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Message-ID: <fywihr6bre6d7kzdgtzjti72sgidfb6p43oujv2pzuxl223qlo@dyovi7er57dr>
Date: Sun, 9 Nov 2025 13:01:49 +0100
From: Alejandro Colomar <alx@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: linux-man@...r.kernel.org, david@...hat.com,
lorenzo.stoakes@...cle.com, hannes@...xchg.org, baohua@...nel.org, shakeel.butt@...ux.dev,
ziy@...dia.com, laoar.shao@...il.com, baolin.wang@...ux.alibaba.com,
Liam.Howlett@...cle.com, linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v2] PR_*ET_THP_DISABLE.2const: document addition of
PR_THP_DISABLE_EXCEPT_ADVISED
Hi Usama,
On Wed, Nov 05, 2025 at 01:48:11PM +0000, Usama Arif wrote:
> PR_THP_DISABLE_EXCEPT_ADVISED extended PR_SET_THP_DISABLE to only provide
> THPs when advised. IOW, it allows individual processes to opt-out of THP =
> "always" into THP = "madvise", without affecting other workloads on the
> system. The series has been merged in [1]. Before [1], the following 2
> calls were allowed with PR_SET_THP_DISABLE:
>
> prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0); // to reset THP setting.
> prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0); // to disable THPs completely.
>
> Now in addition to the 2 calls above, you can do:
>
> prctl(PR_SET_THP_DISABLE, 1, PR_THP_DISABLE_EXCEPT_ADVISED, 0, 0); // to
> disable THPs except madvise.
>
> This patch documents the changes introduced due to the addition of
> PR_THP_DISABLE_EXCEPT_ADVISED flag:
> - PR_GET_THP_DISABLE returns a value whose bits indicate how THP-disable
> is configured for the calling thread (with or without
> PR_THP_DISABLE_EXCEPT_ADVISED).
> - PR_SET_THP_DISABLE now uses arg3 to specify whether to disable THP
> completely for the process, or disable except madvise
> (PR_THP_DISABLE_EXCEPT_ADVISED).
>
> [1] https://github.com/torvalds/linux/commit/9dc21bbd62edeae6f63e6f25e1edb7167452457b
>
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> ---
> v1 -> v2 (Alejandro Colomar):
> - Fixed double negation on when MADV_HUGEPAGE will succeed
> - Turn return values of PR_GET_THP_DISABLE into a table
> - Turn madvise calls into full italics
> - Use semantic newlines
Thanks! I've applied the patch. I've amended a few things (see below).
Have a lovely day!
Alex
> ---
> man/man2/madvise.2 | 6 ++-
> man/man2const/PR_GET_THP_DISABLE.2const | 20 +++++++---
> man/man2const/PR_SET_THP_DISABLE.2const | 52 +++++++++++++++++++++----
> 3 files changed, 64 insertions(+), 14 deletions(-)
>
> diff --git a/man/man2/madvise.2 b/man/man2/madvise.2
> index 7a4310c40..55c6f4a6c 100644
> --- a/man/man2/madvise.2
> +++ b/man/man2/madvise.2
> @@ -372,9 +372,11 @@ or
> .BR VM_PFNMAP ,
> nor can it be stack memory or backed by a DAX-enabled device
> (unless the DAX device is hot-plugged as System RAM).
> -The process must also not have
> +The process can have
> .B PR_SET_THP_DISABLE
> -set (see
> +set only if
> +.B PR_THP_DISABLE_EXCEPT_ADVISED
> +flag is set (see
I've removed 'flag', for consistency.
> .BR prctl (2)).
> .IP
> The
> diff --git a/man/man2const/PR_GET_THP_DISABLE.2const b/man/man2const/PR_GET_THP_DISABLE.2const
> index 38ff3b370..d63cff21c 100644
> --- a/man/man2const/PR_GET_THP_DISABLE.2const
> +++ b/man/man2const/PR_GET_THP_DISABLE.2const
> @@ -6,7 +6,7 @@
> .SH NAME
> PR_GET_THP_DISABLE
> \-
> -get the state of the "THP disable" flag for the calling thread
> +get the state of the "THP disable" flags for the calling thread
> .SH LIBRARY
> Standard C library
> .RI ( libc ,\~ \-lc )
> @@ -18,13 +18,23 @@ Standard C library
> .B int prctl(PR_GET_THP_DISABLE, 0L, 0L, 0L, 0L);
> .fi
> .SH DESCRIPTION
> -Return the current setting of
> -the "THP disable" flag for the calling thread:
> -either 1, if the flag is set, or 0, if it is not.
