Re: [PATCH v4 08/10] mm/vma: improve and document __is_vma_write_locked()

[Vlastimil Babka]

On 1/23/26 21:12, Lorenzo Stoakes wrote:
> We don't actually need to return an output parameter providing mm sequence
> number, rather we can separate that out into another function -
> __vma_raw_mm_seqnum() - and have any callers which need to obtain that
> invoke that instead.
> 
> The access to the raw sequence number requires that we hold the exclusive
> mmap lock such that we know we can't race vma_end_write_all(), so move the
> assert to __vma_raw_mm_seqnum() to make this requirement clear.
> 
> Also while we're here, convert all of the VM_BUG_ON_VMA()'s to
> VM_WARN_ON_ONCE_VMA()'s in line with the convention that we do not invoke
> oopses when we can avoid it.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Few nits:

> ---
>  include/linux/mmap_lock.h | 44 ++++++++++++++++++++++-----------------
>  1 file changed, 25 insertions(+), 19 deletions(-)
> 
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 678f90080fa6..23bde4bd5a85 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -258,17 +258,30 @@ 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)
> +static inline unsigned int __vma_raw_mm_seqnum(struct vm_area_struct *vma)
>  {
> +	const struct mm_struct *mm = vma->vm_mm;
> +
> +	/* We must hold an exclusive write lock for this access to be valid. */
>  	mmap_assert_write_locked(vma->vm_mm);
> +	return mm->mm_lock_seq.sequence;
> +}
>  
> +/*
> + * 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.

This line is no longer applicable.

> + */
> +static inline bool __is_vma_write_locked(struct vm_area_struct *vma)
> +{
>  	/*
>  	 * 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);
> +	return vma->vm_lock_seq == __vma_raw_mm_seqnum(vma);
>  }
>  
>  int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
> @@ -281,12 +294,10 @@ int __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq,
>   */
>  static inline void vma_start_write(struct vm_area_struct *vma)
>  {
> -	unsigned int mm_lock_seq;
> -
> -	if (__is_vma_write_locked(vma, &mm_lock_seq))
> +	if (__is_vma_write_locked(vma))
>  		return;
>  
> -	__vma_start_write(vma, mm_lock_seq, TASK_UNINTERRUPTIBLE);
> +	__vma_start_write(vma, __vma_raw_mm_seqnum(vma), TASK_UNINTERRUPTIBLE);

At this point I think __vma_start_write() could just perform
__vma_raw_mm_seqnum() itself and we can remove the param.
It could possibly make the inline code smaller.