Re: [PATCH v4 12/12] Documentation: mm: update the admin guide for mTHP collapse

Usama Arif
2025-04-24T15:03:55Z

On 17/04/2025 01:02, 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.
>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
> Documentation/admin-guide/mm/transhuge.rst | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> index dff8d5985f0f..06814e05e1d5 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 +144,14 @@ 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. 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.
> +

Hi Nico,

Could you add a bit more explanation of the creep behaviour here in documentation.
I remember you explained in one of the earlier versions that if more than half of the
collapsed mTHP is zero-filled, it for some reason becomes eligible for collapsing to
larger order, but if less than half is zero-filled its not eligible? I cant exactly
remember what the reason was :) Would be good to have it documented more if possible.

Thanks

> 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