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

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

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).

I guess it might be reasonable to have because I am sure it will fail at 
some point and maybe provoke weird issues we didn't think of. In that 
case, having an indication that splitting failed at some point might be 
reasonable.

-- 
Cheers,

David