[PATCH v3 4/6] docs: transhuge: document process level THP controls
From: Usama Arif <hidden>
Date: 2025-08-04 15:43:43
Also in:
linux-fsdevel, linux-mm, lkml
Subsystem:
documentation, memory management - misc, memory management - thp (transparent huge page), the rest · Maintainers:
Jonathan Corbet, Andrew Morton, David Hildenbrand, Lorenzo Stoakes, Linus Torvalds
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 <redacted> --- 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 -------------------
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