linux-kernel - Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in khugepaged_scan_mm

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Message-ID: <5444e983-9e97-4d08-8177-4e7899f8b4c1@linux.dev>
Date: Tue, 16 Sep 2025 18:00:50 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Kiryl Shutsemau <kirill@...temov.name>
Cc: Hugh Dickins <hughd@...gle.com>, akpm@...ux-foundation.org,
 david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
 dev.jain@....com, baohua@...nel.org, ioworker0@...il.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm-new 1/3] mm/khugepaged: skip unsuitable VMAs earlier in
 khugepaged_scan_mm_slot()



On 2025/9/16 17:58, Lorenzo Stoakes wrote:
> On Tue, Sep 16, 2025 at 10:48:22AM +0100, Kiryl Shutsemau wrote:
>> On Tue, Sep 16, 2025 at 10:39:53AM +0100, Lorenzo Stoakes wrote:
>>> On Tue, Sep 16, 2025 at 10:29:11AM +0100, Kiryl Shutsemau wrote:
>>>> On Tue, Sep 16, 2025 at 02:21:26PM +0800, Lance Yang wrote:
>>>>> Users of mlock() expect low and predictable latency. THP collapse is a
>>>>> heavy operation that introduces exactly the kind of unpredictable delays
>>>>> they want to avoid. It has to unmap PTEs, copy data from the small folios
>>>>> to a new THP, and then remap the THP back to the PMD ;)
>>>>
>>>> Generally, we allow minor page faults into mlocked VMAs and avoid major.
>>>> This is minor page fault territory in my view.
>>>
>>> Hm, but we won't be causing minor faults via reclaim right, since they're
>>> not on any LRU?
>>
>> PTEs are still present when we do THP allocation. No reclaim while the
>> access is blocked. We only block the access on copy and PTEs->PMD
>> collapse.
> 
> Right indeed, esp. with compaction being allowed for mlock, I agree with you
> that this patch should be dropped :)

Got it. Will do ;)

Thanks,
Lance