Re: [PATCH mm-hotfixes] mm: do not copy page tables unnecessarily for VM_UFFD_WP

[David Hildenbrand (Red Hat)]

On 1/14/26 12:00, Lorenzo Stoakes wrote:
> Commit ab04b530e7e8 ("mm: introduce copy-on-fork VMAs and make
> VM_MAYBE_GUARD one") aggregates flags checks in vma_needs_copy(), including
> VM_UFFD_WP.
>
> However in doing so, it incorrectly performed this check against
> src_vma. This check was done on the assumption that all relevant flags are
> copied upon fork.
>
> However the userfaultfd logic is very innovative in that it implements
> custom logic on fork in dup_userfaultfd(), including a rather well hidden
> case where lacking UFFD_FEATURE_EVENT_FORK causes VM_UFFD_WP to not be
> propagated to the destination VMA.
>
> And indeed, vma_needs_copy(), prior to this patch, did check this property
> on dst_vma, not src_vma.
>
> Since all the other relevant flags are copied on fork, we can simply fix
> this by checking against dst_vma.
>
> While we're here, we fix a comment against VM_COPY_ON_FORK (noting that it
> did indeed already reference dst_vma) to make it abundantly clear that we
> must check against the destination VMA.
>
> Reported-by: Chris Mason <clm@xxxxxxxx>
> Closes: https://lore.kernel.org/all/20260113231257.3002271-1-clm@xxxxxxxx/
> Fixes: ab04b530e7e8 ("mm: introduce copy-on-fork VMAs and make VM_MAYBE_GUARD one")
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Acked-by: David Hildenbrand (Red Hat) <david@xxxxxxxxxx>

--
Cheers

David