linux-kernel - Re: [PATCH] slab: fix clearing freelist in free_deferred

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Message-ID: <1dbe9c35-9ce7-47b5-ae32-f406bdff58fd@suse.cz>
Date: Tue, 14 Oct 2025 10:44:51 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Andrew Morton <akpm@...ux-foundation.org>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>,
 Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] slab: fix clearing freelist in free_deferred_objects()

On 10/14/25 10:40, Vlastimil Babka wrote:
> defer_free() links pending objects using the slab's freelist offset
> which is fine as they are not free yet. free_deferred_objects() then
> clears this pointer to avoid confusing the debugging consistency checks
> that may be enabled for the cache.
> 
> However, with CONFIG_SLAB_FREELIST_HARDENED, even the NULL pointer needs
> to be encoded appropriately using set_freepointer(), otherwise it's
> decoded as something else and triggers the consistency checks, as found
> by the kernel test robot.
> 
> Use set_freepointer() to prevent the issue.
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202510101652.7921fdc6-lkp@intel.com
> Acked-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
> will be added to slab/for-next-fixes and sent later this week
> ---
>  mm/slub.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index b1f15598fbfd..64c17afc375b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6443,15 +6443,16 @@ static void free_deferred_objects(struct irq_work *work)
>  		slab = virt_to_slab(x);
>  		s = slab->slab_cache;
>  
> +

extra newline moved...

> +		/* Point 'x' back to the beginning of allocated object */
> +		x -= s->offset;

... here. noticed and fixed up locally after sending...

>  		/*
>  		 * We used freepointer in 'x' to link 'x' into df->objects.
>  		 * Clear it to NULL to avoid false positive detection
>  		 * of "Freepointer corruption".
>  		 */
> -		*(void **)x = NULL;
> +		set_freepointer(s, x, NULL);
>  
> -		/* Point 'x' back to the beginning of allocated object */
> -		x -= s->offset;
>  		__slab_free(s, slab, x, x, 1, _THIS_IP_);
>  	}
>  
> 
> ---
> base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
> change-id: 20251014-fix-freelist-29915edcc2f4
> 
> Best regards,