linux-kernel - Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on non-swap entries

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Message-ID: <4640ec4f-57ee-40f1-92c1-6bc703de7933@linux.dev>
Date: Wed, 1 Oct 2025 17:38:22 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, baohua@...nel.org,
 baolin.wang@...ux.alibaba.com, dev.jain@....com, hughd@...gle.com,
 ioworker0@...il.com, kirill@...temov.name, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, mpenttil@...hat.com, npache@...hat.com,
 ryan.roberts@....com, ziy@...dia.com, richard.weiyang@...il.com,
 akpm@...ux-foundation.org
Subject: Re: [PATCH mm-new v2 1/1] mm/khugepaged: abort collapse scan on
 non-swap entries



On 2025/10/1 16:31, David Hildenbrand wrote:
> On 01.10.25 05:22, Lance Yang wrote:
>> From: Lance Yang <lance.yang@...ux.dev>
>>
>> Currently, special non-swap entries (like migration, hwpoison, or PTE
>> markers) are not caught early in hpage_collapse_scan_pmd(), leading to
>> failures deep in the swap-in logic.
>>
>> hpage_collapse_scan_pmd()
>>   `- collapse_huge_page()
>>       `- __collapse_huge_page_swapin() -> fails!
>>
>> As David suggested[1], this patch skips any such non-swap entries
>> early. If any one is found, the scan is aborted immediately with the
>> SCAN_PTE_NON_PRESENT result, as Lorenzo suggested[2], avoiding wasted
>> work.
>>
>> [1] https://lore.kernel.org/linux-mm/7840f68e-7580-42cb- 
>> a7c8-1ba64fd6df69@...hat.com
>> [2] https://lore.kernel.org/linux-mm/7df49fe7-c6b7-426a-8680- 
>> dcd55219c8bd@...ifer.local
>>
>> Suggested-by: David Hildenbrand <david@...hat.com>
>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>> Signed-off-by: Lance Yang <lance.yang@...ux.dev>
>> ---
> 
> Could have mentioned you adjust the flow of code to resemble what we 
> have in __collapse_huge_page_isolate().

Thanks for pointing that out! Let's do that separately. I'm working
on a follow-up patch that will unify the scanning with the
almost-duplicated code as you suggested ;)

> 
> Acked-by: David Hildenbrand <david@...hat.com>

Cheers!