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Message-ID: <20250804154317.1648084-5-usamaarif642@gmail.com>
Date: Mon, 4 Aug 2025 16:40:47 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
david@...hat.com,
linux-mm@...ck.org
Cc: linux-fsdevel@...r.kernel.org,
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Usama Arif <usamaarif642@...il.com>
Subject: [PATCH v3 4/6] docs: transhuge: document process level THP controls
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::
+
+ 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).
+
+ 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:
+ - 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).
+
+ 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
+ - 3 if MMF_DISABLE_THP_EXCEPT_ADVISED flag is set
+ - 0 otherwise.
+
Khugepaged controls
-------------------
--
2.47.3
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