linux-kernel - Re: [PATCH 2/2] mm/khugepaged: return EAGAIN for transient dirty pages in MADV

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Message-ID: <4ab54ae0-2607-443d-8698-788d8e951bdd@arm.com>
Date: Mon, 10 Nov 2025 19:16:40 +0530
From: Dev Jain <dev.jain@....com>
To: Shivank Garg <shivankg@....com>, Andrew Morton
 <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Nico Pache
 <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Barry Song <baohua@...nel.org>, Lance Yang <lance.yang@...ux.dev>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Zach O'Keefe <zokeefe@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm/khugepaged: return EAGAIN for transient dirty
 pages in MADV_COLLAPSE

On 10/11/25 5:02 pm, Shivank Garg wrote:
> When MADV_COLLAPSE encounters dirty file-backed pages, it currently
> returns -EINVAL, this is misleading as EINVAL suggests invalid arguments,
> whereas dirty pages are a transient condition that may resolve on retry.
>
> Introduce SCAN_PAGE_DIRTY and map it to -EAGAIN. For khugepaged, this
> is harmless as it will revisit the range after async writeback completes.

Taking a cursory look at enum scan_result, I am sure there are other error
codes as well which redirect to -EINVAL when they actually should to something
else...

>
> Signed-off-by: Shivank Garg <shivankg@....com>
> ---
>   include/trace/events/huge_memory.h | 3 ++-
>   mm/khugepaged.c                    | 4 +++-
>   2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index dd94d14a2427..9014a9bbe64c 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -38,7 +38,8 @@
>   	EM( SCAN_PAGE_HAS_PRIVATE,	"page_has_private")		\
>   	EM( SCAN_STORE_FAILED,		"store_failed")			\
>   	EM( SCAN_COPY_MC,		"copy_poisoned_page")		\
> -	EMe(SCAN_PAGE_FILLED,		"page_filled")
> +	EM(SCAN_PAGE_FILLED,		"page_filled")			\
> +	EMe(SCAN_PAGE_DIRTY,		"page_dirty")
>   
>   #undef EM
>   #undef EMe
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index d08ed6eb9ce1..7df329c9c87d 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -60,6 +60,7 @@ enum scan_result {
>   	SCAN_STORE_FAILED,
>   	SCAN_COPY_MC,
>   	SCAN_PAGE_FILLED,
> +	SCAN_PAGE_DIRTY,
>   };
>   
>   #define CREATE_TRACE_POINTS
> @@ -1967,7 +1968,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>   				 */
>   				xas_unlock_irq(&xas);
>   				filemap_flush(mapping);
> -				result = SCAN_FAIL;
> +				result = SCAN_PAGE_DIRTY;
>   				goto xa_unlocked;
>   			} else if (folio_test_writeback(folio)) {
>   				xas_unlock_irq(&xas);

Now that you are at it, it would make sense to redirect this folio_test_writeback()
branch also to -EAGAIN, and call the common error code SCAN_PAGE_UNDER_WRITEBACK.

> @@ -2747,6 +2748,7 @@ static int madvise_collapse_errno(enum scan_result r)
>   	case SCAN_PAGE_LRU:
>   	case SCAN_DEL_PAGE_LRU:
>   	case SCAN_PAGE_FILLED:
> +	case SCAN_PAGE_DIRTY:
>   		return -EAGAIN;
>   	/*
>   	 * Other: Trying again likely not to succeed / error intrinsic to