On 11/02/2025 00:40, Nico Pache wrote:
> The new transparent_hugepage=defer option allows for a more conservative
> approach to THPs. Document its usage in the transhuge admin-guide.
>
> Signed-off-by: Nico Pache <npa...@redhat.com>
> ---
> Documentation/admin-guide/mm/transhuge.rst | 22 +++++++++++++++++-----
> 1 file changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/admin-guide/mm/transhuge.rst
> b/Documentation/admin-guide/mm/transhuge.rst
> index dff8d5985f0f..b3b18573bbb4 100644
> --- a/Documentation/admin-guide/mm/transhuge.rst
> +++ b/Documentation/admin-guide/mm/transhuge.rst
> @@ -88,8 +88,9 @@ In certain cases when hugepages are enabled system wide,
> application
> may end up allocating more memory resources. An application may mmap a
> large region but only touch 1 byte of it, in that case a 2M page might
> be allocated instead of a 4k page for no good. This is why it's
> -possible to disable hugepages system-wide and to only have them inside
> -MADV_HUGEPAGE madvise regions.
> +possible to disable hugepages system-wide, only have them inside
> +MADV_HUGEPAGE madvise regions, or defer them away from the page fault
> +handler to khugepaged.
>
> Embedded systems should enable hugepages only inside madvise regions
> to eliminate any risk of wasting any precious byte of memory and to
> @@ -99,6 +100,15 @@ Applications that gets a lot of benefit from hugepages
> and that don't
> risk to lose memory by using hugepages, should use
> madvise(MADV_HUGEPAGE) on their critical mmapped regions.
>
> +Applications that would like to benefit from THPs but would still like a
> +more memory conservative approach can choose 'defer'. This avoids
> +inserting THPs at the page fault handler unless they are MADV_HUGEPAGE.
> +Khugepaged will then scan the mappings for potential collapses into PMD
> +sized pages. Admins using this the 'defer' setting should consider
> +tweaking khugepaged/max_ptes_none. The current default of 511 may
> +aggressively collapse your PTEs into PMDs. Lower this value to conserve
> +more memory (ie. max_ptes_none=64).
> +
maybe remove the "(ie. max_ptes_none=64)", its appearing as a recommendation for
the value, but it might not be optimal for different workloads.
> .. _thp_sysfs:
>
> sysfs
> @@ -136,6 +146,7 @@ The top-level setting (for use with "inherit") can be set
> by issuing
> one of the following commands::
>
> echo always >/sys/kernel/mm/transparent_hugepage/enabled
> + echo defer >/sys/kernel/mm/transparent_hugepage/enabled
> echo madvise >/sys/kernel/mm/transparent_hugepage/enabled
> echo never >/sys/kernel/mm/transparent_hugepage/enabled
>
> @@ -274,7 +285,8 @@ of small pages into one large page::
> A higher value leads to use additional memory for programs.
> A lower value leads to gain less thp performance. Value of
> max_ptes_none can waste cpu time very little, you can
> -ignore it.
> +ignore it. Consider lowering this value when using
> +``transparent_hugepage=defer``
lowering this value even with thp=always makes sense, as there might be cases
when pf might not give a THP, but a VMA becomes eligable to scan via khugepaged
later? I would remove this line.
>
> ``max_ptes_swap`` specifies how many pages can be brought in from
> swap when collapsing a group of pages into a transparent huge page::
> @@ -299,8 +311,8 @@ Boot parameters
>
> You can change the sysfs boot time default for the top-level "enabled"
> control by passing the parameter ``transparent_hugepage=always`` or
> -``transparent_hugepage=madvise`` or ``transparent_hugepage=never`` to the
> -kernel command line.
> +``transparent_hugepage=madvise`` or ``transparent_hugepage=defer`` or
> +``transparent_hugepage=never`` to the kernel command line.
>
> Alternatively, each supported anonymous THP size can be controlled by
> passing
> ``thp_anon=<size>[KMG],<size>[KMG]:<state>;<size>[KMG]-<size>[KMG]:<state>``,
