On 5/14/25 8:22 PM, Nico Pache 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 <bagasdotme@xxxxxxxxx> > Signed-off-by: Nico Pache <npache@xxxxxxxxxx> > --- > Documentation/admin-guide/mm/transhuge.rst | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst > index dff8d5985f0f..5c63fe51b3ad 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. > @@ -144,6 +144,18 @@ 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 a larger mTHP size; When we collapse, we are either size. When or size; when > +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 -- ~Randy
