linux-kernel - Re: [PATCH] slab: replace cache_from

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Message-ID: <efd02f1f-b09b-4786-aa55-aa366f58442f@suse.cz>
Date: Tue, 20 Jan 2026 15:01:23 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Hao Li <hao.li@...ux.dev>
Cc: Harry Yoo <harry.yoo@...cle.com>, Eric Dumazet <edumazet@...gle.com>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] slab: replace cache_from_obj() with inline checks

On 1/20/26 14:56, Hao Li wrote:
> On Tue, Jan 20, 2026 at 01:55:09PM +0100, Vlastimil Babka wrote:
>> On 1/20/26 12:57, Hao Li wrote:
>> 
>> >> @@ -6774,11 +6770,21 @@ static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
>> >>   */
>> >>  void kmem_cache_free(struct kmem_cache *s, void *x)
>> >>  {
>> >> -	s = cache_from_obj(s, x);
>> >> -	if (!s)
>> >> -		return;
>> >> +	struct slab *slab;
>> >> +
>> >> +	slab = virt_to_slab(x);
>> >> +
>> >> +	if (IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) ||
>> >> +	    kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS)) {
>> >> +
>> >> +		if (unlikely(!slab || (slab->slab_cache != s))) {
>> >> +			warn_free_bad_obj(s, x);
>> > 
>> > Just to make sure I'm understanding correctly, are we intentionally not
>> > releasing the object x in this case? Thanks.
>> 
>> Yes, it means something went wrong so it's better not to do anything.
> 
> Got it, that makes sense - thanks!
> 
>> That was true before this patch as well.
> 
> I'm still not entirely sure I follow. I read the original code, and it seems
> like it retrieved the real cache from the object and released the object into
> that correct cache. Did I misunderstand something?

You're right, I misread it as returning NULL, but that was only on
virt_to_slab() failure. So that's a change, but I believe a proper one. But
I'll mention it in the changelog and the comment. Thanks!