linux-kernel - Re: [PATCH v7 12/12] Documentation: mm: update the admin guide for mTHP collapse

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Message-ID: <86845229-d8b3-4dae-89d3-888dc4d73ea2@infradead.org>
Date: Wed, 14 May 2025 21:40:57 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Nico Pache <npache@...hat.com>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...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, kirill.shutemov@...ux.intel.com,
 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,
 Bagas Sanjaya <bagasdotme@...il.com>
Subject: Re: [PATCH v7 12/12] Documentation: mm: update the admin guide for
 mTHP collapse



On 5/14/25 8:22 PM, Nico Pache wrote:
> Now that we can collapse to mTHPs lets update the admin guide to
> reflect these changes and provide proper guidence on how to utilize it.
> 
> Reviewed-by: Bagas Sanjaya <bagasdotme@...il.com>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index dff8d5985f0f..5c63fe51b3ad 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -63,7 +63,7 @@ often.
>  THP can be enabled system wide or restricted to certain tasks or even
>  memory ranges inside task's address space. Unless THP is completely
>  disabled, there is ``khugepaged`` daemon that scans memory and
> -collapses sequences of basic pages into PMD-sized huge pages.
> +collapses sequences of basic pages into huge pages.
>  
>  The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>`
>  interface and using madvise(2) and prctl(2) system calls.
> @@ -144,6 +144,18 @@ hugepage sizes have enabled="never". If enabling multiple hugepage
>  sizes, the kernel will select the most appropriate enabled size for a
>  given allocation.
>  
> +khugepaged uses max_ptes_none scaled to the order of the enabled mTHP size
> +to determine collapses. When using mTHPs it's recommended to set
> +max_ptes_none low-- ideally less than HPAGE_PMD_NR / 2 (255 on 4k page
> +size). This will prevent undesired "creep" behavior that leads to
> +continuously collapsing to a larger mTHP size; When we collapse, we are

either                                      size. When
or                                          size; when

> +bringing in new non-zero pages that will, on a subsequent scan, cause the
> +max_ptes_none check of the +1 order to always be satisfied. By limiting
> +this to less than half the current order, we make sure we don't cause this
> +feedback loop. max_ptes_shared and max_ptes_swap have no effect when
> +collapsing to a mTHP, and mTHP collapse will fail on shared or swapped out
> +pages.
> +
>  It's also possible to limit defrag efforts in the VM to generate
>  anonymous hugepages in case they're not immediately free to madvise
>  regions or to never try to defrag memory and simply fallback to regular

-- 
~Randy