Re: [PATCH V1 2/2] mm/slab: only allow SLAB_OBJ_EXT_IN_OBJ for unmergeable caches

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Feb 04, 2026 at 08:45:19AM +0800, Hao Li wrote:
> On Tue, Feb 03, 2026 at 09:32:06PM +0900, Harry Yoo wrote:
> > On Tue, Feb 03, 2026 at 07:56:16PM +0800, Hao Li wrote:
> > > On Tue, Jan 27, 2026 at 07:31:51PM +0900, Harry Yoo wrote:
> > > > diff --git a/mm/slub.c b/mm/slub.c
> > > > index ae9af184a18b..0581847e7dac 100644
> > > > --- a/mm/slub.c
> > > > +++ b/mm/slub.c
> > > > @@ -7676,7 +7676,8 @@ static int calculate_sizes(struct kmem_cache_args *args, struct kmem_cache *s)
> > > >  	 */
> > > >  	aligned_size = ALIGN(size, s->align);
> > > >  #if defined(CONFIG_SLAB_OBJ_EXT) && defined(CONFIG_64BIT)
> > > > -	if (aligned_size - size >= sizeof(struct slabobj_ext))
> > > > +	if (slab_args_unmergeable(args, s->flags) &&
> > > > +			(aligned_size - size >= sizeof(struct slabobj_ext)))
> > > >  		s->flags |= SLAB_OBJ_EXT_IN_OBJ;
> > > 
> > > Hi Harry,
> > > 
> > > This patch looks reasonable to me. I just noticed a minor point that I
> > > wanted to bring up:
> > > 
> > > It seems a bit self-referential that SLAB_NEVER_MERGE already includes
> > > SLAB_OBJ_EXT_IN_OBJ, but we're using SLAB_NEVER_MERGE to decide whether to set
> > > SLAB_OBJ_EXT_IN_OBJ.
> > 
> > Hi Hao, thanks for bringing it up!
> >  
> > > Do you think it might be helpful to add a comment here for better clarity?
> > 
> > Hmm but I'm not sure what should be clarified here.
> > (perhaps because I wrote it).
> > 
> > Checking SLAB_OBJ_EXT_IN_OBJ (as part of SLAB_NEVER_MERGE) before
> > setting SLAB_OBJ_EXT_IN_OBJ should be fine (because it's not set before
> > we set it), and once you set it, it should prevent merging.
> 
> Yeah, s->flags currently doesn't have SLAB_OBJ_EXT_IN_OBJ, and the functionality
> here is totally fine. I just happened to notice this while reading through the
> code, and from a semantic perspective, it made me pause for a moment. It looks
> like we're checking if s->flags contains SLAB_OBJ_EXT_IN_OBJ (among other flags)
> to decide whether we can set SLAB_OBJ_EXT_IN_OBJ. Maybe we could add a small
> comment like: "SLAB_OBJ_EXT_IN_OBJ hasn't been set yet here; this is just
> checking for other unmergeable reasons."
> 
> Of course, this is just a small thought, it's perfectly fine to leave it as is-I
> just thought it might help slightly.

I think it's probably okay to leave it as-is for now.
But thanks for bringing this up!

-- 
Cheers,
Harry / Hyeonggon