linux-kernel - Re: [PATCH v3 3/3] mm: page_counters: initialize usage using ATOMIC_LONG

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Message-Id: <20240728135447.bd5f7b44e1b7a583bfaaee36@linux-foundation.org>
Date: Sun, 28 Jul 2024 13:54:47 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>, Shakeel
 Butt <shakeel.butt@...ux.dev>, Muchun Song <muchun.song@...ux.dev>
Subject: Re: [PATCH v3 3/3] mm: page_counters: initialize usage using
 ATOMIC_LONG_INIT() macro

On Fri, 26 Jul 2024 19:11:10 -0400 Johannes Weiner <hannes@...xchg.org> wrote:

> On Fri, Jul 26, 2024 at 08:31:10PM +0000, Roman Gushchin wrote:
> > When a page_counter structure is initialized, there is no need to
> > use an atomic set operation to initialize the usage counter because
> > at this point the structure is not visible to anybody else.
> > ATOMIC_LONG_INIT() is what should be used in such cases.
> > 
> > Signed-off-by: Roman Gushchin <roman.gushchin@...ux.dev>
> > ---
> >  include/linux/page_counter.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h
> > index cf837d0f8ed1..5da11392b382 100644
> > --- a/include/linux/page_counter.h
> > +++ b/include/linux/page_counter.h
> > @@ -53,7 +53,7 @@ static inline void page_counter_init(struct page_counter *counter,
> >  				     struct page_counter *parent,
> >  				     bool protection_support)
> >  {
> > -	atomic_long_set(&counter->usage, 0);
> > +	counter->usage = (atomic_long_t)ATOMIC_LONG_INIT(0);
> 
> Pretty cool that ATOMIC_LONG_INIT() return value needs a cast to
> atomic_long_t! ^_^

That's because this wicked patch passed in an `int'. 

	counter->usage = ATOMIC_LONG_INIT((atomic_long_t)0);