Re: [PATCH RESEND v3 08/10] mm/vma: improve and document __is_vma_write_locked()

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 22, 2026 at 5:02 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> The function is a little confusing, clean it up a little then add a
> descriptive comment.

I appreciate the descriptive comment but what exactly was confusing in
this function?

>
> No functional change intended.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>  include/linux/mmap_lock.h | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 873bc5f3c97c..b00d34b5ad10 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -252,17 +252,30 @@ static inline void vma_end_read(struct vm_area_struct *vma)
>         vma_refcount_put(vma);
>  }
>
> -/* WARNING! Can only be used if mmap_lock is expected to be write-locked */
> -static inline bool __is_vma_write_locked(struct vm_area_struct *vma, unsigned int *mm_lock_seq)
> +/*
> + * Determine whether a VMA is write-locked. Must be invoked ONLY if the mmap
> + * write lock is held.
> + *
> + * Returns true if write-locked, otherwise false.
> + *
> + * Note that mm_lock_seq is updated only if the VMA is NOT write-locked.

True, this does not result in a functional change because we do not
use mm_lock_seq if __is_vma_write_locked() succeeds. However this
seems to add additional gotcha that you need to remember. Any reason
why?

> + */
> +static inline bool __is_vma_write_locked(struct vm_area_struct *vma,
> +                                        unsigned int *mm_lock_seq)
>  {
> -       mmap_assert_write_locked(vma->vm_mm);
> +       struct mm_struct *mm = vma->vm_mm;
> +       const unsigned int seq = mm->mm_lock_seq.sequence;
> +
> +       mmap_assert_write_locked(mm);
>
>         /*
>          * current task is holding mmap_write_lock, both vma->vm_lock_seq and
>          * mm->mm_lock_seq can't be concurrently modified.
>          */
> -       *mm_lock_seq = vma->vm_mm->mm_lock_seq.sequence;
> -       return (vma->vm_lock_seq == *mm_lock_seq);
> +       if (vma->vm_lock_seq == seq)
> +               return true;
> +       *mm_lock_seq = seq;
> +       return false;
>  }
>
>  int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
> --
> 2.52.0