linux-kernel - Re: [PATCH] slub: let need_slab_obj_exts() return false if SLAB_NO_OBJ

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Message-ID: <aYXAqQFGfru8aC6M@hyeyoo>
Date: Fri, 6 Feb 2026 19:21:29 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Hao Li <hao.li@...ux.dev>, akpm@...ux-foundation.org, cl@...two.org,
        rientjes@...gle.com, roman.gushchin@...ux.dev, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slub: let need_slab_obj_exts() return false if
 SLAB_NO_OBJ_EXT is set

On Fri, Feb 06, 2026 at 11:03:08AM +0100, Vlastimil Babka wrote:
> On 2/6/26 11:01, Harry Yoo wrote:
> > On Fri, Feb 06, 2026 at 10:40:54AM +0100, Vlastimil Babka wrote:
> >> On 2/6/26 01:53, Harry Yoo wrote:
> >> > On Thu, Feb 05, 2026 at 08:07:23PM +0800, Hao Li wrote:
> >> >> SLAB_NO_OBJ_EXT is set for boot caches, but need_slab_obj_exts() doesn't
> >> >> check this flag. We should return false unconditionally when
> >> >> SLAB_NO_OBJ_EXT is set.
> >> >> 
> >> >> Signed-off-by: Hao Li <hao.li@...ux.dev>
> >> >> ---
> >> > 
> >> > Looks reasonable to me,
> >> > Acked-by: Harry Yoo <harry.yoo@...cle.com>
> >> 
> >> Thanks, added to slab/for-7.0/obj_metadata
> >> I think it doesn't change anything at the moment?
> > 
> > ...unless you enable memory allocation profiling :)
> 
> If I do, would kmem_cache or kmem_cache_node caches gotten obj_exts in
> leftover space?

/me builds and tests a new kernel with mem profiling always enabled...

Yes, but only if merging is disabled.

If you disable slab merging with `slab_nomerge`, kmem_cache gets
obj_exts from SLAB_OBJ_EXTS_IN_OBJ.

When I enable debug feature slab_debug=P (and thus change s->size),
kmem_cache_node gets obj_exts from SLAB_OBJ_EXTS_IN_OBJ.

-- 
Cheers,
Harry / Hyeonggon