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

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Message-ID: <obrup5ejxuy42fpwmzczvo5ubyv62tocoqooqpxg3u5eq2i2sg@qpcialjznogf>
Date: Tue, 20 Jan 2026 23:39:19 +0800
From: Hao Li <hao.li@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
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 Tue, Jan 20, 2026 at 03:01:23PM +0100, Vlastimil Babka wrote:
> 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.

Yes, I agree - the current handling looks good to me.

> But I'll mention it in the changelog and the comment. Thanks!

Sounds great, thanks for taking care of that.

Reviewed-by: Hao Li <hao.li@...ux.dev>

-- 
Thanks,
Hao