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Date:   Wed, 23 Feb 2022 03:37:59 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     David Rientjes <rientjes@google.com>
Cc:     linux-mm@kvack.org, Roman Gushchin <guro@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>
Subject: Re: [PATCH 4/5] mm/slub: Limit min_partial only in cache creation
Message-ID: <YhWsF/Bz89LpLa/g@ip-172-31-19-208.ap-northeast-1.compute.internal>
References: <20220221105336.522086-1-42.hyeyoo@gmail.com>
 <20220221105336.522086-5-42.hyeyoo@gmail.com>
 <91cc8ab-a0f0-2687-df99-10b2267c7a9@google.com>
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On Tue, Feb 22, 2022 at 03:48:16PM -0800, David Rientjes wrote:
> On Mon, 21 Feb 2022, Hyeonggon Yoo wrote:
> 
> > SLUB sets number of minimum partial slabs for node (min_partial) using
> > set_min_partial(). SLUB holds at least min_partial slabs even if they're empty
> > to avoid excessive use of page allocator.
> > 
> > set_min_partial() limits value of min_partial between MIN_PARTIAL and
> > MAX_PARTIAL. As set_min_partial() can be called by min_partial_store()
> > too, Only limit value of min_partial in kmem_cache_open() so that it
> > can be changed to value that a user wants.
> > 
> > Signed-off-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
> 
> I think this makes sense and there is no reason to limit the bounds that 
> may be set at runtime with undocumented behavior.

Thank you for comment.

> 
> However, since set_min_partial() is only setting the value in the 
> kmem_cache, could we remove the helper function entirely and fold it into 
> its two callers?

Right. We don't need to separate this as function. I'll update this
in next version. Thanks!

> 
> > ---
> >  mm/slub.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 3a4458976ab7..a4964deccb61 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -4002,10 +4002,6 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)
> >  
> >  static void set_min_partial(struct kmem_cache *s, unsigned long min)
> >  {
> > -	if (min < MIN_PARTIAL)
> > -		min = MIN_PARTIAL;
> > -	else if (min > MAX_PARTIAL)
> > -		min = MAX_PARTIAL;
> >  	s->min_partial = min;
> >  }
> >  
> > @@ -4184,6 +4180,8 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> >  
> >  static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
> >  {
> > +	int min_partial;
> > +
> >  	s->flags = kmem_cache_flags(s->size, flags, s->name);
> >  #ifdef CONFIG_SLAB_FREELIST_HARDENED
> >  	s->random = get_random_long();
> > @@ -4215,7 +4213,10 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
> >  	 * The larger the object size is, the more slabs we want on the partial
> >  	 * list to avoid pounding the page allocator excessively.
> >  	 */
> > -	set_min_partial(s, ilog2(s->size) / 2);
> > +	min_partial = min(MAX_PARTIAL, ilog2(s->size) / 2);
> > +	min_partial = max(MIN_PARTIAL, min_partial);
> > +
> > +	set_min_partial(s, min_partial);
> >  
> >  	set_cpu_partial(s);
> >  
> > -- 
> > 2.33.1
> > 
> >

-- 
Hyeonggon