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

[Lance Yang]

On 2025/9/29 16:25, David Hildenbrand wrote:
> On 28.09.25 06:52, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > 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