linux-kernel - Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

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Message-ID: <1ec368c4-c4d8-41ea-b8a3-7d1fdb3ec358@redhat.com>
Date: Wed, 11 Jun 2025 14:34:22 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
 hughd@...gle.com
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled

On 05.06.25 10:00, Baolin Wang wrote:
> The MADV_COLLAPSE will ignore the system-wide Anon THP sysfs settings, which
> means that even though we have disabled the Anon THP configuration, MADV_COLLAPSE
> will still attempt to collapse into a Anon THP. This violates the rule we have
> agreed upon: never means never.
> 
> Another rule for madvise, referring to David's suggestion: “allowing for collapsing
> in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
> 
> To address this issue, should check whether the Anon THP configuration is disabled
> in thp_vma_allowable_orders(), even when the TVA_ENFORCE_SYSFS flag is set.
> 
> In summary, the current strategy is:
> 
> 1. If always & orders == 0, and madvise & orders == 0, and hugepage_global_enabled() == false
> (global THP settings are not enabled), it means mTHP of that orders are prohibited
> from being used, then madvise_collapse() is forbidden for that orders.
> 
> 2. If always & orders == 0, and madvise & orders == 0, and hugepage_global_enabled() == true
> (global THP settings are enabled), and inherit & orders == 0, it means mTHP of that
> orders are still prohibited from being used, thus madvise_collapse() is not allowed
> for that orders.
> 
> Reviewed-by: Zi Yan <ziy@...dia.com>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>   include/linux/huge_mm.h | 23 +++++++++++++++++++----
>   1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 2f190c90192d..199ddc9f04a1 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -287,20 +287,35 @@ unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
>   				       unsigned long orders)
>   {
>   	/* 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 (vma_is_anonymous(vma)) {
> +		unsigned long always = READ_ONCE(huge_anon_orders_always);
> +		unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
> +		unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);
> +		unsigned long mask = always | madvise;
> +
> +		/*
> +		 * If the system-wide THP/mTHP sysfs settings are disabled,
> +		 * then we should never allow hugepages.
 > +		 */> +		if (!(mask & orders) && !(hugepage_global_enabled() && 
(inherit & orders)))
> +			return 0;

I'm still trying to digest that. Isn't there a way for us to work with 
the orders,
essentially masking off all orders that are forbidden globally. Similar 
to below, if !orders, then return 0?
/* Orders disabled directly. */
orders &= ~TODO;
/* Orders disabled by inheriting from the global toggle. */
if (!hugepage_global_enabled())
	orders &= ~READ_ONCE(huge_anon_orders_inherit);

TODO is probably a -1ULL and then clearing always/madvise/inherit. Could 
add a simple helper for that

huge_anon_orders_never

-- 
Cheers,

David / dhildenb