linux-kernel - Re: [PATCH] docs: update THP documentation to clarify sysfs "never" setting

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Message-ID: <35df32ae-dc95-48e1-bdb1-90f17bfd4d5c@linux.alibaba.com>
Date: Tue, 22 Jul 2025 09:30:18 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Nico Pache
 <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
 Jonathan Corbet <corbet@....net>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs: update THP documentation to clarify sysfs "never"
 setting



On 2025/7/21 23:55, Lorenzo Stoakes wrote:
> Rather confusingly, setting all Transparent Huge Page sysfs settings to
> "never" does not in fact result in THP being globally disabled.
> 
> Rather, it results in khugepaged being disabled, but one can still obtain
> THP pages using madvise(..., MADV_COLLAPSE).
> 
> This is something that has remained poorly documented for some time, and it
> is likely the received wisdom of most users of THP that never does, in
> fact, mean never.
> 
> It is therefore important to highlight, very clearly, that this is not the
> ase.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
>   Documentation/admin-guide/mm/transhuge.rst | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index dff8d5985f0f..182519197ef7 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -107,7 +107,7 @@ sysfs
>   Global THP controls
>   -------------------
>   
> -Transparent Hugepage Support for anonymous memory can be entirely disabled
> +Transparent Hugepage Support for anonymous memory can be disabled
>   (mostly for debugging purposes) or only enabled inside MADV_HUGEPAGE
>   regions (to avoid the risk of consuming more memory resources) or enabled
>   system wide. This can be achieved per-supported-THP-size with one of::
> @@ -119,6 +119,11 @@ system wide. This can be achieved per-supported-THP-size with one of::
>   where <size> is the hugepage size being addressed, the available sizes
>   for which vary by system.
>   
> +.. note:: Setting "never" in all sysfs THP controls does **not** disable
> +          Transparent Huge Pages globally. This is because ``madvise(...,
> +          MADV_COLLAPSE)`` ignores these settings and collapses ranges to
> +          PMD-sized huge pages unconditionally.
> +
>   For example::
>   
>   	echo always >/sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> @@ -187,7 +192,9 @@ madvise
>   	behaviour.
>   
>   never
> -	should be self-explanatory.
> +	should be self-explanatory. Note that ``madvise(...,
> +	MADV_COLLAPSE)`` can still cause transparent huge pages to be
> +	obtained even if this mode is specified everywhere.

I hope this part of the explanation is also copy-pasted into the 
'Hugepages in tmpfs/shmem' section. Otherwise look good to me. Thanks.