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

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Message-ID: <7696ef91-89c0-427d-951b-56dbb33b4dd6@arm.com>
Date: Thu, 20 Nov 2025 15:25:44 +0530
From: Dev Jain <dev.jain@....com>
To: "Garg, Shivank" <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 1:47 pm, Garg, Shivank wrote:
>
> On 11/20/2025 1:33 PM, Dev Jain wrote:
>> On 20/11/25 12:20 pm, Shivank Garg wrote:
>> 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>
>>
> Thanks for the review.
>
> I chose not to use SCAN_PAGE_DIRTY because dirty and writeback have different meanings[1]:
>
>      Dirty: Memory that is waiting to be written back to disk
> Writeback: Memory that is actively being written back to disk
>
> [1] https://www.kernel.org/doc/Documentation/filesystems/proc.txt
>
> IIUC, a page under writeback is no longer dirty, so using SCAN_PAGE_DIRTY would be misleading
> for pages in the writeback state.
>
> I considered SCAN_PAGE_DIRTY_OR_WRITEBACK initially but felt it was too long.
>
> SCAN_PAGE_NOT_CLEAN covers both states that indicate the page is not in a clean/stable
> state suitable for collapse.
>
> [1] https://www.kernel.org/doc/Documentation/filesystems/proc.txt

Alright, makes sense.

>
> Thanks,
> Shivank