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: <aQK2l29EczfwYQNZ@hyeyoo>
Date: Thu, 30 Oct 2025 09:51:35 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Andrey Ryabinin <ryabinin.a.a@...il.com>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, 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 Wed, Oct 29, 2025 at 07:19:29PM +0100, Andrey Ryabinin wrote:
> 
> 
> 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.

Oh god, yes! Thanks for catching. Will fix in the next version.

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

-- 
Cheers,
Harry / Hyeonggon