linux-kernel - Re: [PATCH 2/2] mm/mempolicy: Skip unnecessary synchronize

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Message-ID: <20250603143101.1231739-1-joshua.hahnjy@gmail.com>
Date: Tue,  3 Jun 2025 07:31:00 -0700
From: Joshua Hahn <joshua.hahnjy@...il.com>
To: "Huang, Ying" <ying.huang@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Gregory Price <gourry@...rry.net>,
	David Hildenbrand <david@...hat.com>,
	Zi Yan <ziy@...dia.com>,
	Matthew Brost <matthew.brost@...el.com>,
	Rakie Kim <rakie.kim@...com>,
	Byungchul Park <byungchul@...com>,
	Alistair Popple <apopple@...dia.com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	kernel-team@...a.com
Subject: Re: [PATCH 2/2] mm/mempolicy: Skip unnecessary synchronize_rcu()

On Tue, 03 Jun 2025 10:10:46 +0800 "Huang, Ying" <ying.huang@...ux.alibaba.com> wrote:

> Joshua Hahn <joshua.hahnjy@...il.com> writes:
> 
> > By unconditionally setting wi_state to NULL and conditionally calling
> > synchronize_rcu(), we can save an unncessary call when there is no
> > old_wi_state.
> 
> Per my understanding, in the original code, if !old_wi_state, we will
> return immediately instead of calling synchronize_rcu() too.  Or I miss
> something?
> 
> The patch itself is a nice cleanup with reduced line number.  Feel free
> to add my
> 
> Reviewed-by: Huang Ying <ying.huang@...ux.alibaba.com>

Hi Ying, you are correct. I was thinking in my mind to write "save an
unnecessary goto" but my mind must have slipped. Thank you for the catch,
and thank you for reviewing always. Have a great day!
Joshua

> in the future version.
> 
> > Suggested-by: David Hildenbrand <david@...hat.com>
> > Signed-off-by: Joshua Hahn <joshua.hahnjy@...il.com>
> > ---
> >  mm/mempolicy.c | 13 +++++--------
> >  1 file changed, 5 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 3b1dfd08338b..b0619d0020c9 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -3703,18 +3703,15 @@ static void wi_state_free(void)
> >  	struct weighted_interleave_state *old_wi_state;
> >  
> >  	mutex_lock(&wi_state_lock);
> > -
> >  	old_wi_state = rcu_dereference_protected(wi_state,
> >  			lockdep_is_held(&wi_state_lock));
> > -	if (!old_wi_state) {
> > -		mutex_unlock(&wi_state_lock);
> > -		return;
> > -	}
> > -
> >  	rcu_assign_pointer(wi_state, NULL);
> >  	mutex_unlock(&wi_state_lock);
> > -	synchronize_rcu();
> > -	kfree(old_wi_state);
> > +
> > +	if (old_wi_state) {
> > +		synchronize_rcu();
> > +		kfree(old_wi_state);
> > +	}
> >  }
> >  
> >  static struct kobj_attribute wi_auto_attr =
> 
> ---
> Best Regards,
> Huang, Ying