Re: [PATCH v2 09/13] mm/slab: change struct slabobj_ext to a union

On 7/23/26 12:05 PM, Hao Li wrote:
> On Mon, Jul 20, 2026 at 04:16:23PM +0200, Vlastimil Babka (SUSE) wrote:
>> Currently, struct slabobj_ext can hold both objcg pointer and
>> codetag_ref (when both are compile-enabled) and there is an array of as
>> many slabobj_ext instances as there are objects in a slab.
>>
>> This makes the layout fixed so even if codetag_ref is unused (because
>> memory allocation profiling is disabled), the space for them is
>> allocated and wasted. Similarly, some caches (currently kmalloc_normal)
>> do not ever need objcg pointers, leading to wasted memory with memory
>> allocation profiling enabled.
>>
>> To make this more flexible, change the layout so that struct slabobj_ext
>> becomes a union of objcg pointer and codetag_ref (to ensure uniform
>> size; in practice both are the same size anyway). The slabobj_ext array
>> then can have twice as many elements as before. For cache locality
>> purposes, the effective memory layout is unchanged, so objcg and codetag
>> ref for a given object are still adjacent.
>>
>> cache_obj_ext_size() returns the effective size of (0-2) struct
>> slabobj_ext's for a cache, slab_obj_ext_size() for a slab. Currently
>> both return a constant value derived from the config options, but will
>> be made dynamic later. Replace all sizeof(slabobj_ext) usage with these.
>>
>> No functional change intended, the layout is still effectively static.
>>
>> Reviewed-by: Suren Baghdasaryan <[email protected]>
>> Signed-off-by: Vlastimil Babka (SUSE) <[email protected]>
>> ---
>>  mm/slab.h | 49 +++++++++++++++++++++++++++++++++++++++----------
>>  mm/slub.c | 19 +++++++++++--------
>>  2 files changed, 50 insertions(+), 18 deletions(-)
>>
>> diff --git a/mm/slab.h b/mm/slab.h
>> index e586798e4f16..f8446167e175 100644
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -550,18 +550,42 @@ static inline bool need_kmalloc_no_objext(void)
>>  }
>>  
>>  /*
>> - * Extended information for slab objects stored as an array in page->memcg_data
>> - * if MEMCG_DATA_OBJEXTS is set.
>> + * Extended information for slab objects stored as a pointer to an array in
>> + * slab->obj_exts (aliasing page->memcg_data) if MEMCG_DATA_OBJEXTS is set.
>>   */
>>  struct slabobj_ext {
>> +	/*
>> +	 * All elements of the union should be pointer-sized to avoid memory
>> +	 * waste
>> +	 */
>> +	union {
>>  #ifdef CONFIG_MEMCG
>> -	struct obj_cgroup *_objcg;
>> +		struct obj_cgroup *_objcg;
>>  #endif
>>  #ifdef CONFIG_MEM_ALLOC_PROFILING
>> -	union codetag_ref _ctref;
>> +		union codetag_ref _ctref;
>>  #endif
>> +	};
>>  } __aligned(8);
> 
> slabobj_ext is now acting as a generic struct, serving as either _objcg or
> _ctref. However, because it's set to __aligned(8), I'm wondering if this might
> waste half the memory on 32-bit?

We could loosen the alignment to
max(__NR_OBJEXTS_FLAGS, alignof(void *))

and drop OBJEXT_FLAG_UNUSED.

...not sure how much we care about this on 32bit though.

-- 
Cheers,
Harry / Hyeonggon

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