linux-kernel - Re: [RFC 2/2] mm: thp: add THP_SPLIT_PMD_PTE_ALLOC

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Message-ID: <918b5c70-1245-437b-857f-0b202f5bb400@kernel.org>
Date: Wed, 11 Feb 2026 14:38:35 +0100
From: "David Hildenbrand (Arm)" <david@...nel.org>
To: Usama Arif <usama.arif@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>, lorenzo.stoakes@...cle.com,
 willy@...radead.org, linux-mm@...ck.org
Cc: fvdl@...gle.com, hannes@...xchg.org, riel@...riel.com,
 shakeel.butt@...ux.dev, kas@...nel.org, baohua@...nel.org, dev.jain@....com,
 baolin.wang@...ux.alibaba.com, npache@...hat.com, Liam.Howlett@...cle.com,
 ryan.roberts@....com, vbabka@...e.cz, lance.yang@...ux.dev,
 linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [RFC 2/2] mm: thp: add THP_SPLIT_PMD_PTE_ALLOC_FAILED counter

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@...ux.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.

-- 
Cheers,

David