linux-kernel - Re: [PATCH v13 mm-new 10/16] khugepaged: add per-order mTHP collapse failure statistics

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Message-ID: <654dc140-3ee6-4c0c-96d6-c1405b048dc1@infradead.org>
Date: Mon, 1 Dec 2025 10:38:55 -0800
From: Randy Dunlap <rdunlap@...radead.org>
To: Nico Pache <npache@...hat.com>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-doc@...r.kernel.org
Cc: david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, ryan.roberts@....com,
 dev.jain@....com, corbet@....net, rostedt@...dmis.org, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com, akpm@...ux-foundation.org,
 baohua@...nel.org, willy@...radead.org, peterx@...hat.com,
 wangkefeng.wang@...wei.com, usamaarif642@...il.com, sunnanyong@...wei.com,
 vishal.moola@...il.com, thomas.hellstrom@...ux.intel.com,
 yang@...amperecomputing.com, kas@...nel.org, aarcange@...hat.com,
 raquini@...hat.com, anshuman.khandual@....com, catalin.marinas@....com,
 tiwai@...e.de, will@...nel.org, dave.hansen@...ux.intel.com, jack@...e.cz,
 cl@...two.org, jglisse@...gle.com, surenb@...gle.com, zokeefe@...gle.com,
 hannes@...xchg.org, rientjes@...gle.com, mhocko@...e.com, hughd@...gle.com,
 richard.weiyang@...il.com, lance.yang@...ux.dev, vbabka@...e.cz,
 rppt@...nel.org, jannh@...gle.com, pfalcato@...e.de
Subject: Re: [PATCH v13 mm-new 10/16] khugepaged: add per-order mTHP collapse
 failure statistics



On 12/1/25 9:46 AM, Nico Pache wrote:
> Add three new mTHP statistics to track collapse failures for different
> orders when encountering swap PTEs, excessive none PTEs, and shared PTEs:
> 
> - collapse_exceed_swap_pte: Increment when mTHP collapse fails due to swap
> 	PTEs
> 
> - collapse_exceed_none_pte: Counts when mTHP collapse fails due to
>   	exceeding the none PTE threshold for the given order
> 
> - collapse_exceed_shared_pte: Counts when mTHP collapse fails due to shared
>   	PTEs
> 
> These statistics complement the existing THP_SCAN_EXCEED_* events by
> providing per-order granularity for mTHP collapse attempts. The stats are
> exposed via sysfs under
> `/sys/kernel/mm/transparent_hugepage/hugepages-*/stats/` for each
> supported hugepage size.
> 
> As we currently dont support collapsing mTHPs that contain a swap or
> shared entry, those statistics keep track of how often we are
> encountering failed mTHP collapses due to these restrictions.
> 
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 24 ++++++++++++++++++++++
>  include/linux/huge_mm.h                    |  3 +++
>  mm/huge_memory.c                           |  7 +++++++
>  mm/khugepaged.c                            | 16 ++++++++++++---
>  4 files changed, 47 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index c51932e6275d..d396d1bfb274 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -714,6 +714,30 @@ nr_anon_partially_mapped
>         an anonymous THP as "partially mapped" and count it here, even though it
>         is not actually partially mapped anymore.
>  
> +collapse_exceed_none_pte
> +       The number of collapse attempts that failed due to exceeding the
> +       max_ptes_none threshold. For mTHP collapse, Currently only max_ptes_none

	                                              currently

> +       values of 0 and (HPAGE_PMD_NR - 1) are supported. Any other value will
> +       emit a warning and no mTHP collapse will be attempted. khugepaged will
> +       try to collapse to the largest enabled (m)THP size, if it fails, it will

	                                                size. If
or
	                                                size; if

> +       try the next lower enabled mTHP size. This counter records the number of
> +       times a collapse attempt was skipped for exceeding the max_ptes_none
> +       threshold, and khugepaged will move on to the next available mTHP size.
> +
> +collapse_exceed_swap_pte
> +       The number of anonymous mTHP pte ranges which were unable to collapse due

	                               PTE

> +       to containing at least one swap PTE. Currently khugepaged does not
> +       support collapsing mTHP regions that contain a swap PTE. This counter can
> +       be used to monitor the number of khugepaged mTHP collapses that failed
> +       due to the presence of a swap PTE.
> +
> +collapse_exceed_shared_pte
> +       The number of anonymous mTHP pte ranges which were unable to collapse due

	                               PTE

> +       to containing at least one shared PTE. Currently khugepaged does not
> +       support collapsing mTHP pte ranges that contain a shared PTE. This

	                          PTE

> +       counter can be used to monitor the number of khugepaged mTHP collapses
> +       that failed due to the presence of a shared PTE.
> +
>  As the system ages, allocating huge pages may be expensive as the
>  system uses memory compaction to copy data around memory to free a
>  huge page for use. There are some counters in ``/proc/vmstat`` to help


-- 
~Randy