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Mon, 01 Sep 2025 09:10:39 -0700 (PDT) Received: from localhost ([2a03:2880:20ff:2::]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7fc16341017sm683721285a.63.2025.09.01.09.10.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Sep 2025 09:10:38 -0700 (PDT) From: Usama Arif To: alx@kernel.org Cc: linux-man@vger.kernel.org, david@redhat.com, lorenzo.stoakes@oracle.com, hannes@cmpxchg.org, baohua@kernel.org, shakeel.butt@linux.dev, ziy@nvidia.com, laoar.shao@gmail.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, linux-kernel@vger.kernel.org, kernel-team@meta.com, Usama Arif Subject: [PATCH] PR_*ET_THP_DISABLE.2const: document addition of PR_THP_DISABLE_EXCEPT_ADVISED Date: Mon, 1 Sep 2025 17:09:03 +0100 Message-ID: <20250901160903.2801339-1-usamaarif642@gmail.com> X-Mailer: git-send-email 2.47.3 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 = =3D "always" into THP =3D "madvise", without affecting other workloads on the system. The series has been merged in [1]. 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://lore.kernel.org/all/20250815135549.130506-1-usamaarif642@gmail.= com/ Signed-off-by: Usama Arif --- man/man2/madvise.2 | 4 +- man/man2const/PR_GET_THP_DISABLE.2const | 18 ++++++--- man/man2const/PR_SET_THP_DISABLE.2const | 52 +++++++++++++++++++++---- 3 files changed, 61 insertions(+), 13 deletions(-) diff --git a/man/man2/madvise.2 b/man/man2/madvise.2 index 10cc21fa4..6a5290f67 100644 --- a/man/man2/madvise.2 +++ b/man/man2/madvise.2 @@ -373,7 +373,9 @@ nor can it be stack memory or backed by a DAX-enabled d= evice (unless the DAX device is hot-plugged as System RAM). The process must also not have .B PR_SET_THP_DISABLE -set (see +set without the +.B PR_THP_DISABLE_EXCEPT_ADVISED +flag (see .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..df239700f 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 +18,21 @@ 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. +Returns a value whose bits indicate how THP-disable is configured +for the calling thread. +The returned value is interpreted as follows: +.P +.nf +.B "Bits" +.B " 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. +.fi .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 564e005d4..9f0f17702 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 ) @@ -15,24 +15,62 @@ Standard C library .BR "#include " " /* Definition of " PR_* " constants */" .B #include .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 " f= lags ", 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, +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 malloc 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: +.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] 2 +Global THP controls are set to "always" or "madvise" and +.BR madvise (..., +.BR MADV_HUGEPAGE ) +or +.BR madvise (..., +.BR MADV_COLLAPSE ) +is used. +.IP \[bu] +Global THP controls are set to "never" and +.BR madvise (..., +.BR 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. --=20 2.47.3