Re: [PATCH v10 13/13] Documentation: mm: update the admin guide for mTHP collapse

[Nico Pache]

On Tue, Aug 19, 2025 at 7:49 AM Nico Pache <npache@xxxxxxxxxx> 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>
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-npache@xxxxxxxxxx/
--in-reply-to=20250819141742.626517-1-npache@xxxxxxxxxx



-- 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
>