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 4wLFw/T5 t1ExZ1np13kbd0lrAe8qcF/YWTRdBxTurkz52p3wyTzxud9h6CVj1MSZJJwQ0wIgF1C0Ih7S/wmkmZaW+lpom99k2e1bfnzdFMnpQNWdd8NaEmGTK2j8j5j04h6epgYnLJX+/ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > > Signed-off-by: Harry Yoo > > --- > > 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