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 <bagasdo...@gmail.com>
> Signed-off-by: Nico Pache <npa...@redhat.com>
> ---
> 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
