Re: [PATCH v4 4/7] docs: transhuge: document process level THP controls

[Zi Yan <ziy@nvidia.com>]

On 13 Aug 2025, at 9:55, 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@gmail.com>
> ---
>  Documentation/admin-guide/mm/transhuge.rst | 37 ++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index 370fba1134606..fa8242766e430 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -225,6 +225,43 @@ 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. The THP behaviour set using
> +``PR_SET_THP_DISABLE`` is inherited across fork(2) and execve(2). These calls
> +support the following arguments::
> +
> +	prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0):
> +		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 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.

It is better to change the above sentence to:

madvise(..., MADV_HUGEPAGE) or madvise(..., MADV_COLLAPSE) is used.

Since this document is for sysadmin, who does not need to know the implementation
details like VM_HUGEPAGE. And I do not see any kernel internal is mentioned
in the rest of the document.

> +		- 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.

The same for the above sentence.

Something like:

Note that MADV_COLLAPSE is always rejected if madvise(..., MADV_NOHUGEPAGE) is
used.



> +
> +	prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0):
> +		This will re-enabled THPs for the process, as if they would never have

s/re-enabled/re-enable/

> +		been disabled. Whether THPs will actually be used depends on global THP
> +		controls.

and madvise() calls.

> +
> +	prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0):
> +		This returns a value whose bit indicate how THP-disable is configured:

s/bit/bits

> +		Bits
> +		 1 0  Value  Description
> +		|0|0|   0    No THP-disable behaviour specified.
> +		|0|1|   1    THP is entirely disabled for this process.
> +		|1|1|   3    THP-except-advised mode is set for this process.
> +
>  Khugepaged controls
>  -------------------
>
> -- 
> 2.47.3

Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi