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

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

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.

-- 
Cheers,
Harry / Hyeonggon