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

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Message-ID: <ef4a0c2c-62d1-46de-9ea2-dd946857a03a@linux.dev>
Date: Thu, 20 Nov 2025 20:24:19 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: "Garg, Shivank" <shivankg@....com>, Dev Jain <dev.jain@....com>
Cc: Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 David Hildenbrand <david@...nel.org>, Nico Pache <npache@...hat.com>,
 Ryan Roberts <ryan.roberts@....com>, Barry Song <baohua@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Steven Rostedt <rostedt@...dmis.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 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 2025/11/20 16:17, 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.

Nit: If SCAN_PAGE_DIRTY_OR_WRITEBACK is too verbose, how about 
SCAN_PAGE_DIRTY_WB?

It keeps the specificity without the length, and is arguably more 
descriptive
than NOT_CLEAN ;)

That said, LGTM.

Reviewed-by: Lance Yang <lance.yang@...ux.dev>

> 
> 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
> 
> Thanks,
> Shivank