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

[Hao Li <hao.li@linux.dev>]

On Thu, Feb 05, 2026 at 02:13:23PM +0900, Harry Yoo wrote:
> 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!
> 

Ah, no problem, thanks!

-- 
Thanks,
Hao