On Thu, May 15, 2025 at 12:39 PM Nico Pache <npa...@redhat.com> wrote:
>
> setting /transparent_hugepages/enabled=always allows applications
> to benefit from THPs without having to madvise. However, the page fault
> handler takes very few considerations to decide weather or not to actually
> use a 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.
>
> Acked-by: Zi Yan <z...@nvidia.com>
> Co-developed-by: Rafael Aquini <raqu...@redhat.com>
> Signed-off-by: Rafael Aquini <raqu...@redhat.com>
> Signed-off-by: Nico Pache <npa...@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..02038e3db829 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_FLAG,
> 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_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_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);
> -
> if (vm_flags & VM_HUGEPAGE)
> mask |= READ_ONCE(huge_anon_orders_madvise);
> - if (hugepage_global_always() ||
> + if (hugepage_global_always() || hugepage_global_defer() ||
> ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
> mask |= READ_ONCE(huge_anon_orders_inherit);
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 700988a0d5cf..ce0ee74753af 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -297,12 +297,15 @@ static ssize_t enabled_show(struct kobject *kobj,
> const char *output;
>
> if (test_bit(TRANSPARENT_HUGEPAGE_FLAG, &transparent_hugepage_flags))
> - output = "[always] madvise never";
> + output = "[always] madvise defer never";
a small nit: alphabetical ordering might improve readability here.
--
Regards
Yafang
