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

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Message-ID: <f7b3b941-ed24-4471-959c-d25601de8366@linux.dev>
Date: Wed, 11 Feb 2026 13:43:50 +0000
From: Usama Arif <usama.arif@...ux.dev>
To: "David Hildenbrand (Arm)" <david@...nel.org>,
 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 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@...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.
> 

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