Re: [PATCH RFC 08/12] mm/slab: change struct slabobj_ext to a union

[Vlastimil Babka (SUSE)]

On 7/16/26 06:11, Suren Baghdasaryan wrote:
> On Wed, Jul 15, 2026 at 3:11 AM Vlastimil Babka (SUSE)
> <vbabka@xxxxxxxxxx> wrote:
>> No functional change intended, the layout is still effectively static.
>>
>> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>
> 
> This is much cleaner than what I was preparing. Nicely done!
> 
> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Thanks!

>> ---
>>  mm/slab.h | 41 +++++++++++++++++++++++++++++++++--------
>>  mm/slub.c | 17 +++++++++--------
>>  2 files changed, 42 insertions(+), 16 deletions(-)
>>
>> diff --git a/mm/slab.h b/mm/slab.h
>> index 3ad9777ad600..359ab8caf61e 100644
>> --- a/mm/slab.h
>> +++ b/mm/slab.h
>> @@ -554,14 +554,34 @@ static inline bool need_kmalloc_no_objext(void)
>>   * if MEMCG_DATA_OBJEXTS is set.
>>   */
>>  struct slabobj_ext {
> 
> Perhaps we should add a comment here stating that every element of
> this union should be pointer-sized?
> 

OK, how about this?

--- a/mm/slab.h
+++ b/mm/slab.h
@@ -550,10 +550,14 @@ 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;