Re: [PATCH v2 2/3] mm/huge_memory: Prevent huge zeropage refcount corruption in PMD move

[Lorenzo Stoakes]

TL;DR - To make life easier, I squashed the two patches and asked Andrew to take
it with my R-b,T-b tags attached, hope that's ok with you Chris.

It takes your work, combines commit msgs and the code into one patch with
correct attribution to you.

See https://lore.kernel.org/linux-mm/a1e787dd-b911-474d-8570-f37685357d86@lucifer.local/

Thanks, Lorenzo

On Mon, Mar 02, 2026 at 05:34:47PM +0000, Lorenzo Stoakes wrote:
> On Thu, Feb 26, 2026 at 10:16:47PM +0800, Chris Down wrote:
> > After commit d82d09e48219 ("mm/huge_memory: mark PMD mappings of the
> > huge zero folio special"), moved huge zero PMDs must remain special so
> > vm_normal_page_pmd() continues to treat them as special mappings.
> >
> > move_pages_huge_pmd() currently reconstructs the destination PMD in the
> > huge zero page branch, which drops PMD state such as pmd_special() on
> > architectures with CONFIG_ARCH_HAS_PTE_SPECIAL. As a result,
> > vm_normal_page_pmd() can treat the moved huge zero PMD as a normal page
> > and corrupt its refcount.
> >
> > Instead of reconstructing the PMD from the folio, derive the destination
> > entry from src_pmdval after pmdp_huge_clear_flush(), then handle the PMD
> > metadata the same way move_huge_pmd() does for moved entries by marking
> > it soft-dirty and clearing uffd-wp.
> >
> > Fixes: d82d09e48219 ("mm/huge_memory: mark PMD mappings of the huge zero folio special")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Chris Down <chris@xxxxxxxxxxxxxx>
> > ---
> >  mm/huge_memory.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index fed57951a7cd..8166b5e871ad 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2794,7 +2794,8 @@ int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pm
> >  		_dst_pmd = pmd_mkwrite(pmd_mkdirty(_dst_pmd), dst_vma);
> >  	} else {
> >  		src_pmdval = pmdp_huge_clear_flush(src_vma, src_addr, src_pmd);
> > -		_dst_pmd = folio_mk_pmd(page_folio(src_page), dst_vma->vm_page_prot);
> > +		_dst_pmd = move_soft_dirty_pmd(src_pmdval);
> > +		_dst_pmd = clear_uffd_wp_pmd(_dst_pmd);
>
> I'm confused as to what's going on here, it seems like the 2/3 is simply
> updating the 1/3 with a different fixes?
>
> I agree with David that just moving it is probably completely fine, so I think
> this should be the only actual patch you need, and you can just Fixes:
> e3981db444a0 with it? Then make this a v3 series with 2 patches this + the test
> right (but maybe best not backport the test :)?
>
> >  	}
> >  	set_pmd_at(mm, dst_addr, dst_pmd, _dst_pmd);
> >
> > --
> > 2.51.2
> >
> >
> >