linux-kernel - Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

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Date: Thu, 4 Apr 2024 12:03:56 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org
Cc: mgorman@...hsingularity.net, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: page_alloc: use the correct THP order for THP PCP

On 4/3/24 3:47 PM, Baolin Wang wrote:
> Commit 44042b449872 ("mm/page_alloc: allow high-order pages to be stored
> on the per-cpu lists") extends the PCP allocator to store THP pages, and
> it determines whether to cache THP pags in PCP by comparing with pageblock_order.
> But the pageblock_order is not always equal to THP order, it might also
> be MAX_PAGE_ORDER, which could prevent PCP from caching THP pages.
> 
> Therefore, using HPAGE_PMD_ORDER instead to determine the need for caching
> THP for PCP can fix this issue
> 
> Fixes: 44042b449872 ("mm/page_alloc: allow high-order pages to be stored on the per-cpu lists")
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

IIUC this happens with CONFIG_HUGETLB_PAGE disabled because HUGETLBFS is
disabled? But THPs are still enabled? I think there might be more of THP
working suboptimally in that case with pageblock_order being larger
(MAX_PAGE_ORDER).

In other words, should be rather make pageblock_order itself defined as

 min_t(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_ORDER)

in case with !CONFIG_HUGETLB_PAGE but THP enabled.

> ---
>  mm/page_alloc.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 1beb56f75319..915f4ef070da 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -506,7 +506,7 @@ static inline unsigned int order_to_pindex(int migratetype, int order)
>  {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	if (order > PAGE_ALLOC_COSTLY_ORDER) {
> -		VM_BUG_ON(order != pageblock_order);
> +		VM_BUG_ON(order != HPAGE_PMD_ORDER);
>  		return NR_LOWORDER_PCP_LISTS;
>  	}
>  #else
> @@ -522,7 +522,7 @@ static inline int pindex_to_order(unsigned int pindex)
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	if (pindex == NR_LOWORDER_PCP_LISTS)
> -		order = pageblock_order;
> +		order = HPAGE_PMD_ORDER;
>  #else
>  	VM_BUG_ON(order > PAGE_ALLOC_COSTLY_ORDER);
>  #endif
> @@ -535,7 +535,7 @@ static inline bool pcp_allowed_order(unsigned int order)
>  	if (order <= PAGE_ALLOC_COSTLY_ORDER)
>  		return true;
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -	if (order == pageblock_order)
> +	if (order == HPAGE_PMD_ORDER)
>  		return true;
>  #endif
>  	return false;