Re: [PATCH v5 1/4] mm: defer THP insertion to khugepaged

[Zi Yan <ziy@nvidia.com>]

On 28 Apr 2025, at 14:29, Nico Pache wrote:

> setting /transparent_hugepages/enabled=always allows applications
> to benefit from THPs without having to madvise. However, the pf handler

s/pf/page fault

> takes very few considerations to decide weather or not to actually use a

s/weather/whether

> THP. This can lead to a lot of wasted memory. khugepaged only operates
> on memory that was either allocated with enabled=always or MADV_HUGEPAGE.
>
> Introduce the ability to set enabled=defer, which will prevent THPs from
> being allocated by the page fault handler unless madvise is set,
> leaving it up to khugepaged to decide which allocations will collapse to a
> THP. This should allow applications to benefits from THPs, while curbing
> some of the memory waste.
>
> Co-developed-by: Rafael Aquini <raquini@redhat.com>
> Signed-off-by: Rafael Aquini <raquini@redhat.com>
> Signed-off-by: Nico Pache <npache@redhat.com>
> ---
>  include/linux/huge_mm.h | 15 +++++++++++++--
>  mm/huge_memory.c        | 31 +++++++++++++++++++++++++++----
>  2 files changed, 40 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e3d15c737008..57e6c962afb1 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -48,6 +48,7 @@ enum transparent_hugepage_flag {
>  	TRANSPARENT_HUGEPAGE_UNSUPPORTED,
>  	TRANSPARENT_HUGEPAGE_FLAG,
>  	TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG,
> +	TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG,

What does INST mean here? Can you add one sentence on this new flag
in the commit log to explain what it is short for?


>  	TRANSPARENT_HUGEPAGE_DEFRAG_DIRECT_FLAG,
>  	TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_FLAG,
>  	TRANSPARENT_HUGEPAGE_DEFRAG_KSWAPD_OR_MADV_FLAG,
> @@ -186,6 +187,7 @@ static inline bool hugepage_global_enabled(void)
>  {
>  	return transparent_hugepage_flags &
>  			((1<<TRANSPARENT_HUGEPAGE_FLAG) |
> +			(1<<TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG) |
>  			(1<<TRANSPARENT_HUGEPAGE_REQ_MADV_FLAG));
>  }
>
> @@ -195,6 +197,12 @@ static inline bool hugepage_global_always(void)
>  			(1<<TRANSPARENT_HUGEPAGE_FLAG);
>  }
>
> +static inline bool hugepage_global_defer(void)
> +{
> +	return transparent_hugepage_flags &
> +			(1<<TRANSPARENT_HUGEPAGE_DEFER_PF_INST_FLAG);
> +}
> +
>  static inline int highest_order(unsigned long orders)
>  {
>  	return fls_long(orders) - 1;
> @@ -291,13 +299,16 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
>  				       unsigned long tva_flags,
>  				       unsigned long orders)
>  {
> +	if ((tva_flags & TVA_IN_PF) && hugepage_global_defer() &&
> +			!(vm_flags & VM_HUGEPAGE))
> +		return 0;
> +
>  	/* Optimization to check if required orders are enabled early. */
>  	if ((tva_flags & TVA_ENFORCE_SYSFS) && vma_is_anonymous(vma)) {
>  		unsigned long mask = READ_ONCE(huge_anon_orders_always);
> -

This newline should stay, right?

The rest looks good to me. Thanks. Acked-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi