linux-kernel - Re: [PATCH 1/1] mm/ksm: fix spurious soft-dirty bit on zero-filled page merging

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Message-ID: <556e072d-6bec-421a-a78c-fcde61af67f9@linux.dev>
Date: Tue, 30 Sep 2025 15:36:36 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: David Hildenbrand <david@...hat.com>
Cc: xu.xin16@....com.cn, chengming.zhou@...ux.dev, ran.xiaokai@....com.cn,
 yang.yang29@....com.cn, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 ioworker0@...il.com, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] mm/ksm: fix spurious soft-dirty bit on zero-filled
 page merging



On 2025/9/30 15:32, David Hildenbrand wrote:
> On 29.09.25 12:08, Lance Yang wrote:
>>
>>
>> On 2025/9/29 16:25, David Hildenbrand wrote:
>>> On 28.09.25 06:52, Lance Yang wrote:
>>>> From: Lance Yang <lance.yang@...ux.dev>
>>>>
>>>> When KSM merges a zero-filled page with the shared zeropage, it uses
>>>> pte_mkdirty() to mark the new PTE for internal accounting. However,
>>>> pte_mkdirty() unconditionally sets both the hardware dirty bit and the
>>>> soft-dirty bit.
>>>>
>>>
>>> Right, that's one think we should clean up at one point.
>>
>> Got it. I’ll take a look when I get a chance ;)
>>
>>>
>>>> This behavior causes false positives in userspace tools like CRIU that
>>>> rely on the soft-dirty mechanism for tracking memory changes.
>>>
>>> IIRC, false positives are not a problem. We get them all of the time
>>> when merging VMAs etc.
>>
>> Right, Indeed.
>>
>>> So I am not sure if this here is really worth fixing. Soft-dirty is not,
>>> and never will be false-positive free.
>>
>> Makes sense to me. It doesn’t seem worth the trouble to fix. Let’s go
>> ahead and drop it.
> 
> Yeah, I would prefer if we can just decouple soft-dirty from dirty 
> handling at some point. I recall I had a use case around 
> PageAnonExclusive at some point.

Cool. I'll give it a try ;)