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

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

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:
> > > While SLAB_OBJ_EXT_IN_OBJ allows to reduce memory overhead to account
> > > slab objects, it prevents slab merging because merging can change
> > > the metadata layout.
> > > 
> > > As pointed out Vlastimil Babka, disabling merging solely for this memory
> > > optimization may not be a net win, because disabling slab merging tends
> > > to increase overall memory usage.
> > > 
> > > Restrict SLAB_OBJ_EXT_IN_OBJ to caches that are already unmergeable for
> > > other reasons (e.g., those with constructors or SLAB_TYPESAFE_BY_RCU).
> > > 
> > > Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> > > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> > > ---
> > >  mm/slab.h        | 1 +
> > >  mm/slab_common.c | 3 +--
> > >  mm/slub.c        | 3 ++-
> > >  3 files changed, 4 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/mm/slab.h b/mm/slab.h
> > > index 8593c506cbf1..a5c4f981ee8b 100644
> > > --- a/mm/slab.h
> > > +++ b/mm/slab.h
> > > @@ -388,6 +388,7 @@ extern void create_boot_cache(struct kmem_cache *, const char *name,
> > >  			unsigned int useroffset, unsigned int usersize);
> > >  
> > >  int slab_unmergeable(struct kmem_cache *s);
> > > +bool slab_args_unmergeable(struct kmem_cache_args *args, slab_flags_t flags);
> > >  
> > >  slab_flags_t kmem_cache_flags(slab_flags_t flags, const char *name);
> > >  
> > > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > > index 904414c3ebb8..d5a70a831a2a 100644
> > > --- a/mm/slab_common.c
> > > +++ b/mm/slab_common.c
> > > @@ -174,8 +174,7 @@ int slab_unmergeable(struct kmem_cache *s)
> > >  	return 0;
> > >  }
> > >  
> > > -static bool slab_args_unmergeable(struct kmem_cache_args *args,
> > > -				  slab_flags_t flags)
> > > +bool slab_args_unmergeable(struct kmem_cache_args *args, slab_flags_t flags)
> > >  {
> > >  	if (slab_nomerge)
> > >  		return true;
> > > 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.

-- 
Thanks,
Hao

> 
> -- 
> Cheers,
> Harry / Hyeonggon