On 13 Aug 2025, at 9:55, 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 | 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. It is better to change the above sentence to: madvise(..., MADV_HUGEPAGE) or madvise(..., MADV_COLLAPSE) is used. Since this document is for sysadmin, who does not need to know the implementation details like VM_HUGEPAGE. And I do not see any kernel internal is mentioned in the rest of the document. > + - 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. The same for the above sentence. Something like: Note that MADV_COLLAPSE is always rejected if madvise(..., MADV_NOHUGEPAGE) is used. > + > + prctl(PR_SET_THP_DISABLE, 0, 0, 0, 0): > + This will re-enabled THPs for the process, as if they would never have s/re-enabled/re-enable/ > + been disabled. Whether THPs will actually be used depends on global THP > + controls. and madvise() calls. > + > + prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0): > + This returns a value whose bit indicate how THP-disable is configured: s/bit/bits > + 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 > ------------------- > > -- > 2.47.3 Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@xxxxxxxxxx> Best Regards, Yan, Zi
