linux-kernel - Re: [PATCH V5 8/8] mm/slab: place slabobj_ext metadata in unused space within s->size

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Message-ID: <aV9tnLlsecX8ukMv@hyeyoo>
Date: Thu, 8 Jan 2026 17:41:00 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Hao Li <hao.li@...ux.dev>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, andreyknvl@...il.com,
        cl@...two.org, 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,
        ryabinin.a.a@...il.com, 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, cgroups@...r.kernel.org
Subject: Re: [PATCH V5 8/8] mm/slab: place slabobj_ext metadata in unused
 space within s->size

On Thu, Jan 08, 2026 at 01:52:09PM +0800, Hao Li wrote:
> On Mon, Jan 05, 2026 at 05:02:30PM +0900, Harry Yoo wrote:
> > When a cache has high s->align value and s->object_size is not aligned
> > to it, each object ends up with some unused space because of alignment.
> > If this wasted space is big enough, we can use it to store the
> > slabobj_ext metadata instead of wasting it.
> 
> Hi, Harry,

Hi Hao,

> When we save obj_ext in s->size space, it seems that slab_ksize() might
> be missing the corresponding handling.

Oops.

> It still returns s->size, which could cause callers of slab_ksize()
> to see unexpected data (i.e. obj_ext), or even overwrite the obj_ext data.

Yes indeed.
Great point, thanks!

I'll fix it by checking if the slab has obj_exts within the object
layout and returning s->object_size if so.

-- 
Cheers,
Harry / Hyeonggon