linux-kernel - Re: [PATCH RESEND v3 03/10] mm/vma: rename is_vma_write

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Message-ID: <70a156e8-9c39-489e-9072-16b1ab483d08@lucifer.local>
Date: Mon, 26 Jan 2026 10:04:14 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.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 RESEND v3 03/10] mm/vma: rename is_vma_write_only(),
 separate out shared refcount put

On Fri, Jan 23, 2026 at 02:41:42PM +0000, Lorenzo Stoakes wrote:
> > > > +{
> > > > +     int oldcnt;
> > > > +     bool detached;
> > > > +
> > > > +     detached = __refcount_dec_and_test(&vma->vm_refcnt, &oldcnt);
> > > > +     if (refcnt)
> > > > +             *refcnt = oldcnt - 1;
> > > > +     return detached;
> >
> > IIUC there is always a connection between detached and *refcnt
> > resulting value. If detached==true then the resulting *refcnt has to
> > be 0. If so, __vma_refcount_put() can simply return (oldcnt - 1) as
> > new count:
> >
> > static inline int __vma_refcount_put(struct vm_area_struct *vma)
> > {
> >        int oldcnt;
> >
> >        __refcount_dec_and_test(&vma->vm_refcnt, &oldcnt);
>
> You can't do this as it's __must_check... :)
>
> So have to replace with __refcount_dec(), which is a void function.
>
> >        return oldcnt - 1;

Actually this doesn't work as __refcount_dec() won't let you decrement to zero
and will flag a saturated error if you do.

In the end the code looks like this:

static inline __must_check unsigned int
__vma_refcount_put_return(struct vm_area_struct *vma)
{
	int oldcnt;

	if (__refcount_dec_and_test(&vma->vm_refcnt, &oldcnt))
		return 0;

	return oldcnt - 1;
}

Which combines the __must_check, abstraction of oldcnt - 1 and xxx_return()
naming requested on review.

Cheers, Lorenzo