Re: [PATCH v3 4/6] docs: transhuge: document process level THP controls
From: David Hildenbrand <hidden>
Date: 2025-08-05 10:24:58
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linux-fsdevel, linux-mm, lkml
On 04.08.25 17:40, Usama Arif wrote:
quoted hunk ↗ jump to hunk
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:: +
Not sure if we really want to talk about MMF_ internals.
+ 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).
" 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 set the MMF_DISABLE_THP_EXCEPT_ADVISED mm flag which + will result in THPs being faulted in or collapsed only for + the following cases:
Maybe: 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. ... > + or MADV_COLLAPSE.
+ - 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).
"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 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
if PR_SET_THP_DISABLE was called without any flags
+ - 3 if MMF_DISABLE_THP_EXCEPT_ADVISED flag is set
if PR_SET_THP_DISABLE was called with PR_THP_DISABLE_EXCEPT_ADVISED
+ - 0 otherwise.
+ Khugepaged controls -------------------
-- Cheers, David / dhildenb