On Tue, Aug 19, 2025 at 7:49 AM Nico Pache <npa...@redhat.com> wrote: > > Now that we can collapse to mTHPs lets update the admin guide to > reflect these changes and provide proper guidence on how to utilize it. > > Reviewed-by: Bagas Sanjaya <bagasdo...@gmail.com> > Signed-off-by: Nico Pache <npa...@redhat.com> I had git send email error and had to resend this patch (13), but i forgot the in-reply-to please ignore this one and reply to correct version
https://lore.kernel.org/lkml/20250819141742.626517-1-npa...@redhat.com/ --in-reply-to=20250819141742.626517-1-npa...@redhat.com -- Nico > --- > Documentation/admin-guide/mm/transhuge.rst | 19 +++++++++++++------ > 1 file changed, 13 insertions(+), 6 deletions(-) > > diff --git a/Documentation/admin-guide/mm/transhuge.rst > b/Documentation/admin-guide/mm/transhuge.rst > index b85547ac4fe9..1f9e6a32052c 100644 > --- a/Documentation/admin-guide/mm/transhuge.rst > +++ b/Documentation/admin-guide/mm/transhuge.rst > @@ -63,7 +63,7 @@ often. > THP can be enabled system wide or restricted to certain tasks or even > memory ranges inside task's address space. Unless THP is completely > disabled, there is ``khugepaged`` daemon that scans memory and > -collapses sequences of basic pages into PMD-sized huge pages. > +collapses sequences of basic pages into huge pages. > > The THP behaviour is controlled via :ref:`sysfs <thp_sysfs>` > interface and using madvise(2) and prctl(2) system calls. > @@ -149,6 +149,18 @@ hugepage sizes have enabled="never". If enabling > multiple hugepage > sizes, the kernel will select the most appropriate enabled size for a > given allocation. > > +khugepaged uses max_ptes_none scaled to the order of the enabled mTHP size > +to determine collapses. When using mTHPs it's recommended to set > +max_ptes_none low-- ideally less than HPAGE_PMD_NR / 2 (255 on 4k page > +size). This will prevent undesired "creep" behavior that leads to > +continuously collapsing to the largest mTHP size; when we collapse, we are > +bringing in new non-zero pages that will, on a subsequent scan, cause the > +max_ptes_none check of the +1 order to always be satisfied. By limiting > +this to less than half the current order, we make sure we don't cause this > +feedback loop. max_ptes_shared and max_ptes_swap have no effect when > +collapsing to a mTHP, and mTHP collapse will fail on shared or swapped out > +pages. > + > It's also possible to limit defrag efforts in the VM to generate > anonymous hugepages in case they're not immediately free to madvise > regions or to never try to defrag memory and simply fallback to regular > @@ -264,11 +276,6 @@ support the following arguments:: > Khugepaged controls > ------------------- > > -.. note:: > - khugepaged currently only searches for opportunities to collapse to > - PMD-sized THP and no attempt is made to collapse to other THP > - sizes. > - > khugepaged runs usually at low frequency so while one may not want to > invoke defrag algorithms synchronously during the page faults, it > should be worth invoking defrag at least in khugepaged. However it's > -- > 2.50.1 >
