Re: [PATCH 1/1] mm/rmap: fix soft-dirty bit loss when remapping zero-filled mTHP subpage to shared zeropage

[Lance Yang]

On 2025/9/29 15:25, David Hildenbrand wrote:
> On 28.09.25 06:48, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > When splitting an mTHP and replacing a zero-filled subpage with the 
> > shared
> > zeropage, try_to_map_unused_to_zeropage() currently drops the soft-dirty
> > bit.
> >
> > For userspace tools like CRIU, which rely on the soft-dirty mechanism for
> > incremental snapshots, losing this bit means modified pages are missed,
> > leading to inconsistent memory state after restore.
> >
> > Preserve the soft-dirty bit from the old PTE when creating the zeropage
> > mapping to ensure modified pages are correctly tracked.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage 
> > when splitting isolated thp")
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> >   mm/migrate.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index ce83c2c3c287..bf364ba07a3f 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -322,6 +322,10 @@ static bool try_to_map_unused_to_zeropage(struct 
> > page_vma_mapped_walk *pvmw,
> >       newpte = pte_mkspecial(pfn_pte(my_zero_pfn(pvmw->address),
> >                       pvmw->vma->vm_page_prot));
> > +
> > +    if (pte_swp_soft_dirty(ptep_get(pvmw->pte)))
> > +        newpte = pte_mksoft_dirty(newpte);
> > +
> >       set_pte_at(pvmw->vma->vm_mm, pvmw->address, pvmw->pte, newpte);
> >       dec_mm_counter(pvmw->vma->vm_mm, mm_counter(folio));
>
> It's interesting that there isn't a single occurrence of the stof-dirty 
> flag in khugepaged code. I guess it all works because we do the
>
>      _pmd = maybe_pmd_mkwrite(pmd_mkdirty(_pmd), vma);
>
> and the pmd_mkdirty() will imply marking it soft-dirty.
>
> Now to the problem at hand: I don't think this is particularly 
> problematic in the common case: if the page is zero, it likely was never 
> written to (that's what the unerused shrinker is targeted at), so the 
> soft-dirty setting on the PMD is actually just an over-indication for 
> this page.

Cool. Thanks for the insight! Good to know that ;)

> For example, when we just install the shared zeropage directly in 
> do_anonymous_page(), we obviously also don't set it dirty/soft-dirty.
>
> Now, one could argue that if the content was changed from non-zero to 
> zero, it ould actually be soft-dirty.

Exactly. A false negative could be a problem for the userspace tools, IMO.

> Long-story short: I don't think this matters much in practice, but it's 
> an easy fix.
>
> As said by dev, please avoid double ptep_get() if possible.

Sure, will do. I'll refactor it in the next version.

> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks!

> @Lance, can you double-check that the uffd-wp bit is handled correctly? 
> I strongly assume we lose that as well here.

Certainly, I'll check the uffd-wp bit as well and get back to you soon.

Cheers,
Lance