Re: [PATCH RFC v4 04/12] mm/gup: let check_vma_flags() ignore selected VMA flags

[Suren Baghdasaryan]

On Fri, Jul 24, 2026 at 3:30 PM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> check_vma_flags() rejects a VMA whose flags gup cannot handle. A caller
> that wants to reach a COWed page in a VM_IO or VM_PFNMAP VMA, where that
> page does have a struct page, cannot express the exception and has to
> repeat the flag test itself.
>
> Add an ignore_flags argument that check_vma_flags() clears from its local
> copy of vma->vm_flags before the flag checks. A caller can drop a single
> rejection this way while keeping every other check. All current callers
> pass 0.
>
> This prepares for get_user_page_vma(), which passes VM_IO | VM_PFNMAP to
> reach those COWed pages.
>
> No functional change intended.
>
> Assisted-by: Claude:claude-opus-4.8
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

> ---
>  mm/gup.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 0692119b7904..dcece7b5253c 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1197,13 +1197,21 @@ static bool writable_file_mapping_allowed(struct vm_area_struct *vma,
>         return !vma_needs_dirty_tracking(vma);
>  }
>
> -static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> +static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags,
> +                          vm_flags_t ignore_flags)
>  {
>         vm_flags_t vm_flags = vma->vm_flags;
>         int write = (gup_flags & FOLL_WRITE);
>         int foreign = (gup_flags & FOLL_REMOTE);
>         bool vma_anon = vma_is_anonymous(vma);
>
> +       /*
> +        * Opt out of the flag checks that read this local copy (the
> +        * VM_IO/VM_PFNMAP gate and the write/read/cow bits); checks that
> +        * re-read vma->vm_flags through helpers are unaffected.
> +        */
> +       vm_flags &= ~ignore_flags;
> +
>         if (vm_flags & (VM_IO | VM_PFNMAP))
>                 return -EFAULT;
>
> @@ -1387,7 +1395,7 @@ static long __get_user_pages(struct mm_struct *mm,
>                                         ret = -ENOMEM;
>                                         goto out;
>                                 }
> -                               if (check_vma_flags(vma, gup_flags)) {
> +                               if (check_vma_flags(vma, gup_flags, 0)) {
>                                         ret = -EINVAL;
>                                         goto out;
>                                 }
> @@ -1408,7 +1416,7 @@ static long __get_user_pages(struct mm_struct *mm,
>                                 ret = -EFAULT;
>                                 goto out;
>                         }
> -                       ret = check_vma_flags(vma, gup_flags);
> +                       ret = check_vma_flags(vma, gup_flags, 0);
>                         if (ret)
>                                 goto out;
>                 }
> --
> 2.53.0-Meta
>
>