linux-kernel - Re: [PATCH v8 mm-new 06/12] mm: thp: enable THP allocation exclusively through khugepaged

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Message-ID: <035a8839-c786-45b6-8458-87ac1c48f3bc@gmail.com>
Date: Fri, 26 Sep 2025 16:27:25 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Yafang Shao <laoar.shao@...il.com>, akpm@...ux-foundation.org,
 david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, hannes@...xchg.org,
 gutierrez.asier@...wei-partners.com, willy@...radead.org, ast@...nel.org,
 daniel@...earbox.net, andrii@...nel.org, ameryhung@...il.com,
 rientjes@...gle.com, corbet@....net, 21cnbao@...il.com,
 shakeel.butt@...ux.dev, tj@...nel.org, lance.yang@...ux.dev
Cc: bpf@...r.kernel.org, linux-mm@...ck.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 mm-new 06/12] mm: thp: enable THP allocation
 exclusively through khugepaged



On 26/09/2025 10:33, Yafang Shao wrote:
> khugepaged_enter_vma() ultimately invokes any attached BPF function with
> the TVA_KHUGEPAGED flag set when determining whether or not to enable
> khugepaged THP for a freshly faulted in VMA.
> 
> Currently, on fault, we invoke this in do_huge_pmd_anonymous_page(), as
> invoked by create_huge_pmd() and only when we have already checked to
> see if an allowable TVA_PAGEFAULT order is specified.
> 
> Since we might want to disallow THP on fault-in but allow it via
> khugepaged, we move things around so we always attempt to enter
> khugepaged upon fault.
> 
> This change is safe because:
> - the checks for thp_vma_allowable_order(TVA_KHUGEPAGED) and
>   thp_vma_allowable_order(TVA_PAGEFAULT) are functionally equivalent

hmm I dont think this is the case. __thp_vma_allowable_orders
deals with TVA_PAGEFAULT (in_pf) differently from TVA_KHUGEPAGED.

> - khugepaged operates at the MM level rather than per-VMA. The THP
>   allocation might fail during page faults due to transient conditions
>   (e.g., memory pressure), it is safe to add this MM to khugepaged for
>   subsequent defragmentation.
> 
> While we could also extend prctl() to utilize this new policy, such a
> change would require a uAPI modification to PR_SET_THP_DISABLE.
> 
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Acked-by: Lance Yang <lance.yang@...ux.dev>
> ---
>  mm/huge_memory.c |  1 -
>  mm/memory.c      | 13 ++++++++-----
>  2 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 08372dfcb41a..2b155a734c78 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1346,7 +1346,6 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>  	ret = vmf_anon_prepare(vmf);
>  	if (ret)
>  		return ret;
> -	khugepaged_enter_vma(vma);
>  
>  	if (!(vmf->flags & FAULT_FLAG_WRITE) &&
>  			!mm_forbids_zeropage(vma->vm_mm) &&
> diff --git a/mm/memory.c b/mm/memory.c
> index 58ea0f93f79e..64f91191ffff 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6327,11 +6327,14 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
>  	if (pud_trans_unstable(vmf.pud))
>  		goto retry_pud;
>  
> -	if (pmd_none(*vmf.pmd) &&
> -	    thp_vma_allowable_order(vma, TVA_PAGEFAULT, PMD_ORDER)) {
> -		ret = create_huge_pmd(&vmf);
> -		if (!(ret & VM_FAULT_FALLBACK))
> -			return ret;
> +	if (pmd_none(*vmf.pmd)) {
> +		if (vma_is_anonymous(vma))
> +			khugepaged_enter_vma(vma);
> +		if (thp_vma_allowable_order(vma, TVA_PAGEFAULT, PMD_ORDER)) {
> +			ret = create_huge_pmd(&vmf);
> +			if (!(ret & VM_FAULT_FALLBACK))
> +				return ret;
> +		}
>  	} else {
>  		vmf.orig_pmd = pmdp_get_lockless(vmf.pmd);
>