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: <7uiizca4ejiqw6zegjwmou5va4kw7na7wivy4kxebrju7dsdwo@5brr7vhwf5oh>
Date: Thu, 8 Jan 2026 17:52:27 +0800
From: Hao Li <hao.li@...ux.dev>
To: Harry Yoo <harry.yoo@...cle.com>
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 05:41:00PM +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.

With that ordering in mind, SLAB_OBJ_EXT_IN_OBJ should behave like
another "tail metadata" cap: put the check right before `return s->size`,
and if it's set, return s->inuse instead. Curious what you think.

-- 
Thanks,
Hao

> 
> -- 
> Cheers,
> Harry / Hyeonggon