linux-kernel - Re: [PATCH] slub: Fix Off-By-One in the While condition in on

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Message-ID: <Z7blsPJiOPTFWEL2@harry>
Date: Thu, 20 Feb 2025 17:20:00 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Lilitha Persefoni Gkini <lilithpgkini@...il.com>
Cc: Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slub: Fix Off-By-One in the While condition in
 on_freelist()

On Sat, Feb 15, 2025 at 06:57:01PM +0200, Lilitha Persefoni Gkini wrote:
> The condition `nr <= slab->objects` in the `on_freelist()` serves as
> bound while walking through the `freelist` linked list because we can't
> have more free objects than the maximum amount of objects in the slab.
> But the `=` can result in an extra unnecessary iteration.
> 
> The patch changes it to `nr < slab->objects` to ensure it iterates
> at most `slab->objects` number of times.
> 
> Signed-off-by: Lilitha Persefoni Gkini <lilithpgkini@...il.com>
> ---
>  mm/slub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 1f50129dcfb3..ad42450d4b0f 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1435,7 +1435,7 @@ static int on_freelist(struct kmem_cache *s, struct slab *slab, void *search)
>  	int max_objects;
>  
>  	fp = slab->freelist;
> -	while (fp && nr <= slab->objects) {
> +	while (fp && nr < slab->objects) {

Hi, this makes sense to me.

But based on what the name of the variable suggests (nr of objects),
I think it makes clearer to initialize it to 1 instead?

--
Cheers,
Harry