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

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Message-ID: <8476e9be-3f80-4d7a-9a38-f910661e61c6@linux.dev>
Date: Mon, 29 Sep 2025 18:08:45 +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 2025/9/29 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.

Thanks,
Lance