linux-kernel - Re: [PATCH v3 12/12] Documentation: mm: update the admin guide for mTHP collapse

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Message-ID: <984797ec-9073-4a83-b3ee-7c5041f2d423@infradead.org>
Date: Mon, 14 Apr 2025 18:09:22 -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: akpm@...ux-foundation.org, corbet@....net, rostedt@...dmis.org,
 mhiramat@...nel.org, mathieu.desnoyers@...icios.com, david@...hat.com,
 baohua@...nel.org, baolin.wang@...ux.alibaba.com, ryan.roberts@....com,
 willy@...radead.org, peterx@...hat.com, ziy@...dia.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,
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 anshuman.khandual@....com, catalin.marinas@....com, tiwai@...e.de,
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Subject: Re: [PATCH v3 12/12] Documentation: mm: update the admin guide for
 mTHP collapse



On 4/14/25 3:05 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.
> 
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index dff8d5985f0f..f0d4e78cedaa 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 +144,13 @@ 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 its recommended to set max_ptes_none low.

                                         it's

> +Ideally less than HPAGE_PMD_NR / 2 (255 on 4k page size). This will prevent

   ^^^ not a sentence

> +undesired "creep" behavior that leads to continuously collapsing to a larger
> +mTHP size. 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