> +Return a value whose bits indicate how THP-disable is configured
> +for the calling thread.
> +The returned value is interpreted as follows:
> +.P
> +.TS
> +allbox;
> +cb cb cb l
> +c c c l.
> +Bit 1 Bit 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.
> +.TE
I've replaced this by something simpler:
.TP
.B 0b00
No THP-disable behaviour specified.
.TP
.B 0b01
THP is entirely disabled for this process.
.TP
.B 0b11
THP-except-advised mode is set for this process.
(0b is a binary prefix standardized in ISO C23, and it is now supported
by printf(3) and strtol(3).)
> .SH RETURN VALUE
> On success,
> .BR PR_GET_THP_DISABLE ,
> -returns the boolean value described above.
> +returns the value described above.
> On error, \-1 is returned, and
> .I errno
> is set to indicate the error.
> diff --git a/man/man2const/PR_SET_THP_DISABLE.2const b/man/man2const/PR_SET_THP_DISABLE.2const
> index 532beac66..75e17fa6a 100644
> --- a/man/man2const/PR_SET_THP_DISABLE.2const
> +++ b/man/man2const/PR_SET_THP_DISABLE.2const
> @@ -6,7 +6,7 @@
> .SH NAME
> PR_SET_THP_DISABLE
> \-
> -set the state of the "THP disable" flag for the calling thread
> +set the state of the "THP disable" flags for the calling thread
> .SH LIBRARY
> Standard C library
> .RI ( libc ,\~ \-lc )
> @@ -15,15 +15,20 @@ Standard C library
> .BR "#include <linux/prctl.h>" " /* Definition of " PR_* " constants */"
> .B #include <sys/prctl.h>
> .P
> -.BI "int prctl(PR_SET_THP_DISABLE, long " flag ", 0L, 0L, 0L);"
> +.BI "int prctl(PR_SET_THP_DISABLE, long " thp_disable ", unsigned long " flags ", 0L, 0L);"
> .fi
> .SH DESCRIPTION
> -Set the state of the "THP disable" flag for the calling thread.
> +Set the state of the "THP disable" flags for the calling thread.
> If
> -.I flag
> -has a nonzero value, the flag is set, otherwise it is cleared.
> +.I thp_disable
> +has a nonzero value,
> +the THP disable flag is set according to the value of
> +.I flags,
This should be
.IR flags ,
> +otherwise it is cleared.
> .P
> -Setting this flag provides a method
> +This
> +.BR prctl (2)
> +provides a method
> for disabling transparent huge pages
> for jobs where the code cannot be modified,
> and using a
> @@ -31,10 +36,43 @@ and using a
> hook with
> .BR madvise (2)
> is not an option (i.e., statically allocated data).
> -The setting of the "THP disable" flag is inherited by a child created via
> +The setting of the "THP disable" flags is inherited by a child created via
> .BR fork (2)
> and is preserved across
> .BR execve (2).
> +.P
> +The behavior depends on the value of
> +.IR flags:
This should be:
.IR flags :
> +.TP
> +.B 0
> +The
> +.BR prctl (2)
> +call will disable THPs completely for the process,
> +irrespective of global THP controls or
> +.BR MADV_COLLAPSE .
> +.TP
> +.B PR_THP_DISABLE_EXCEPT_ADVISED
> +The
> +.BR prctl (2)
> +call will disable THPs for the process
> +except when the usage of THPs is
> +advised.
> +Consequently, THPs will only be used when:
> +.RS
> +.IP \[bu] 3
> +Global THP controls are set to "always" or "madvise" and
> +.I \%madvise(...,\~MADV_HUGEPAGE)
> +or
> +.I \%madvise(...,\~MADV_COLLAPSE)
> +is used.
> +.IP \[bu]
> +Global THP controls are set to "never" and
> +.I \%madvise(...,\~MADV_COLLAPSE)
> +is used.
> +This is the same behavior
> +as if THPs would not be disabled on
> +a process level.
> +.RE
> .SH RETURN VALUE
> On success,
> 0 is returned.
> --
> 2.47.3
>
--
<https://www.alejandro-colomar.es>
Use port 80 (that is, <...:80/>).
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