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

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

On 11/02/2026 13:38, David Hildenbrand (Arm) wrote:
> On 2/11/26 14:31, Usama Arif wrote:
>>
>>
>> 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@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.
>>>
>>
>> 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).
> 
> Btw, you can use the allocation fault injection framework to find weird issues, if you haven't heard of that yet.
> 

This looks very interesting, Thanks! Let me have a look.