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 18:29, Lance Yang wrote:
> 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.

Yes, the uffd-wp bit was indeed being dropped, but ...

The shared zeropage is read-only, which triggers a fault. IIUC,
The kernel then falls back to checking the VM_UFFD_WP flag on
the VMA and correctly generates a uffd-wp event, masking the
fact that the uffd-wp bit on the PTE was lost.

IMHO, explicitly preserving the uffd-wp bit on the PTE is still
necessary, since we're not sure if losing that bit is safe in
all cases :)

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