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

[David Hildenbrand <david@redhat.com>]

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@linux.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.

-- 
Cheers

David / dhildenb