linux-kernel - Re: [PATCH v3 4/6] docs: transhuge: document process level THP controls

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Message-ID: <7828753c-98fc-4f96-bf7c-0d94f3b99e4b@redhat.com>
Date: Tue, 5 Aug 2025 12:24:52 +0200
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc: linux-fsdevel@...r.kernel.org, corbet@....net, rppt@...nel.org,
 surenb@...gle.com, mhocko@...e.com, hannes@...xchg.org, baohua@...nel.org,
 shakeel.butt@...ux.dev, riel@...riel.com, ziy@...dia.com,
 laoar.shao@...il.com, dev.jain@....com, baolin.wang@...ux.alibaba.com,
 npache@...hat.com, lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
 ryan.roberts@....com, vbabka@...e.cz, jannh@...gle.com,
 Arnd Bergmann <arnd@...db.de>, sj@...nel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v3 4/6] docs: transhuge: document process level THP
 controls

On 04.08.25 17:40, Usama Arif wrote:
> This includes the PR_SET_THP_DISABLE/PR_GET_THP_DISABLE pair of
> prctl calls as well the newly introduced PR_THP_DISABLE_EXCEPT_ADVISED
> flag for the PR_SET_THP_DISABLE prctl call.
> 
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> ---
>   Documentation/admin-guide/mm/transhuge.rst | 38 ++++++++++++++++++++++
>   1 file changed, 38 insertions(+)
> 
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 370fba113460..a36a04394ff5 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -225,6 +225,44 @@ to "always" or "madvise"), and it'll be automatically shutdown when
>   PMD-sized THP is disabled (when both the per-size anon control and the
>   top-level control are "never")
>   
> +process THP controls
> +--------------------
> +
> +A process can control its own THP behaviour using the ``PR_SET_THP_DISABLE``
> +and ``PR_GET_THP_DISABLE`` pair of prctl(2) calls. These calls support the
> +following arguments::
> +

Not sure if we really want to talk about MMF_ internals.

> +	prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0):
> +		This will set the MMF_DISABLE_THP_COMPLETELY mm flag which will
> +		result in no THPs being faulted in or collapsed, irrespective
> +		of global THP controls. This flag and hence the behaviour is
> +		inherited across fork(2) and execve(2).

"
This will disable THPs completely for the process, irrespective
of global THP controls or MADV_COLLAPSE.
"

?

> +
> +	prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0):
> +		This will set the MMF_DISABLE_THP_EXCEPT_ADVISED mm flag which
> +		will result in THPs being faulted in or collapsed only for
> +		the following cases:

Maybe:

This will disable THPs for the process except when the usage of THPs is 
advised. Consequently, THPs will only be used when:
- Global THP controls are set to "always" or "madvise" and
   the area either has VM_HUGEPAGE set (e.g., due do MADV_HUGEPAGE) or
   MADV_COLLAPSE is used.
- Global THP controls are set to "never" and MADV_COLLAPSE is used. This
   is the same behavior as if THPs would not be disabled on a process
   level.
Note that MADV_COLLAPSE is currently always rejected if VM_NOHUGEPAGE is 
set on an area.
...

 > +		  or MADV_COLLAPSE.
> +		- Global THP controls are set to "always" or "madvise" and
> +		  the process has madvised the region with either MADV_HUGEPAGE
> +		  or MADV_COLLAPSE.
> +		- Global THP controls is set to "never" and the process has
> +		  madvised the region with MADV_COLLAPSE.
> +		This flag and hence the behaviour is inherited across fork(2)
> +		and execve(2).
> +
> +	prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0):
> +		This will clear the MMF_DISABLE_THP_COMPLETELY and
> +		MMF_DISABLE_THP_EXCEPT_ADVISED mm flags. The process will
> +		behave according to the global THP controls. This behaviour
> +		will be inherited across fork(2) and execve(2).

"This will re-enabled THPs for the process, as if they would never have 
been disabled. Whether THPs will actually be used depends on global THP 
controls."

?

> +
> +	prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0):
> +		This will return the THP disable mm flag status of the process
> +		that was set by prctl(PR_SET_THP_DISABLE, ...). i.e.
> +		- 1 if MMF_DISABLE_THP_COMPLETELY flag is set

if PR_SET_THP_DISABLE was called without any flags

> +		- 3 if MMF_DISABLE_THP_EXCEPT_ADVISED flag is set

if PR_SET_THP_DISABLE was called with PR_THP_DISABLE_EXCEPT_ADVISED

> +		- 0 otherwise.

> +
>   Khugepaged controls
>   -------------------
>   


-- 
Cheers,

David / dhildenb