing and I wanted to share.
It was always 0 as I couldnt get split to fail :) But I can rename it as THP_SPLIT_PMD_FAILED
as suggested and we can use for future split failures (hopefully none).

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