linux-kernel - Re: [PATCH v5 3/4] khugepaged: add defer option to mTHP options

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Message-ID: <BE08D7F9-716B-4EE8-AC4E-2D9F4C5D4A05@nvidia.com>
Date: Wed, 30 Apr 2025 16:34:18 -0400
From: Zi Yan <ziy@...dia.com>
To: Nico Pache <npache@...hat.com>
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Subject: Re: [PATCH v5 3/4] khugepaged: add defer option to mTHP options

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

> Now that we have defer to globally disable THPs at fault time, lets add
> a defer setting to the mTHP options. This will allow khugepaged to
> operate at that order, while avoiding it at PF time.
>
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>  include/linux/huge_mm.h |  5 +++++
>  mm/huge_memory.c        | 38 +++++++++++++++++++++++++++++++++-----
>  mm/khugepaged.c         |  8 ++++----
>  3 files changed, 42 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 57e6c962afb1..a877c59bea67 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -96,6 +96,7 @@ extern struct kobj_attribute thpsize_shmem_enabled_attr;
>  #define TVA_SMAPS		(1 << 0)	/* Will be used for procfs */
>  #define TVA_IN_PF		(1 << 1)	/* Page fault handler */
>  #define TVA_ENFORCE_SYSFS	(1 << 2)	/* Obey sysfs configuration */
> +#define TVA_IN_KHUGEPAGE	((1 << 2) | (1 << 3)) /* Khugepaged defer support */

Why is TVA_IN_KHUGEPAGE a superset of TVA_ENFORCE_SYSFS? Because khugepaged
also obeys sysfs configuration?

I wonder if explicitly coding the behavior is better. For example,
in __thp_vma_allowable_orders(), enforce_sysfs = tva_flags & (TVA_ENFORCE_SYSFS | TVA_IN_KHUGEPAGE).

>
>  #define thp_vma_allowable_order(vma, vm_flags, tva_flags, order) \
>  	(!!thp_vma_allowable_orders(vma, vm_flags, tva_flags, BIT(order)))
> @@ -182,6 +183,7 @@ extern unsigned long transparent_hugepage_flags;
>  extern unsigned long huge_anon_orders_always;
>  extern unsigned long huge_anon_orders_madvise;
>  extern unsigned long huge_anon_orders_inherit;
> +extern unsigned long huge_anon_orders_defer;
>
>  static inline bool hugepage_global_enabled(void)
>  {
> @@ -306,6 +308,9 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
>  	/* Optimization to check if required orders are enabled early. */
>  	if ((tva_flags & TVA_ENFORCE_SYSFS) && vma_is_anonymous(vma)) {

And code here becomes tva_flags & (TVA_ENFORCE_SYSFS | TVA_IN_KHUGEPAGE).

Otherwise, LGTM. Reviewed-by: Zi Yan <ziy@...dia.com>

--
Best Regards,
Yan, Zi