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

[Pedro Falcato]

On Wed, Jan 14, 2026 at 11:00:06AM +0000, 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: Pedro Falcato <pfalcato@xxxxxxx>

> ---
>  include/linux/mm.h | 6 +++++-
>  mm/memory.c        | 6 +++++-
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index cb3de0c73d03..44a2a9c0a92f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -609,7 +609,11 @@ enum {
>  /*
>   * Flags which should result in page tables being copied on fork. These are
>   * flags which indicate that the VMA maps page tables which cannot be
> - * reconsistuted upon page fault, so necessitate page table copying upon
> + * reconsistuted upon page fault, so necessitate page table copying upon fork.
> + *
> + * Note that these flags should be compared with the DESTINATION VMA not the
> + * source, as VM_UFFD_WP may not be propagated to destination, while all other
> + * flags will be.
>   *
>   * VM_PFNMAP / VM_MIXEDMAP - These contain kernel-mapped data which cannot be
>   *                           reasonably reconstructed on page fault.
> diff --git a/mm/memory.c b/mm/memory.c
> index 4b0790c8fa48..2839000cd26f 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1465,7 +1465,11 @@ copy_p4d_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
>  static bool
>  vma_needs_copy(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>  {
> -	if (src_vma->vm_flags & VM_COPY_ON_FORK)
> +	/*
> +	 * We check against dst_vma as while sane VMA flags will have been

s/sane/most/?
I understand you're annoyed but perhaps we should leave this out of the code
comments themselves :)

> +	 * copied, VM_UFFD_WP may be set only on dst_vma.
> +	 */
> +	if (dst_vma->vm_flags & VM_COPY_ON_FORK)
>  		return true;
>  	/*
>  	 * The presence of an anon_vma indicates an anonymous VMA has page
> --
> 2.52.0

-- 
Pedro