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: <aV-Kn8vfyL5mnlJv@hyeyoo>
Date: Thu, 8 Jan 2026 19:44:47 +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 05:52:27PM +0800, Hao Li wrote:
> On Thu, Jan 08, 2026 at 05:41:00PM +0900, Harry Yoo wrote:
> > 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.
> 
> Makes sense - I think there's one more nuance worth capturing.
> slab_ksize() seems to compute the maximum safe size by applying layout
> constraints from most-restrictive to least-restrictive:
> redzones/poison/KASAN clamp it to object_size, tail metadata
> (SLAB_TYPESAFE_BY_RCU / SLAB_STORE_USER) clamps it to inuse, and only
> when nothing metadata lives does it return s->size.

Waaaait, SLAB_TYPESAFE_BY_RCU isn't the only case where we put freelist
pointer after the object.

What about caches with constructor?
We do place it after object, but slab_ksize() may return s->size? 

-- 
Cheers,
Harry / Hyeonggon