linux-kernel - Re: [PATCH v9 09/14] khugepaged: avoid unnecessary mTHP collapse attempts

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Message-ID: <85b2cfd8-4aeb-4e98-8065-b6594783de62@linux.alibaba.com>
Date: Fri, 18 Jul 2025 10:14:51 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Nico Pache <npache@...hat.com>, linux-mm@...ck.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org
Cc: david@...hat.com, ziy@...dia.com, lorenzo.stoakes@...cle.com,
 Liam.Howlett@...cle.com, ryan.roberts@....com, dev.jain@....com,
 corbet@....net, rostedt@...dmis.org, mhiramat@...nel.org,
 mathieu.desnoyers@...icios.com, akpm@...ux-foundation.org,
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 wangkefeng.wang@...wei.com, usamaarif642@...il.com, sunnanyong@...wei.com,
 vishal.moola@...il.com, thomas.hellstrom@...ux.intel.com,
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Subject: Re: [PATCH v9 09/14] khugepaged: avoid unnecessary mTHP collapse
 attempts



On 2025/7/14 08:32, Nico Pache wrote:
> There are cases where, if an attempted collapse fails, all subsequent
> orders are guaranteed to also fail. Avoid these collapse attempts by
> bailing out early.
> 
> Signed-off-by: Nico Pache <npache@...hat.com>
> ---
>   mm/khugepaged.c | 17 +++++++++++++++++
>   1 file changed, 17 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index a701d9f0f158..7a9c4edf0e23 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1367,6 +1367,23 @@ static int collapse_scan_bitmap(struct mm_struct *mm, unsigned long address,
>   				collapsed += (1 << order);
>   				continue;
>   			}

After doing more testing, I think you need to add the following changes 
after patch 8.

Because when collapsing mTHP, if we encounter a PTE-mapped large folio 
within the PMD range, we should continue scanning to complete that PMD, 
in case there is another mTHP that can be collapsed within that PMD range.

+                       if (ret == SCAN_PTE_MAPPED_HUGEPAGE)
+                               continue;

> +			/*
> +			 * Some ret values indicate all lower order will also
> +			 * fail, dont trying to collapse smaller orders
> +			 */
> +			if (ret == SCAN_EXCEED_NONE_PTE ||
> +				ret == SCAN_EXCEED_SWAP_PTE ||
> +				ret == SCAN_EXCEED_SHARED_PTE ||
> +				ret == SCAN_PTE_NON_PRESENT ||
> +				ret == SCAN_PTE_UFFD_WP ||
> +				ret == SCAN_ALLOC_HUGE_PAGE_FAIL ||
> +				ret == SCAN_CGROUP_CHARGE_FAIL ||
> +				ret == SCAN_COPY_MC ||
> +				ret == SCAN_PAGE_LOCK ||
> +				ret == SCAN_PAGE_COUNT)
> +				goto next;
> +			else

Nit: the 'else' statement can be dropped.

> +				break;
>   		}
>   
>   next: