On 04.08.25 17:40, Usama Arif wrote:
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@xxxxxxxxx>
---
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
