linux-kernel - Re: [PATCH v4 08/10] mm/vma: improve and document __is_vma_write

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Message-ID: <CAJuCfpEBHsuGJFAGyXdASpaV_FY0_RBShKZM3QA_9z7ZvAtu6g@mail.gmail.com>
Date: Mon, 26 Jan 2026 11:21:29 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...nel.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Jann Horn <jannh@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	linux-rt-devel@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>, 
	Waiman Long <longman@...hat.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
	Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v4 08/10] mm/vma: improve and document __is_vma_write_locked()

On Mon, Jan 26, 2026 at 8:29 AM Lorenzo Stoakes
<lorenzo.stoakes@...cle.com> wrote:
>
> 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@...cle.com>
> >
> > Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Sorry but I have one more comment below.

Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

>
> 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)

This function returns the mm->mm_lock_seq.sequence attribute of mm, so
no real commection to VMA. IMO it's better to rename it into
__raw_mm_lock_seqnum(const struct mm_struct *mm) and have the callers
pass vma->vm_mm.

> > >  {
> > > +   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);

If for some reason you need to keep this function VMA-centric, then in
the above line please s/vma->vm_mm/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