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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Jan 26, 2026 at 12:30:04PM +0100, Vlastimil Babka wrote:
> 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@oracle.com>
>
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

Thanks!

>
> 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 This line is no longer applicable.

Is there for nostalgia's sake! :P

OK maybe not...

>
> > + */
> > +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.
>

Good idea!

Will send fix-patch for both.

Thanks, Lorenzo