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: <pyhgduyryvfoxfb72hdnojd63fkgwx2jbn5r2h4cuthkki2ouf@ntuesnpnterc>
Date: Thu, 8 Jan 2026 20:48:46 +0800
From: Hao Li <hao.li@...ux.dev>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, akpm@...ux-foundation.org, 
	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 11:52:36AM +0100, Vlastimil Babka wrote:
> On 1/8/26 11:44, Harry Yoo wrote:
> > 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? 
> 
> I think the freelist pointer is fine because it's not used by allocated objects?
> Also ksize() should no longer be used to fill more of the object than that
> was requested in the first place.

Yes - being conservative here seems safest. Exposing extra bytes that
callers don't expect could easily break assumptions and lead to subtle
bugs.

-- 
Thanks,
Hao

> 
> 
>