linux-kernel - Re: [PATCH] mm/slub: Removing unnecessary variable accesses in the get

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Message-ID: <aMDW1urp03myzZFi@hyeyoo>
Date: Wed, 10 Sep 2025 10:39:34 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: JaeJoon Jung <rgbi3307@...il.com>
Cc: cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org, vbabka@...e.cz,
        roman.gushchin@...ux.dev, 42.hyeyoo@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, rgbi3307@...er.com
Subject: Re: [PATCH] mm/slub: Removing unnecessary variable accesses in the
 get_freelist()

Hi Jaejoon,

I updated my email from 42.hyeyoo@...il.com to harry.yoo@...cle.com
a while ago. Please check up-to-date MAINTAINERS file when sending a patch.

On Wed, Sep 10, 2025 at 09:59:56AM +0900, JaeJoon Jung wrote:
> It pass a NULL pointer to the freelist_new variable
> in the __slab_update_freelist() function so that it don't have to re-fetch
> the variable values inside the while loop.

No, it needs to re-fetch values when cmpxchg fails.
Otherwise it would fall into an infinite loop, no?

at a high level overview, cmpxchg works like this (atomically, of course):

retry:
    old = var;
    // modify some bits in 'old' and store it to 'new'
    new = old + something;
    if (var == old) { // compare
         var = new; // exchange if the value is expected
    } else {
	// if var != old, someone else updated the variable. retry
        goto retry;
    }

and this retry will certainly fail if you don't you re-fetch the value,
modify it, and try cmpxchg again. The 'old' value fetched before failing
cmpxchg will not match anymore because other CPUs already updated that
variable.

> Removing unnecessary variable accesses as shown below
> will reduce the code size of the get_freelist() function and make it faster.
> 
> Signed-off-by: JaeJoon Jung <rgbi3307@...il.com>
> ---
>  mm/slub.c | 21 ++++-----------------
>  1 file changed, 4 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index d257141896c9..2e305a17a9d7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3654,27 +3654,14 @@ __update_cpu_freelist_fast(struct kmem_cache *s,
>   */
>  static inline void *get_freelist(struct kmem_cache *s, struct slab *slab)
>  {
> -	struct slab new;
> -	unsigned long counters;
> -	void *freelist;
> -
>  	lockdep_assert_held(this_cpu_ptr(&s->cpu_slab->lock));
>  
> -	do {
> -		freelist = slab->freelist;
> -		counters = slab->counters;
> -
> -		new.counters = counters;
> -
> -		new.inuse = slab->objects;
> -		new.frozen = freelist != NULL;

...and the frozen and inuse bits are part of counters field,
so they are not updated anymore?

> -
> -	} while (!__slab_update_freelist(s, slab,
> -		freelist, counters,
> -		NULL, new.counters,
> +	while (!__slab_update_freelist(s, slab,
> +		slab->freelist, slab->counters,
> +		NULL, slab->counters,
>  		"get_freelist"));
>  
> -	return freelist;
> +	return slab->freelist;
>  }
>  
>  /*
> -- 
> 2.43.0

-- 
Cheers,
Harry / Hyeonggon