linux-kernel - Re: [PATCH V2 2/2] mm/khugepaged: map dirty/writeback pages failures to EAGAIN

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6c1d6b80-d290-4110-9a49-53e7404136bc@arm.com>
Date: Thu, 20 Nov 2025 13:33:58 +0530
From: Dev Jain <dev.jain@....com>
To: Shivank Garg <shivankg@....com>, Andrew Morton
 <akpm@...ux-foundation.org>, David Hildenbrand <david@...nel.org>,
 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 V2 2/2] mm/khugepaged: map dirty/writeback pages failures
 to EAGAIN


On 20/11/25 12:20 pm, Shivank Garg wrote:
> When collapse_file encounters dirty or writeback pages in file-backed
> mappings, it currently SCAN_FAIL which maps to -EINVAL. This is
> misleading as EINVAL suggests invalid arguments, whereas dirty/writeback
> pages represent transient conditions that may resolve on retry.
>
> Introduce SCAN_PAGE_NOT_CLEAN to cover both dirty and writeback states,
> mapping it to -EAGAIN. For MADV_COLLAPSE, this provides userspace with
> a clear signal that retry may succeed after writeback completes, making
> -EAGAIN semantically correct. For khugepaged, this is harmless as it
> will naturally revisit the range during periodic scans after async
> writeback completes.
>
> Signed-off-by: Shivank Garg <shivankg@....com>
> ---
>   include/trace/events/huge_memory.h | 3 ++-
>   mm/khugepaged.c                    | 8 +++++---
>   2 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> index 4cde53b45a85..1caf24b951e1 100644
> --- a/include/trace/events/huge_memory.h
> +++ b/include/trace/events/huge_memory.h
> @@ -37,7 +37,8 @@
>   	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_NOT_CLEAN,	"page_not_clean")
>   
>   #undef EM
>   #undef EMe
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 066a332c76ad..282b413d17e8 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -59,6 +59,7 @@ enum scan_result {
>   	SCAN_STORE_FAILED,
>   	SCAN_COPY_MC,
>   	SCAN_PAGE_FILLED,
> +	SCAN_PAGE_NOT_CLEAN,
>   };
>   
>   #define CREATE_TRACE_POINTS
> @@ -1968,11 +1969,11 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>   				 */
>   				xas_unlock_irq(&xas);
>   				filemap_flush(mapping);
> -				result = SCAN_FAIL;
> +				result = SCAN_PAGE_NOT_CLEAN;
>   				goto xa_unlocked;
>   			} else if (folio_test_writeback(folio)) {
>   				xas_unlock_irq(&xas);
> -				result = SCAN_FAIL;
> +				result = SCAN_PAGE_NOT_CLEAN;
>   				goto xa_unlocked;
>   			} else if (folio_trylock(folio)) {
>   				folio_get(folio);
> @@ -2019,7 +2020,7 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
>   			 * folio is dirty because it hasn't been flushed
>   			 * since first write.
>   			 */
> -			result = SCAN_FAIL;
> +			result = SCAN_PAGE_NOT_CLEAN;
>   			goto out_unlock;
>   		}
>   
> @@ -2748,6 +2749,7 @@ 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_NOT_CLEAN:
>   		return -EAGAIN;
>   	/*
>   	 * Other: Trying again likely not to succeed / error intrinsic to

SCAN_PAGE_NOT_CLEAN is confusing - NOT_CLEAN literally means dirty, so why not SCAN_PAGE_DIRTY?
Or SCAN_PAGE_DIRTY_OR_UNDER_WRITEBACK? Since folio_test_writeback() is true as a result of
the folio being dirty, maybe just SCAN_PAGE_DIRTY can do.

Reviewed-by: Dev Jain <dev.jain@....com>