linux-kernel - Re: [RFC PATCH V3 7/7] mm/slab: place slabobj_ext metadata in unused space within s->size

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Message-ID: <bf8f2ed4-0b3b-4ddc-9ce9-0d0b1a64c914@gmail.com>
Date: Wed, 29 Oct 2025 19:19:29 +0100
From: Andrey Ryabinin <ryabinin.a.a@...il.com>
To: Harry Yoo <harry.yoo@...cle.com>, akpm@...ux-foundation.org,
 vbabka@...e.cz
Cc: andreyknvl@...il.com, cl@...ux.com, dvyukov@...gle.com,
 glider@...gle.com, hannes@...xchg.org, linux-mm@...ck.org,
 mhocko@...nel.org, muchun.song@...ux.dev, rientjes@...gle.com,
 roman.gushchin@...ux.dev, shakeel.butt@...ux.dev, surenb@...gle.com,
 vincenzo.frascino@....com, yeoreum.yun@....com, tytso@....edu,
 adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V3 7/7] mm/slab: place slabobj_ext metadata in unused
 space within s->size



On 10/27/25 1:28 PM, Harry Yoo wrote:

>  			slab->obj_exts |= MEMCG_DATA_OBJEXTS;
>  		slab_set_stride(slab, sizeof(struct slabobj_ext));
> +	} else if (obj_exts_in_object(s)) {
> +		unsigned int offset = obj_exts_offset_in_object(s);
> +
> +		slab->obj_exts = (unsigned long)slab_address(slab);
> +		slab->obj_exts += s->red_left_pad;
> +		slab->obj_exts += obj_exts_offset_in_object(s);
> +		if (IS_ENABLED(CONFIG_MEMCG))
> +			slab->obj_exts |= MEMCG_DATA_OBJEXTS;
> +		slab_set_stride(slab, s->size);
> +
> +		for_each_object(addr, s, slab_address(slab), slab->objects) {
> +			kasan_unpoison_range(addr + offset,
> +					     sizeof(struct slabobj_ext));

Is this leftover from previous version? Otherwise I don't get why we unpoison this.

> +			memset(addr + offset, 0, sizeof(struct slabobj_ext));
> +		}
>  	}
>  	metadata_access_disable();
>  }