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

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Message-ID: <33577DDE-D88E-44F9-9B91-7AA46EACCCE8@nvidia.com>
Date: Thu, 29 May 2025 11:10:42 -0400
From: Zi Yan <ziy@...dia.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled

On 29 May 2025, at 4:23, 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.
>
> 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.
>
> 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)))

Can you explain the logic here? Is it equivalent to:
1. if THP is set to always, always_mask & orders == 0, or
2. if THP if set to madvise, madvise_mask & order == 0, or
3. if THP is set to inherit, inherit_mask & order == 0?

I cannot figure out why (always | madvise) & orders does not check
THP enablement case, but inherit & orders checks hugepage_global_enabled().

Thanks.

> +			return 0;
> +
> +		if (!(tva_flags & TVA_ENFORCE_SYSFS))
> +			goto skip;
>
> +		mask = always;
>  		if (vm_flags & VM_HUGEPAGE)
> -			mask |= READ_ONCE(huge_anon_orders_madvise);
> +			mask |= madvise;
>  		if (hugepage_global_always() ||
>  		    ((vm_flags & VM_HUGEPAGE) && hugepage_global_enabled()))
> -			mask |= READ_ONCE(huge_anon_orders_inherit);
> +			mask |= inherit;
>
>  		orders &= mask;
>  		if (!orders)
>  			return 0;
>  	}
>
> +skip:
>  	return __thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
>  }
>
> -- 
> 2.43.5


Best Regards,
Yan, Zi