[PATCH v4 4/7] docs: transhuge: document process level THP controls
From: Usama Arif <hidden>
Date: 2025-08-13 13:57:33
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 | 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. + - 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. + + prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0): + This will re-enabled THPs for the process, as if they would never have + been disabled. Whether THPs will actually be used depends on global THP + controls. + + prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0): + This returns a value whose bit indicate how THP-disable is configured: + 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 -------------------
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